Re: [PATCH] xfs: verify buffer contents when we skip log replay

[Lee Jones <lee@kernel.org>]

Hi Darrick,

On Tue, 11 Apr 2023, Darrick J. Wong wrote:

> From: Darrick J. Wong <djwong@kernel.org>
> 
> syzbot detected a crash during log recovery:
> 
> XFS (loop0): Mounting V5 Filesystem bfdc47fc-10d8-4eed-a562-11a831b3f791
> XFS (loop0): Torn write (CRC failure) detected at log block 0x180. Truncating head block from 0x200.
> XFS (loop0): Starting recovery (logdev: internal)
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in xfs_btree_lookup_get_block+0x15c/0x6d0 fs/xfs/libxfs/xfs_btree.c:1813
> Read of size 8 at addr ffff88807e89f258 by task syz-executor132/5074
> 
> CPU: 0 PID: 5074 Comm: syz-executor132 Not tainted 6.2.0-rc1-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0x1b1/0x290 lib/dump_stack.c:106
>  print_address_description+0x74/0x340 mm/kasan/report.c:306
>  print_report+0x107/0x1f0 mm/kasan/report.c:417
>  kasan_report+0xcd/0x100 mm/kasan/report.c:517
>  xfs_btree_lookup_get_block+0x15c/0x6d0 fs/xfs/libxfs/xfs_btree.c:1813
>  xfs_btree_lookup+0x346/0x12c0 fs/xfs/libxfs/xfs_btree.c:1913
>  xfs_btree_simple_query_range+0xde/0x6a0 fs/xfs/libxfs/xfs_btree.c:4713
>  xfs_btree_query_range+0x2db/0x380 fs/xfs/libxfs/xfs_btree.c:4953
>  xfs_refcount_recover_cow_leftovers+0x2d1/0xa60 fs/xfs/libxfs/xfs_refcount.c:1946
>  xfs_reflink_recover_cow+0xab/0x1b0 fs/xfs/xfs_reflink.c:930
>  xlog_recover_finish+0x824/0x920 fs/xfs/xfs_log_recover.c:3493
>  xfs_log_mount_finish+0x1ec/0x3d0 fs/xfs/xfs_log.c:829
>  xfs_mountfs+0x146a/0x1ef0 fs/xfs/xfs_mount.c:933
>  xfs_fs_fill_super+0xf95/0x11f0 fs/xfs/xfs_super.c:1666
>  get_tree_bdev+0x400/0x620 fs/super.c:1282
>  vfs_get_tree+0x88/0x270 fs/super.c:1489
>  do_new_mount+0x289/0xad0 fs/namespace.c:3145
>  do_mount fs/namespace.c:3488 [inline]
>  __do_sys_mount fs/namespace.c:3697 [inline]
>  __se_sys_mount+0x2d3/0x3c0 fs/namespace.c:3674
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f89fa3f4aca
> Code: 83 c4 08 5b 5d c3 66 2e 0f 1f 84 00 00 00 00 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fffd5fb5ef8 EFLAGS: 00000206 ORIG_RAX: 00000000000000a5
> RAX: ffffffffffffffda RBX: 00646975756f6e2c RCX: 00007f89fa3f4aca
> RDX: 0000000020000100 RSI: 0000000020009640 RDI: 00007fffd5fb5f10
> RBP: 00007fffd5fb5f10 R08: 00007fffd5fb5f50 R09: 000000000000970d
> R10: 0000000000200800 R11: 0000000000000206 R12: 0000000000000004
> R13: 0000555556c6b2c0 R14: 0000000000200800 R15: 00007fffd5fb5f50
>  </TASK>
> 
> The fuzzed image contains an AGF with an obviously garbage
> agf_refcount_level value of 32, and a dirty log with a buffer log item
> for that AGF.  The ondisk AGF has a higher LSN than the recovered log
> item.  xlog_recover_buf_commit_pass2 reads the buffer, compares the
> LSNs, and decides to skip replay because the ondisk buffer appears to be
> newer.
> 
> Unfortunately, the ondisk buffer is corrupt, but recovery just read the
> buffer with no buffer ops specified:
> 
> 	error = xfs_buf_read(mp->m_ddev_targp, buf_f->blf_blkno,
> 			buf_f->blf_len, buf_flags, &bp, NULL);
> 
> Skipping the buffer leaves its contents in memory unverified.  This sets
> us up for a kernel crash because xfs_refcount_recover_cow_leftovers
> reads the buffer (which is still around in XBF_DONE state, so no read
> verification) and creates a refcountbt cursor of height 32.  This is
> impossible so we run off the end of the cursor object and crash.
> 
> Fix this by invoking the verifier on all skipped buffers and aborting
> log recovery if the ondisk buffer is corrupt.  It might be smarter to
> force replay the log item atop the buffer and then see if it'll pass the
> write verifier (like ext4 does) but for now let's go with the
> conservative option where we stop immediately.
> 
> Link: https://syzkaller.appspot.com/bug?extid=7e9494b8b399902e994e
> Fixes: 67dc288c2106 ("xfs: ensure verifiers are attached to recovered buffers")
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/xfs_buf_item_recover.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)

Rightly or wrongly, CVE-2023-212 has been raised against this issue.

It looks as though the Fixes: tag above was stripped when applied.

Should this still be submitted to Stable?

> diff --git a/fs/xfs/xfs_buf_item_recover.c b/fs/xfs/xfs_buf_item_recover.c
> index 5368a0d34452..ebe7f2c3cf63 100644
> --- a/fs/xfs/xfs_buf_item_recover.c
> +++ b/fs/xfs/xfs_buf_item_recover.c
> @@ -971,6 +971,16 @@ xlog_recover_buf_commit_pass2(
>  	if (lsn && lsn != -1 && XFS_LSN_CMP(lsn, current_lsn) >= 0) {
>  		trace_xfs_log_recover_buf_skip(log, buf_f);
>  		xlog_recover_validate_buf_type(mp, bp, buf_f, NULLCOMMITLSN);
> +
> +		/*
> +		 * We're skipping replay of this buffer log item due to the log
> +		 * item LSN being behind the ondisk buffer.  Verify the buffer
> +		 * contents since we aren't going to run the write verifier.
> +		 */
> +		if (bp->b_ops) {
> +			bp->b_ops->verify_read(bp);
> +			error = bp->b_error;
> +		}
>  		goto out_release;
>  	}
>  

-- 
Lee Jones [李琼斯]