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

On 2021/11/12 12:17, Josef Bacik wrote:

On Fri, Nov 12, 2021 at 10:22:53AM +0800, Qu Wenruo wrote:

quoted

[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 <redacted>
Fixes: d4088803f511 ("btrfs: subpage: make lzo_compress_pages() compatible")
Signed-off-by: Qu Wenruo <redacted>
---
  fs/btrfs/lzo.c | 12 +++++++++++-
  1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/lzo.c b/fs/btrfs/lzo.c
index 00cffc183ec0..f410ceabcdbd 100644
--- a/fs/btrfs/lzo.c
+++ b/fs/btrfs/lzo.c

@@ -125,6 +125,7 @@ static inline size_t read_compress_length(const char *buf)
  static int copy_compressed_data_to_page(char *compressed_data,
  					size_t compressed_size,
  					struct page **out_pages,
+					unsigned long max_nr_page,

If you want to do const down below you should use const here probably?  Thanks,

Right, max_nr_page should also be const.

Thanks for catching this,
Qu

Josef