Message-ID: <20200420201615.GC302402@kroah.com>
Date:   Mon, 20 Apr 2020 22:16:15 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
        ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org,
        mhocko@...e.com, yukuai3@...wei.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>,
        syzbot+603294af2d01acfdd6da@...kaller.appspotmail.com
Subject: Re: [PATCH v2 03/10] blktrace: fix debugfs use after free

On Sun, Apr 19, 2020 at 07:45:22PM +0000, Luis Chamberlain wrote:
> On commit 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
> merged on v4.12 Omar fixed the original blktrace code for request-based
> drivers (multiqueue). This however left in place a possible crash, if you
> happen to abuse blktrace in a way it was not intended, and even more so
> with our current asynchronous request_queue removal.
> 
> Namely, if you loop adding a device, setup the blktrace with BLKTRACESETUP,
> forget to BLKTRACETEARDOWN, and then just remove the device you end up
> with a panic:
> 
> [  107.193134] debugfs: Directory 'loop0' with parent 'block' already present!
> [  107.254615] BUG: kernel NULL pointer dereference, address: 00000000000000a0
> [  107.258785] #PF: supervisor write access in kernel mode
> [  107.262035] #PF: error_code(0x0002) - not-present page
> [  107.264106] PGD 0 P4D 0
> [  107.264404] Oops: 0002 [#1] SMP NOPTI
> [  107.264803] CPU: 8 PID: 674 Comm: kworker/8:2 Tainted: G            E 5.6.0-rc7-next-20200327 #1
> [  107.265712] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> [  107.266553] Workqueue: events __blk_release_queue
> [  107.267051] RIP: 0010:down_write+0x15/0x40
> [  107.267488] Code: eb ca e8 ee a5 8d ff cc cc cc cc cc cc cc cc cc cc cc cc cc cc 0f 1f 44 00 00 55 48 89 fd e8 52 db ff ff 31 c0 ba 01 00 00 00 <f0> 48 0f b1 55 00 75 0f  65 48 8b 04 25 c0 8b 01 00 48 89 45 08 5d
> [  107.269300] RSP: 0018:ffff9927c06efda8 EFLAGS: 00010246
> [  107.269841] RAX: 0000000000000000 RBX: ffff8be7e73b0600 RCX: ffffff8100000000
> [  107.270559] RDX: 0000000000000001 RSI: ffffff8100000000 RDI: 00000000000000a0
> [  107.271281] RBP: 00000000000000a0 R08: ffff8be7ebc80fa8 R09: ffff8be7ebc80fa8
> [  107.272001] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> [  107.272722] R13: ffff8be7efc30400 R14: ffff8be7e0571200 R15: 00000000000000a0
> [  107.273475] FS:  0000000000000000(0000) GS:ffff8be7efc00000(0000) knlGS:0000000000000000
> [  107.274346] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  107.274968] CR2: 00000000000000a0 CR3: 000000042abee003 CR4: 0000000000360ee0
> [  107.275710] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [  107.276465] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [  107.277214] Call Trace:
> [  107.277532]  simple_recursive_removal+0x4e/0x2e0
> [  107.278049]  ? debugfs_remove+0x60/0x60
> [  107.278493]  debugfs_remove+0x40/0x60
> [  107.278922]  blk_trace_free+0xd/0x50
> [  107.279339]  __blk_trace_remove+0x27/0x40
> [  107.279797]  blk_trace_shutdown+0x30/0x40
> [  107.280256]  __blk_release_queue+0xab/0x110
> [  107.280734]  process_one_work+0x1b4/0x380
> [  107.281194]  worker_thread+0x50/0x3c0
> [  107.281622]  kthread+0xf9/0x130
> [  107.281994]  ? process_one_work+0x380/0x380
> [  107.282467]  ? kthread_park+0x90/0x90
> [  107.282895]  ret_from_fork+0x1f/0x40
> [  107.283316] Modules linked in: loop(E) <etc>
> [  107.288562] CR2: 00000000000000a0
> [  107.288957] ---[ end trace b885d243d441bbce ]---
> 
> This splat happens to be very similar to the one reported via
> kernel.org korg#205713, only that korg#205713 was for v4.19.83
> and the above now includes the simple_recursive_removal() introduced
> via commit a3d1e7eb5abe ("simple_recursive_removal(): kernel-side rm
> -rf for ramfs-style filesystems") merged on v5.6.
> 
> korg#205713 then was used to create CVE-2019-19770 and claims that
> the bug is in a use-after-free in the debugfs core code. The
> implications of this being a generic UAF on debugfs would be
> much more severe, as it would imply parent dentries can sometimes
> not be positive, which we hold by design is just not possible.
> 
> Below is the splat explained with a bit more details, explaining
> what is happening in userspace, kernel, and a print of the CPU on,
> which the code runs on:
> 
> load loopback module
> [   13.603371] == blk_mq_debugfs_register(12) start
> [   13.604040] == blk_mq_debugfs_register(12) q->debugfs_dir created
> [   13.604934] == blk_mq_debugfs_register(12) end
> [   13.627382] == blk_mq_debugfs_register(12) start
> [   13.628041] == blk_mq_debugfs_register(12) q->debugfs_dir created
> [   13.629240] == blk_mq_debugfs_register(12) end
> [   13.651667] == blk_mq_debugfs_register(12) start
> [   13.652836] == blk_mq_debugfs_register(12) q->debugfs_dir created
