linux-kernel - Re: [2.6.39-rc2, framebuffer] use after free oops

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Date:	Wed, 20 Apr 2011 08:05:35 +0200
From:	Bruno Prémont <bonbons@...ux-vserver.org>
To:	Daniel J Blueman <daniel.blueman@...il.com>
Cc:	Paul Mundt <lethal@...ux-sh.org>, linux-fbdev@...r.kernel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [2.6.39-rc2, framebuffer] use after free oops

On Wed, 20 Apr 2011 13:50:10 Daniel J Blueman <daniel.blueman@...il.com> wrote:
> Any ideas on how best to address this issue [0], since it causes
> silent corruption, or at best crashes?

There is probably no easy short-term fix to this... The complex fix
would be to implement some deferred free with driver-local ref-counting
though to be complete it would have to be done for pretty much every FB
driver!

The mid-term fix would be to have fb_info ref-counted by FB
subsystem (I'm working on it, slowly). This way freeing would be
deferred to when last references is being dropped.
The driver then just has to know when it can't touch underlying
hardware anymore (and do the right thing with framebuffer memory in
case it is still mmapped somewhere).

Bruno



> Thanks,
>   Daniel
> 
> --- [0]
> 
> When building in the eg vga16 and intelfb framebuffers and page
> debugging, we can get hit by a use-after-free oops [1]; with page
> debugging disabled, this slips by unnoticed.
> 
> When intelfb registers, the vga16 framebuffer is detected covering
> it's I/O regions, so it is removed (in
> remove_conflicting_framebuffers->unregister_framebuffer). The problem
> is the fb_info structure is freed (in
> unregister_framebuffer->vga16fb_destroy->framebuffer_release->kfree)
> before all open files to it are closed, so we get a use-after-free
> scenario.
> 
> Freeing should be deferred until the last user has gone away. What
> approach would make sense here?
> 
> Daniel
> 
> --- [1]
> 
> checking generic (a0000 10000) vs hw (d0000000 10000000)
> fb: conflicting fb hw usage inteldrmfb vs VGA16 VGA - removing generic driver
> Console: switching to colour VGA+ 80x25
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
> IP: [<ffffffff81708394>] mutex_lock_nested+0x94/0x390
> PGD 2203ab067 PUD 2203ac067 PMD 0
> Oops: 0002 [#2] PREEMPT SMP DEBUG_PAGEALLOC
> last sysfs file: /sys/devices/pci0000:00/0000:00:1b.0/sound/card0/uevent
> CPU 3
> Modules linked in: arc4 ecb uvcvideo videodev v4l2_compat_ioctl32
> i915(+) drm_kms_helper iwlagn mmc_block mac80211 drm video sdhci_pci
> sdhci mmc_core
> 
> Pid: 271, comm: plymouthd Tainted: G      D     2.6.39-rc2-350cd+ #1
> Dell Inc. Latitude E5420/0H5TG2
> RIP: 0010:[<ffffffff81708394>]  [<ffffffff81708394>]
> mutex_lock_nested+0x94/0x390
> RSP: 0018:ffff88021ee77c08  EFLAGS: 00010046
> RAX: 0000000000010000 RBX: 0000000000000008 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffff8170838b
> RBP: ffff88021ee77c78 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: ffff88021e574020
> R13: 0000000000000010 R14: 0000000000000246 R15: ffff88021de6f478
> FS:  00007f50d6e9c720(0000) GS:ffff88022ec60000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000010 CR3: 00000002203aa000 CR4: 00000000000406e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Process plymouthd (pid: 271, threadinfo ffff88021ee76000, task ffff88021e574020)
> Stack:
>  ffffffff81375552 0000000000000000 00000000fffffff2 ffff88021de6f478
>  ffff88021e574020 0000000000000246 0000000000000000 00000000fffffff2
>  ffff88021ee77c68 0000000000000000 0000000000000008 0000000000000000
> Call Trace:
>  [<ffffffff81375552>] ? lock_fb_info+0x22/0x60
>  [<ffffffff81375552>] lock_fb_info+0x22/0x60
>  [<ffffffff8137a24b>] fb_set_user_cmap+0x15b/0x1a0
>  [<ffffffff81376e28>] do_fb_ioctl+0x4b8/0x5e0
>  [<ffffffff81105dae>] ? might_fault+0x4e/0xa0
>  [<ffffffff81058e61>] ? get_parent_ip+0x11/0x50
>  [<ffffffff81058f3d>] ? sub_preempt_count+0x9d/0xd0
>  [<ffffffff8170a26d>] ? _raw_spin_unlock_irqrestore+0x3d/0x80
>  [<ffffffff8104840e>] ? __wake_up+0x4e/0x70
>  [<ffffffff813e47b9>] ? put_ldisc+0x59/0xd0
>  [<ffffffff813e4839>] ? tty_ldisc_deref+0x9/0x10
>  [<ffffffff813def59>] ? tty_ioctl+0x299/0xa00
>  [<ffffffff81377214>] fb_ioctl+0x24/0x30
>  [<ffffffff81152ec7>] do_vfs_ioctl+0x87/0x330
>  [<ffffffff81140e85>] ? fget+0x95/0x240
>  [<ffffffff81140df0>] ? fget_raw+0x240/0x240
>  [<ffffffff811531ba>] sys_ioctl+0x4a/0x80
>  [<ffffffff8170adfb>] system_call_fastpath+0x16/0x1b
> Code: 48 8b 04 25 c8 b5 00 00 8b 80 44 e0 ff ff a9 00 ff ff 07 0f 85
> d7 02 00 00 9c 41 5e fa e8 15 d2 98 ff 4c 8d 6b 08 b8 00 00 01 00 <f0>
> 0f c1 43 08 0f b7 d0 c1 e8 10 39 c2 74 08 f3 90 0f b7 53 08
> RIP  [<ffffffff81708394>] mutex_lock_nested+0x94/0x390
>  RSP <ffff88021ee77c08>
> CR2: 0000000000000010
> ---[ end trace cdacd65f5f1b187d ]---
> note: plymouthd[271] exited with preempt_count 1
> --
> Daniel J Blueman
> 
> 
> 

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