From: Qu Wenruo wqu@suse.com
[ Upstream commit 3670e6451bc9c39ab3a46f1da19360219e4319f3 ]
[BUG] When testing experimental subpage compressed write support, it hits a NULL pointer dereference inside read path:
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000018 pc : __pi_memcmp+0x28/0x1ec lr : check_data_csum+0xd0/0x274 [btrfs] Call trace: __pi_memcmp+0x28/0x1ec btrfs_verify_data_csum+0xf4/0x244 [btrfs] end_bio_extent_readpage+0x1d0/0x6b0 [btrfs] bio_endio+0x15c/0x1dc end_workqueue_fn+0x44/0x64 [btrfs] btrfs_work_helper+0x74/0x250 [btrfs] process_one_work+0x1d4/0x47c worker_thread+0x180/0x400 kthread+0x11c/0x120 ret_from_fork+0x10/0x30 Code: 54000261 d100044c d343fd8c f8408403 (f8408424) ---[ end trace 9e2c59f33ea40866 ]---
[CAUSE] When reading two compressed extents inside the same page, like the following layout, we trigger above crash:
0 32K 64K |-------|\\\| | - Compressed extent (A) --------- Compressed extent (B)
For compressed read, we don't need to populate its io_bio->csum, as we rely on compressed_bio->csum to verify the compressed data, and then copy the decompressed to inode pages.
Normally btrfs_verify_data_csum() skip such page by checking and clearing its PageChecked flag
But since that flag is still for the full page, when endio for inode page range [0, 32K) gets executed, it clears PageChecked flag for the full page.
Then when endio for inode page range [32K, 64K) gets executed, since the page no longer has PageChecked flag, it just continues checking, even though io_bio->csum is NULL.
[FIX] Thankfully there are only two users of PageChecked bit:
- Cow fixup Since subpage has its own way to trace page dirty (dirty_bitmap) and ordered bit (ordered_bitmap), it should never trigger cow fixup.
- Compressed read We can distinguish such read by just checking io_bio->csum.
So just check io_bio->csum before doing the verification to avoid such NULL pointer dereference.
Signed-off-by: Qu Wenruo wqu@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Sasha Levin sashal@kernel.org --- fs/btrfs/inode.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index ab04f9dd465e..ef6e1798fc2d 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -3257,6 +3257,20 @@ unsigned int btrfs_verify_data_csum(struct btrfs_io_bio *io_bio, u32 bio_offset, return 0; }
+ /* + * For subpage case, above PageChecked is not safe as it's not subpage + * compatible. + * But for now only cow fixup and compressed read utilize PageChecked + * flag, while in this context we can easily use io_bio->csum to + * determine if we really need to do csum verification. + * + * So for now, just exit if io_bio->csum is NULL, as it means it's + * compressed read, and its compressed data csum has already been + * verified. + */ + if (io_bio->csum == NULL) + return 0; + if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM) return 0;