> [   13.655107] == blk_mq_debugfs_register(12) end
> [   13.684917] == blk_mq_debugfs_register(12) start
> [   13.687876] == blk_mq_debugfs_register(12) q->debugfs_dir created
> [   13.691588] == blk_mq_debugfs_register(13) end
> [   13.707320] == blk_mq_debugfs_register(13) start
> [   13.707863] == blk_mq_debugfs_register(13) q->debugfs_dir created
> [   13.708856] == blk_mq_debugfs_register(13) end
> [   13.735623] == blk_mq_debugfs_register(13) start
> [   13.736656] == blk_mq_debugfs_register(13) q->debugfs_dir created
> [   13.738411] == blk_mq_debugfs_register(13) end
> [   13.763326] == blk_mq_debugfs_register(13) start
> [   13.763972] == blk_mq_debugfs_register(13) q->debugfs_dir created
> [   13.765167] == blk_mq_debugfs_register(13) end
> [   13.779510] == blk_mq_debugfs_register(13) start
> [   13.780522] == blk_mq_debugfs_register(13) q->debugfs_dir created
> [   13.782338] == blk_mq_debugfs_register(13) end
> [   13.783521] loop: module loaded
> 
> LOOP_CTL_DEL(loop0) #1
> [   13.803550] = __blk_release_queue(4) start
> [   13.807772] == blk_trace_shutdown(4) start
> [   13.810749] == blk_trace_shutdown(4) end
> [   13.813437] = __blk_release_queue(4) calling blk_mq_debugfs_unregister()
> [   13.817593] ==== blk_mq_debugfs_unregister(4) begin
> [   13.817621] ==== blk_mq_debugfs_unregister(4) debugfs_remove_recursive(q->debugfs_dir)
> [   13.821203] ==== blk_mq_debugfs_unregister(4) end q->debugfs_dir is NULL
> [   13.826166] = __blk_release_queue(4) blk_mq_debugfs_unregister() end
> [   13.832992] = __blk_release_queue(4) end
> 
> LOOP_CTL_ADD(loop0) #1
> [   13.843742] == blk_mq_debugfs_register(7) start
> [   13.845569] == blk_mq_debugfs_register(7) q->debugfs_dir created
> [   13.848628] == blk_mq_debugfs_register(7) end
> 
> BLKTRACE_SETUP(loop0) #1
> [   13.850924] == blk_trace_ioctl(7, BLKTRACESETUP) start
> [   13.852852] === do_blk_trace_setup(7) start
> [   13.854580] === do_blk_trace_setup(7) creating directory
> [   13.856620] === do_blk_trace_setup(7) using what debugfs_lookup() gave
> [   13.860635] === do_blk_trace_setup(7) end with ret: 0
> [   13.862615] == blk_trace_ioctl(7, BLKTRACESETUP) end
> 
> LOOP_CTL_DEL(loop0) #2
> [   13.883304] = __blk_release_queue(7) start
> [   13.885324] == blk_trace_shutdown(7) start
> [   13.887197] == blk_trace_shutdown(7) calling __blk_trace_remove()
> [   13.889807] == __blk_trace_remove(7) start
> [   13.891669] === blk_trace_cleanup(7) start
> [   13.911656] ====== blk_trace_free(7) start
> 
> This is a scheduled __blk_release_queue(). Note that at this point
> we know that the old device is gone as the gendisk removal is
> synchronous.
> 
> LOOP_CTL_ADD(loop0) #2
> [   13.912709] == blk_mq_debugfs_register(2) start
> 
> This is a *new* device, as we know the last one was removed synchronously.
> 
> ---> From LOOP_CTL_DEL(loop0) #2
> [   13.915887] ====== blk_trace_free(7) end
> 
> ---> From LOOP_CTL_ADD(loop0) #2
> [   13.918359] debugfs: Directory 'loop0' with parent 'block' already present!
> 
> Above we have a case where the upon LOOP_CTL_ADD(loop) #2 the
> debugfs_create_dir() start_creating() call will have found an existing dentry,
> and return ERR_PTR(-EEXIST), and so the q->debugfs_dir will be initialized to
> NULL. This clash happens because an asynchronous __blk_release_queue() has
> been scheduled from the above LOOP_CTL_DEL(loop0) #2 call, and it has not yet
> removed the old loop0 debugfs directory.
> 
> [   13.926433] == blk_mq_debugfs_register(2) q->debugfs_dir created
> 
> This is a huge assumption, since we never check for errors on
> blk_mq_debugfs_register(), and so the child files we need to
> create *can* simply be created using NULL as as the parent, in
> which case debugfs would use debugfs_mount->mnt_root as the parent.
> 
> [   13.930373] == blk_mq_debugfs_register(2) end
> 
> BLKTRACE_SETUP(loop0) #2
> [   13.933961] == blk_trace_ioctl(2, BLKTRACESETUP) start
> [   13.936758] === do_blk_trace_setup(2) start
> [   13.938944] === do_blk_trace_setup(2) creating directory
> [   13.941029] === do_blk_trace_setup(2) using what debugfs_lookup() gave
> 
> Here debugfs_lookup() will have found the old debugfs directory, even
> though q->debugfs_dir was empty when initializing this device. So the
> files created can actually be created where they should have.
> 
> ---> From LOOP_CTL_DEL(loop0) #2
> [   13.971046] === blk_trace_cleanup(7) end
> [   13.973175] == __blk_trace_remove(7) end
> [   13.975352] == blk_trace_shutdown(7) end
> [   13.977415] = __blk_release_queue(7) calling blk_mq_debugfs_unregister()
> [   13.980645] ==== blk_mq_debugfs_unregister(7) begin
> [   13.980696] ==== blk_mq_debugfs_unregister(7) debugfs_remove_recursive(q->debugfs_dir)
> 
