Re: [PATCH] btrfs: fix a out-of-boundary access for copy_compressed_data_to_page()

[Omar Sandoval <osandov@osandov.com>]

On Fri, Nov 12, 2021 at 10:22:53AM +0800, Qu Wenruo wrote:
> [BUG]
> The following script can cause btrfs to crash:
> 
>  mount -o compress-force=lzo $DEV /mnt
>  dd if=/dev/urandom of=/mnt/foo bs=4k count=1
>  sync
> 
> The calltrace looks like this:
> 
>  general protection fault, probably for non-canonical address 0xe04b37fccce3b000: 0000 [#1] PREEMPT SMP NOPTI
>  CPU: 5 PID: 164 Comm: kworker/u20:3 Not tainted 5.15.0-rc7-custom+ #4
>  Workqueue: btrfs-delalloc btrfs_work_helper [btrfs]
>  RIP: 0010:__memcpy+0x12/0x20
>  Call Trace:
>   lzo_compress_pages+0x236/0x540 [btrfs]
>   btrfs_compress_pages+0xaa/0xf0 [btrfs]
>   compress_file_range+0x431/0x8e0 [btrfs]
>   async_cow_start+0x12/0x30 [btrfs]
>   btrfs_work_helper+0xf6/0x3e0 [btrfs]
>   process_one_work+0x294/0x5d0
>   worker_thread+0x55/0x3c0
>   kthread+0x140/0x170
>   ret_from_fork+0x22/0x30
>  ---[ end trace 63c3c0f131e61982 ]---
> 
> [CAUSE]
> In lzo_compress_pages(), parameter @out_pages is not only an output
> parameter (for the compressed pages), but also an input parameter, for
> the maximum amount of pages we can utilize.
> 
> In commit d4088803f511 ("btrfs: subpage: make lzo_compress_pages()
> compatible"), the refactor doesn't take @out_pages as an input, thus
> completely ignoring the limit.
> 
> And for compress-force case, we could hit incompressible data that
> compressed size would go beyond the page limit, and cause above crash.
> 
> [FIX]
> Save @out_pages as @max_nr_page, and pass it to lzo_compress_pages(),
> and check if we're beyond the limit before accessing the pages.
> 
> Reported-by: Omar Sandoval <osandov@fb.com>
> Fixes: d4088803f511 ("btrfs: subpage: make lzo_compress_pages() compatible")
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/lzo.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)

This fixed the issue for me, and it looks correct.

Reviewed-by: Omar Sandoval <osandov@fb.com>