Message-ID: <23a3b7a6c26f61d101ba71a83c6541c21d332da9.camel@kernel.org>
Date:   Wed, 05 Jul 2023 10:10:55 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        syzbot <syzbot+a67fc5321ffb4b311c98@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-unionfs@...r.kernel.org, miklos@...redi.hu,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [overlayfs?] general protection fault in d_path

On Wed, 2023-07-05 at 16:54 +0300, Amir Goldstein wrote:
> On Wed, Jul 5, 2023 at 4:41 PM Jeff Layton <jlayton@...nel.org> wrote:
> > 
> > On Wed, 2023-07-05 at 15:05 +0200, Christian Brauner wrote:
> > > On Wed, Jul 05, 2023 at 05:00:45AM -0700, syzbot wrote:
> > > > Hello,
> > > > 
> > > > syzbot found the following issue on:
> > > > 
> > > > HEAD commit:    d528014517f2 Revert ".gitignore: ignore *.cover and *.mbx"
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=14fad002a80000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1085b4238c9eb6ba
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=a67fc5321ffb4b311c98
> > > > compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
> > > > 
> > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > 
> > > > Downloadable assets:
> > > > disk image: https://storage.googleapis.com/syzbot-assets/fef94e788067/disk-d5280145.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/576412ea518b/vmlinux-d5280145.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/685a0e4be06b/bzImage-d5280145.xz
> > > > 
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+a67fc5321ffb4b311c98@...kaller.appspotmail.com
> > > > 
> > > > general protection fault, probably for non-canonical address 0xdffffc000000000a: 0000 [#1] PREEMPT SMP KASAN
> > > > KASAN: null-ptr-deref in range [0x0000000000000050-0x0000000000000057]
> > > > CPU: 1 PID: 10127 Comm: syz-executor.3 Not tainted 6.4.0-syzkaller-11478-gd528014517f2 #0
> > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/27/2023
> > > > RIP: 0010:__lock_acquire+0x10d/0x7f70 kernel/locking/lockdep.c:5012
> > > > Code: 85 75 18 00 00 83 3d 15 c8 2c 0d 00 48 89 9c 24 10 01 00 00 0f 84 f8 0f 00 00 83 3d 5c de b3 0b 00 74 34 48 89 d0 48 c1 e8 03 <42> 80 3c 00 00 74 1a 48 89 d7 e8 b4 51 79 00 48 8b 94 24 80 00 00
> > > > RSP: 0018:ffffc900169be9e0 EFLAGS: 00010006
> > > > RAX: 000000000000000a RBX: 1ffff92002d37d60 RCX: 0000000000000002
> > > > RDX: 0000000000000050 RSI: 0000000000000000 RDI: 0000000000000050
> > > > RBP: ffffc900169beca8 R08: dffffc0000000000 R09: 0000000000000001
> > > > R10: dffffc0000000000 R11: fffffbfff1d2fe76 R12: 0000000000000000
> > > > R13: 0000000000000001 R14: 0000000000000002 R15: ffff88802091d940
> > > > FS:  0000000000000000(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 00007fa22a3fe000 CR3: 000000004b5e1000 CR4: 00000000003506e0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > Call Trace:
> > > >  <TASK>
> > > >  lock_acquire+0x1e3/0x520 kernel/locking/lockdep.c:5761
> > > >  seqcount_lockdep_reader_access+0x139/0x220 include/linux/seqlock.h:102
> > > >  get_fs_root_rcu fs/d_path.c:244 [inline]
> > > >  d_path+0x2f0/0x6e0 fs/d_path.c:286
> > > >  audit_log_d_path+0xd3/0x310 kernel/audit.c:2139
> > > >  dump_common_audit_data security/lsm_audit.c:224 [inline]
> > > >  common_lsm_audit+0x7cf/0x1a90 security/lsm_audit.c:458
> > > >  smack_log+0x421/0x540 security/smack/smack_access.c:383
> > > >  smk_tskacc+0x2ff/0x360 security/smack/smack_access.c:253
> > > >  smack_inode_getattr+0x203/0x270 security/smack/smack_lsm.c:1202
> > > >  security_inode_getattr+0xd3/0x120 security/security.c:2114
> > > >  vfs_getattr+0x25/0x70 fs/stat.c:167
> > > >  ovl_getattr+0x1b1/0xf70 fs/overlayfs/inode.c:174
> > > >  ima_check_last_writer security/integrity/ima/ima_main.c:171 [inline]
> > > >  ima_file_free+0x26e/0x4b0 security/integrity/ima/ima_main.c:203
> > > 
> > > Ugh, I think the root of this might all be the call back into
> > > vfs_getattr() that happens on overlayfs:
> > > 
> > > __fput()
> > > -> ima_file_free()
> > >    -> mutex_lock()
> > >    -> vfs_getattr_nosec()
> > >       -> i_op->getattr() == ovl_getattr()
> > >          -> vfs_getattr()
> > >           -> security_inode_getattr()
> > >    -> mutex_unlock()
> > > 
> > > So either overlayfs needs to call vfs_getattr_nosec() when the request
> > > comes from vfs_getattr_nosec() or this needs to use
> > > backing_file_real_path() to operate on the real underlying path.
> > > 
> > > Thoughts?
> > > 
> > 
> > When you say "this needs to use backing_file_real_path()", what do you
> > mean by "this"? IMA?
> > 
> > That said, passing some sort of NOSEC flag to vfs_getattr that
> > designates the call as kernel-internal seems like the more correct thing
> > to do here, and might be useful in other weird stacking cases like this.
> > 
> 
> I don't think that NOSEC is the root cause.
> 
> If you ever noticed file_dentry() sprinkled through fs code,
> it is only there because if that code were to call use helpers
> that rely on file_inode() and d_inode(file->f_path.dentry) being
> the same - bad things will happen and NOSEC will not cover
> all those bad things.
> 
> IMA code also has file_dentry() sprinkled.
> But it still accesses file->f_path in a few places and that
> can result in bad things.
> 

Ok, that makes sense, and is a lot less invasive than having to rework
vfs_getattr.
-- 
Jeff Layton <jlayton@...nel.org>

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