Re: [PATCH v2] btrfs: Check the first key and level for cached extent buffer

[Nikolay Borisov <nborisov@suse.com>]

On 12.03.19 г. 11:10 ч., Qu Wenruo wrote:
> [BUG]
> When reading a file from a fuzzed image, kernel can panic like:
>   BTRFS warning (device loop0): csum failed root 5 ino 270 off 0 csum 0x98f94189 expected csum 0x00000000 mirror 1
>   assertion failed: !memcmp_extent_buffer(b, &disk_key, offsetof(struct btrfs_leaf, items[0].key), sizeof(disk_key)), file: fs/btrfs/ctree.c, line: 2544
>   ------------[ cut here ]------------
>   kernel BUG at fs/btrfs/ctree.h:3500!
>   invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
>   RIP: 0010:btrfs_search_slot.cold.24+0x61/0x63 [btrfs]
>   Call Trace:
>    btrfs_lookup_csum+0x52/0x150 [btrfs]
>    __btrfs_lookup_bio_sums+0x209/0x640 [btrfs]
>    btrfs_submit_bio_hook+0x103/0x170 [btrfs]
>    submit_one_bio+0x59/0x80 [btrfs]
>    extent_read_full_page+0x58/0x80 [btrfs]
>    generic_file_read_iter+0x2f6/0x9d0
>    __vfs_read+0x14d/0x1a0
>    vfs_read+0x8d/0x140
>    ksys_read+0x52/0xc0
>    do_syscall_64+0x60/0x210
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> [CAUSE]
> The fuzzed image has a corrupted leaf whose first key doesn't match with its parent:
>   checksum tree key (CSUM_TREE ROOT_ITEM 0)
>   node 29741056 level 1 items 14 free 107 generation 19 owner CSUM_TREE
>   fs uuid 3381d111-94a3-4ac7-8f39-611bbbdab7e6
>   chunk uuid 9af1c3c7-2af5-488b-8553-530bd515f14c
>   	...
>           key (EXTENT_CSUM EXTENT_CSUM 79691776) block 29761536 gen 19
> 
>   leaf 29761536 items 1 free space 1726 generation 19 owner CSUM_TREE
>   leaf 29761536 flags 0x1(WRITTEN) backref revision 1
>   fs uuid 3381d111-94a3-4ac7-8f39-611bbbdab7e6
>   chunk uuid 9af1c3c7-2af5-488b-8553-530bd515f14c
>           item 0 key (EXTENT_CSUM EXTENT_CSUM 8798638964736) itemoff 1751 itemsize 2244
>                   range start 8798638964736 end 8798641262592 length 2297856
> 
> When reading above tree block, we have extent_buffer->refs = 2 in the
> context:
> - initial one from __alloc_extent_buffer()
>   alloc_extent_buffer()
>   |- __alloc_extent_buffer()
>      |- atomic_set(&eb->refs, 1)
> 
> - one being added to fs_info->buffer_radix
>   alloc_extent_buffer()
>   |- check_buffer_tree_ref()
>      |- atomic_inc(&eb->refs)
> 
> So even we call free_extent_buffer() in read_tree_block or other similar
> situation, we only decrease the refs by 1, it doesn't reach 0 and won't
> be freed right now.
> 
> The staled eb and its corrupted content will still be kept cached.
> 
> Further more, we have several extra cases where we either don't do
> first key check or the check is not proper for all callers:
> - scrub
>   We just don't have first key in this context.
> 
> - shared tree block
>   One tree block can be shared by several snapshot/subvolume trees.
>   In that case, the first key check for one subvolume doesn't apply to
>   another.
> 
> So for above reasons, a corrupted extent buffer can sneak into the
> buffer cache.
> 
> [FIX]
> Export verify_level_key() as btrfs_verify_level_key() and call it in
> read_block_for_search() to fill the hole.
> 
> Due to above described reasons, even we can free corrupted extent buffer
> from cache, we still need the check in read_block_for_search(), for
> scrub and shared tree blocks.
> 
> Reported-by: Yoon Jungyeon <jungyeon@gatech.edu>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202755
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202757
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202759
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202761
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202767
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202769
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> changelog:
> v2:
> - Commit message update to show the reason why stale ebs are kept in
>   cache.
> - Commit message update to show extra reasons why we still need the
>   check, mainly for scrub and shared tree blocks.
> ---
>  fs/btrfs/ctree.c   | 10 ++++++++++
>  fs/btrfs/disk-io.c | 10 +++++-----
>  fs/btrfs/disk-io.h |  3 +++
>  3 files changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index 5a6c39b44c84..7672932aa5b4 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -2401,6 +2401,16 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p,
>  	if (tmp) {
>  		/* first we do an atomic uptodate check */
>  		if (btrfs_buffer_uptodate(tmp, gen, 1) > 0) {
> +			/*
> +			 * Do extra check for first_key, eb can be stale due to
> +			 * being cached, read from scrub, or have multiple
> +			 * parents (shared tree blocks).
> +			 */
> +			if (btrfs_verify_level_key(fs_info, tmp,
> +					parent_level - 1, &first_key, gen)) {
> +				free_extent_buffer(tmp);
> +				return -EUCLEAN;
> +			}

What about verify_parent_transid? Shouldn't it also be checked here?

>  			*eb_ret = tmp;
>  			return 0;
>  		}
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 298b34721bc0..e2a0cb362d28 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -423,9 +423,9 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
>  	return ret;
>  }
>  
> -static int verify_level_key(struct btrfs_fs_info *fs_info,
> -			    struct extent_buffer *eb, int level,
> -			    struct btrfs_key *first_key, u64 parent_transid)
> +int btrfs_verify_level_key(struct btrfs_fs_info *fs_info,
> +			   struct extent_buffer *eb, int level,
> +			   struct btrfs_key *first_key, u64 parent_transid)
>  {
>  	int found_level;
>  	struct btrfs_key found_key;
> @@ -500,8 +500,8 @@ static int btree_read_extent_buffer_pages(struct btrfs_fs_info *fs_info,
>  			if (verify_parent_transid(io_tree, eb,
>  						   parent_transid, 0))
>  				ret = -EIO;
> -			else if (verify_level_key(fs_info, eb, level,
> -						  first_key, parent_transid))
> +			else if (btrfs_verify_level_key(fs_info, eb, level,
> +						first_key, parent_transid))
>  				ret = -EUCLEAN;
>  			else
>  				break;
> diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
> index 987a64bc0c66..67a9fe2d29c7 100644
> --- a/fs/btrfs/disk-io.h
> +++ b/fs/btrfs/disk-io.h
> @@ -39,6 +39,9 @@ static inline u64 btrfs_sb_offset(int mirror)
>  struct btrfs_device;
>  struct btrfs_fs_devices;
>  
> +int btrfs_verify_level_key(struct btrfs_fs_info *fs_info,
> +			   struct extent_buffer *eb, int level,
> +			   struct btrfs_key *first_key, u64 parent_transid);
>  struct extent_buffer *read_tree_block(struct btrfs_fs_info *fs_info, u64 bytenr,
>  				      u64 parent_transid, int level,
>  				      struct btrfs_key *first_key);
>