Re: [PATCH 2/5] blktrace: fix debugfs use after free

[Eric Sandeen]

On 4/13/20 11:18 PM, 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.
> 
> 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:

I think this patch makes this all cleaner anyway, but - without the apparent
loop bug mentioned by Bart which allows removal of the loop device while blktrace
is active (if I read that right), can this still happen?

-Eric

> [  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 ]---