> This is a continuation from the old scheduled  __blk_release_queue(). It
> is using another request_queue structure than what the currently activated
> blktrace setup is using.
> 
> Here now the dentry where the blktrace setup files were created on top
> of is gone.  In fact, since debugfs_remove_recursive() is used, *all* of
> those blktrace debugfs files should have been removed as well, including
> the relay. This is despite the fact that the removal and blktrace setup
> are working on two different request_queue structures.
> 
> [   13.983118] ==== blk_mq_debugfs_unregister(7) end q->debugfs_dir is NULL
> [   13.986945] = __blk_release_queue(7) blk_mq_debugfs_unregister() end
> [   13.993155] = __blk_release_queue(7) end
> 
> The scheduled __blk_release_queue completes finally.
> 
> ---> From BLKTRACE_SETUP(loop0) #2
> [   13.995928] === do_blk_trace_setup(2) end with ret: 0
> [   13.997623] == blk_trace_ioctl(2, BLKTRACESETUP) end
> 
> At this point the old blktrace *setup* is complete though, and let us
> recall that the old dentry is still used. But recall that we just had
> our old device removed, and since debugfs_remove_recursive() was used,
> so are all of its files. This request_queue however is still active.
> 
> LOOP_CTL_DEL(loop0) #3
> [   14.035119] = __blk_release_queue(2) start
> [   14.036925] == blk_trace_shutdown(2) start
> [   14.038518] == blk_trace_shutdown(2) calling __blk_trace_remove()
> [   14.040829] == __blk_trace_remove(2) start
> [   14.042413] === blk_trace_cleanup(2) start
> 
> This removal will cleanup the blktrace setup. Note the trap though,
> the blktrace setup will already have been cleaned up since the
> scheduled removal from instance #2 removed the files on the same
> debugfs path.
> 
> The __blk_release_queue() is scheduled and is asynchronous so can
> continue later, and that is where the crash happens, on CPU 2.
> 
> LOOP_CTL_ADD(loop0) #3
> [   14.072522] == blk_mq_debugfs_register(6) start
> 
> ---> From LOOP_CTL_DEL(loop0) #3
> [   14.075151] ====== blk_trace_free(2) start
> 
> ---> From LOOP_CTL_ADD(loop0) #3
> [   14.075882] == blk_mq_debugfs_register(6) q->debugfs_dir created
> 
> ---> From LOOP_CTL_DEL(loop0) #3
> The panic happens on CPU 2 when we continue with the old scheduled
> removal of the request_queue, which will run into a blktrace setup
> with NULL pointers.
> 
> [   14.078624] BUG: kernel NULL pointer dereference, address: 00000000000000a0
> [   14.084332] == blk_mq_debugfs_register(6) end
> [   14.086971] #PF: supervisor write access in kernel mode
> [   14.086974] #PF: error_code(0x0002) - not-present page
> [   14.086977] PGD 0 P4D 0
> [   14.086984] Oops: 0002 [#1] SMP NOPTI
> [   14.086990] CPU: 2 PID: 287 Comm: kworker/2:2 Tainted: G            E 5.6.0-next-20200403+ #54
> [   14.086991] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> [   14.087002] Workqueue: events __blk_release_queue
> [   14.087011] RIP: 0010:down_write+0x15/0x40
> [   14.090300] == blk_trace_ioctl(6, BLKTRACESETUP) start
> [   14.093277] Code: eb ca e8 3e 34 8d ff cc cc cc cc cc cc cc cc cc cc
> cc cc cc cc 0f 1f 44 00 00 55 48 89 fd e8 52 db ff ff 31 c0 ba 01 00 00
> 00 <f0> 48 0f b1 55 00 75 0f 65 48 8b 04 25 c0 8b 01 00 48 89 45 08 5d
> [   14.093280] RSP: 0018:ffffc28a00533da8 EFLAGS: 00010246
> [   14.093284] RAX: 0000000000000000 RBX: ffff9f7a24d07980 RCX: ffffff8100000000
> [   14.093286] RDX: 0000000000000001 RSI: ffffff8100000000 RDI: 00000000000000a0
> [   14.093287] RBP: 00000000000000a0 R08: 0000000000000000 R09: 0000000000000019
> [   14.093289] R10: 0000000000000774 R11: 0000000000000000 R12: 0000000000000000
> [   14.093291] R13: ffff9f7a2fab0400 R14: ffff9f7a21dd1140 R15: 00000000000000a0
> [   14.093294] FS:  0000000000000000(0000) GS:ffff9f7a2fa80000(0000) knlGS:0000000000000000
> [   14.093296] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   14.093298] CR2: 00000000000000a0 CR3: 00000004293d2003 CR4: 0000000000360ee0
> [   14.093307] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   14.093308] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [   14.093310] Call Trace:
> [   14.093324]  simple_recursive_removal+0x4e/0x2e0
> [   14.093330]  ? debugfs_remove+0x60/0x60
> [   14.093334]  debugfs_remove+0x40/0x60
> [   14.093339]  blk_trace_free+0x20/0x70
> [   14.093346]  __blk_trace_remove+0x54/0x90
> [   14.096704] === do_blk_trace_setup(6) start
> [   14.098534]  blk_trace_shutdown+0x74/0x80
> [   14.100958] === do_blk_trace_setup(6) creating directory
> [   14.104575]  __blk_release_queue+0xbe/0x160
> [   14.104580]  process_one_work+0x1b4/0x380
> [   14.104585]  worker_thread+0x50/0x3c0
> [   14.104589]  kthread+0xf9/0x130
> [   14.104593]  ? process_one_work+0x380/0x380
> [   14.104596]  ? kthread_park+0x90/0x90
> [   14.104599]  ret_from_fork+0x1f/0x40
