Messages in this thread | | | Subject | Re: [PATCH] bdev: fix NULL pointer dereference in sync()/close() race | From | Vegard Nossum <> | Date | Sat, 27 Aug 2016 11:30:22 +0200 |
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On 08/27/2016 11:03 AM, Rabin Vincent wrote: > On Sat, Aug 27, 2016 at 09:07:28AM +0200, Vegard Nossum wrote: >> I got this with syzkaller: >> >> general protection fault: 0000 [#1] PREEMPT SMP KASAN >> Dumping ftrace buffer: >> (ftrace buffer empty) >> CPU: 0 PID: 11941 Comm: syz-executor Not tainted 4.8.0-rc2+ #169 >> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org 04/01/2014 >> task: ffff880110762cc0 task.stack: ffff880102290000 >> RIP: 0010:[<ffffffff81f04b7a>] [<ffffffff81f04b7a>] blk_get_backing_dev_info+0x4a/0x70 >> RSP: 0018:ffff880102297cd0 EFLAGS: 00010202 >> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffc90000bb4000 >> RDX: 0000000000000097 RSI: 0000000000000000 RDI: 00000000000004b8 >> RBP: ffff880102297cd8 R08: 0000000000000000 R09: 0000000000000001 >> R10: 0000000000000000 R11: 0000000000000001 R12: ffff88011a010a90 >> R13: ffff88011a594568 R14: ffff88011a010890 R15: 7fffffffffffffff >> FS: 00007f2445174700(0000) GS:ffff88011aa00000(0000) knlGS:0000000000000000 >> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> CR2: 00000000200047c8 CR3: 0000000107eb5000 CR4: 00000000000006f0 >> DR0: 000000000000001e DR1: 000000000000001e DR2: 0000000000000000 >> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600 >> Stack: >> 1ffff10020452f9e ffff880102297db8 ffffffff81508daa 0000000000000000 >> 0000000041b58ab3 ffffffff844e89e1 ffffffff81508b30 ffffed0020452001 >> 7fffffffffffffff 0000000000000000 0000000000000000 7fffffffffffffff >> Call Trace: >> [<ffffffff81508daa>] __filemap_fdatawrite_range+0x27a/0x2e0 >> [<ffffffff81508b30>] ? filemap_check_errors+0xe0/0xe0 >> [<ffffffff83c24b47>] ? preempt_schedule+0x27/0x30 >> [<ffffffff810020ae>] ? ___preempt_schedule+0x16/0x18 >> [<ffffffff81508e36>] filemap_fdatawrite+0x26/0x30 >> [<ffffffff817191b0>] fdatawrite_one_bdev+0x50/0x70 >> [<ffffffff817341b4>] iterate_bdevs+0x194/0x210 >> [<ffffffff81719160>] ? fdatawait_one_bdev+0x70/0x70 >> [<ffffffff817195f0>] ? sync_filesystem+0x240/0x240 >> [<ffffffff817196be>] sys_sync+0xce/0x160 >> [<ffffffff817195f0>] ? sync_filesystem+0x240/0x240 >> [<ffffffff81002b60>] ? exit_to_usermode_loop+0x190/0x190 >> [<ffffffff8150455a>] ? __context_tracking_exit.part.4+0x3a/0x1e0 >> [<ffffffff81005524>] do_syscall_64+0x1c4/0x4e0 >> [<ffffffff83c3276a>] entry_SYSCALL64_slow_path+0x25/0x25 >> Code: 89 fa 48 c1 ea 03 80 3c 02 00 75 35 48 8b 9b e0 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 8d bb b8 04 00 00 48 89 fa 48 c1 ea 03 <80> 3c 02 00 75 17 48 8b 83 b8 04 00 00 5b 5d 48 05 10 02 00 00 >> RIP [<ffffffff81f04b7a>] blk_get_backing_dev_info+0x4a/0x70 >> RSP <ffff880102297cd0> >> >> The problem is that sync() calls down to blk_get_backing_dev_info() >> without necessarily having the block device opened (if it races with >> another process doing close()): >> >> /** >> * blk_get_backing_dev_info - get the address of a queue's backing_dev_info >> * @bdev: device >> * >> * Locates the passed device's request queue and returns the address of its >> * backing_dev_info. This function can only be called if @bdev is opened <---- >> * and the return value is never NULL. >> */ >> struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev) >> { >> struct request_queue *q = bdev_get_queue(bdev); >> >> return &q->backing_dev_info; >> } >> >> bdev_get_queue() crashes on dereferencing bdev->bd_disk->queue because >> ->bd_disk was set to NULL when close() reached __blkdev_put(). >> >> Taking bdev->bd_mutex and checking bdev->bd_disk actually seems to be a >> reliable test of whether it's safe to call filemap_fdatawrite() for the >> block device inode and completely fixes the crash for me. >> >> What I don't like about this patch is that it simply skips block devices >> which we don't have any open file descriptors for. That seems wrong to >> me because sync() should do writeback on (and wait for) _all_ devices, >> not just the ones that we happen to have an open file descriptor for. >> Imagine if we opened a device, wrote a lot of data to it, closed it, >> called sync(), and sync() returns. Now we should be guaranteed the data >> was written, but I'm not sure we are in this case. But maybe I've >> misunderstood how the writeback mechanism works. >> >> Another ugly thing is that we're now holding a new mutex over a >> potentially big chunk of code (everything that happens inside >> filemap_fdatawrite()). The only thing I can say is that it seems to >> work here. >> >> I have a reproducer that reliably triggers the problem in ~2 seconds so >> I can easily test other proposed fixes if there are any. I would also be >> happy to submit a new patch with some guidance on the Right Way to fix >> this. >> >> Cc: stable@vger.kernel.org >> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com> > > Don't know what's the right fix, but I posted a slightly different one > for the same crash some months ago: > https://patchwork.kernel.org/patch/8556941/ >
Ah, I'm sorry, I didn't see that.
Your patch is 100% identical to my first attempt at a fix and I can confirm that it also fixes the problem for me.
If people who are more savvy in block/fs code could ack the locking bits I think we should apply the patch ASAP because it's an easy local DOS if you have (open/read) access to any block device.
Vegard
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