> [   14.104603] Modules linked in: loop(E) xfs(E) libcrc32c(E)
> crct10dif_pclmul(E) crc32_pclmul(E) ghash_clmulni_intel(E) joydev(E)
> serio_raw(E) aesni_intel(E) glue_helper(E) virtio_balloon(E) evdev(E)
> crypto_simd(E) pcspkr(E) cryptd(E) i6300esb(E) button(E) ip_tables(E)
> x_tables(E) autofs4(E) ext4(E) crc32c_generic(E) crc16(E) mbcache(E)
> jbd2(E) virtio_net(E) net_failover(E) failover(E) virtio_blk(E)
> ata_generic(E) uhci_hcd(E) ata_piix(E) ehci_hcd(E) nvme(E) libata(E)
> crc32c_intel(E) usbcore(E) psmouse(E) nvme_core(E) virtio_pci(E)
> scsi_mod(E) virtio_ring(E) t10_pi(E) virtio(E) i2c_piix4(E) floppy(E)
> [   14.107400] === do_blk_trace_setup(6) using what debugfs_lookup() gave
> [   14.108939] CR2: 00000000000000a0
> [   14.110589] === do_blk_trace_setup(6) end with ret: 0
> [   14.111592] ---[ end trace 7a783b33b9614db9 ]---
> 
> The root cause to this issue is that debugfs_lookup() can find a
> previous incarnation's dir of the same name which is about to get
> removed from a not yet schedule work. When that happens, the the files
> are taken underneath the nose of the blktrace, and when it comes time to
> cleanup, these dentries are long gone because of a scheduled removal.
> 
> This issue is happening because of two reasons:
> 
>   1) The request_queue is currently removed asynchronously as of commit
>      dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
>      v4.12, this allows races with userspace which were not possible
>      before unless as removal of a block device used to happen
>      synchronously with its request_queue. One could however still
>      parallelize blksetup calls while one loops on device addition and
>      removal.
> 
>   2) There are no errors checks when we create the debugfs directory,
>      be it on init or for blktrace. The concept of when the directory
>      *should* really exist is further complicated by the fact that
>      we have asynchronous request_queue removal. And, we have no
>      real sanity checks to ensure we don't re-create the queue debugfs
>      directory.
> 
> We can fix the UAF by using a debugfs directory which moving forward
> will always be accessible if debugfs is enabled, this way, its allocated
> and avaialble always for both request-based block drivers or
> make_request drivers (multiqueue) block drivers.
> 
> We also put sanity checks in place to ensure that if the directory is
> found with debugfs_lookup() it is the dentry we expect. When doing a
> blktrace against a parition, we will always be creating a temporary
> debugfs directory, so ensure that only exists once as well to avoid
> issues against concurrent blktrace runs.
> 
> Lastly, since we are now always creating the needed request_queue
> debugfs directory upon init, we can also take the initiative to
> proactively check against errors. We currently do not check for
> errors on add_disk() and friends, but we shouldn't make the issue
> any worse.
> 
> This also simplifies the code considerably, with the only penalty now
> being that we're always creating the request queue debugfs directory for
> the request-based block device drivers.
> 
> The UAF then is not a core debugfs issue, but instead a complex misuse
> of debugfs, and this issue can only be triggered if you are root.
> 
> This issue can be reproduced with break-blktrace [2] using:
> 
>   break-blktrace -c 10 -d -s
> 
> This patch fixes this issue. Note that there is also another
> respective UAF but from the ioctl path [3], this should also fix
> that issue.
> 
> This patch then also disputes the severity of CVE-2019-19770 as
> this issue is only possible by being root and using blktrace.
> 
> It is not a core debugfs issue.
> 
> [0] https://bugzilla.kernel.org/show_bug.cgi?id=205713
> [1] https://nvd.nist.gov/vuln/detail/CVE-2019-19770
> [2] https://github.com/mcgrof/break-blktrace
> [3] https://lore.kernel.org/lkml/000000000000ec635b059f752700@google.com/
> 
> Cc: Bart Van Assche <bvanassche@....org>
> Cc: Omar Sandoval <osandov@...com>
> Cc: Hannes Reinecke <hare@...e.com>
> Cc: Nicolai Stange <nstange@...e.de>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: yu kuai <yukuai3@...wei.com>
> Reported-by: syzbot+603294af2d01acfdd6da@...kaller.appspotmail.com
> Fixes: 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
> Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> ---
>  block/blk-debugfs.c          | 30 +++++++++++
>  block/blk-mq-debugfs.c       |  5 --
>  block/blk-sysfs.c            |  9 ++++
>  block/blk.h                  | 11 ++++
>  include/linux/blkdev.h       |  5 +-
>  include/linux/blktrace_api.h |  1 +
>  kernel/trace/blktrace.c      | 98 +++++++++++++++++++++++++++++++++---
>  7 files changed, 146 insertions(+), 13 deletions(-)

This patch triggered gmail's spam detection, your changelog text is
whack...

> diff --git a/block/blk-debugfs.c b/block/blk-debugfs.c
> index 19091e1effc0..d84038bce0a5 100644
> --- a/block/blk-debugfs.c
> +++ b/block/blk-debugfs.c
> @@ -3,6 +3,9 @@
>  /*
>   * Shared request-based / make_request-based functionality
>   */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
>  #include <linux/kernel.h>
>  #include <linux/blkdev.h>
>  #include <linux/debugfs.h>
> @@ -13,3 +16,30 @@ void blk_debugfs_register(void)
>  {
>  	blk_debugfs_root = debugfs_create_dir("block", NULL);
>  }
> +
> +int __must_check blk_queue_debugfs_register(struct request_queue *q)
> +{
> +	struct dentry *dir = NULL;
> +
> +	/* This can happen if we have a bug in the lower layers */
> +	dir = debugfs_lookup(kobject_name(q->kobj.parent), blk_debugfs_root);
> +	if (dir) {
> +		pr_warn("%s: registering request_queue debugfs directory twice is not allowed\n",
> +			kobject_name(q->kobj.parent));
> +		dput(dir);
> +		return -EALREADY;
> +	}
> +
> +	q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
> +					    blk_debugfs_root);
> +	if (!q->debugfs_dir)
> +		return -ENOMEM;

Why doesn't the directory just live in the request queue, or somewhere
else, so that you save it when it is created and then that's it.  No
need to "look it up" anywhere else.

Or do you do that in later patches?  I only see this one at the moment,
sorry...

>  static struct dentry *blk_trace_debugfs_dir(struct blk_user_trace_setup *buts,
> +					    struct request_queue *q,
>  					    struct blk_trace *bt)
>  {
>  	struct dentry *dir = NULL;
>  
> +	/* This can only happen if we have a bug on our lower layers */
> +	if (!q->kobj.parent) {
> +		pr_warn("%s: request_queue parent is gone\n", buts->name);

A kobject always has a parent, unless it has not been registered yet, so
I don't know what you are testing could ever happen.



> +		return NULL;
> +	}
> +
> +	/*
> +	 * From a sysfs kobject perspective, the request_queue sits on top of
> +	 * the gendisk, which has the name of the disk. We always create a
> +	 * debugfs directory upon init for this gendisk kobject, so we re-use
> +	 * that if blktrace is going to be done for it.
> +	 */
> +	if (blk_trace_target_disk(buts->name, kobject_name(q->kobj.parent))) {
> +		if (!q->debugfs_dir) {
> +			pr_warn("%s: expected request_queue debugfs_dir is not set\n",
> +				buts->name);

What is userspace supposed to be able to do if they see this warning?

> +			return NULL;
> +		}
> +		/*
> +		 * debugfs_lookup() is used to ensure the directory is not
> +		 * taken from underneath us. We must dput() it later once
> +		 * done with it within blktrace.
> +		 */
> +		dir = debugfs_lookup(buts->name, blk_debugfs_root);
> +		if (!dir) {
> +			pr_warn("%s: expected request_queue debugfs_dir dentry is gone\n",
> +				buts->name);

Again, can't we just save the pointer when we create it and not have to
look it up again?

> +			return NULL;
> +		}
> +		 /*
> +		 * This is a reaffirmation that debugfs_lookup() shall always
> +		 * return the same dentry if it was already set.
> +		 */

I'm all for reaffirmation and the like, but really, is this needed???

> +		if (dir != q->debugfs_dir) {
> +			dput(dir);
> +			pr_warn("%s: expected dentry dir != q->debugfs_dir\n",
> +				buts->name);
> +			return NULL;

Why are you testing to see if debugfs really is working properly?
SHould all users do crazy things like this (hint, rhetorical
question...)

> +		}
> +		bt->backing_dir = q->debugfs_dir;
> +		return bt->backing_dir;
> +	}
> +
> +	/*
> +	 * If not using blktrace on the gendisk, we are going to create a
> +	 * temporary debugfs directory for it, however this cannot be shared
> +	 * between two concurrent blktraces since the path is not unique, so
> +	 * ensure this is only done once.
> +	 */
>  	dir = debugfs_lookup(buts->name, blk_debugfs_root);
> -	if (!dir)
> -		bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> +	if (dir) {
> +		pr_warn("%s: temporary blktrace debugfs directory already present\n",
> +			buts->name);
> +		dput(dir);
> +		return NULL;
> +	}
> +
> +	bt->dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> +	if (!bt->dir) {
> +		pr_warn("%s: temporary blktrace debugfs directory could not be created\n",
> +			buts->name);

Again, do not test the return value, you do not care.  I've been
removing these checks from everywhere.

thanks,

greg k-h

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