[PATCH AUTOSEL 5.14 192/252] btrfs: subpage: check if there are compressed extents inside one page

From: Sasha Levin
Date: Thu Sep 09 2021 - 07:59:45 EST

From: Qu Wenruo <wqu@xxxxxxxx>

[ Upstream commit 3670e6451bc9c39ab3a46f1da19360219e4319f3 ]

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:
btrfs_verify_data_csum+0xf4/0x244 [btrfs]
end_bio_extent_readpage+0x1d0/0x6b0 [btrfs]
end_workqueue_fn+0x44/0x64 [btrfs]
btrfs_work_helper+0x74/0x250 [btrfs]
Code: 54000261 d100044c d343fd8c f8408403 (f8408424)
---[ end trace 9e2c59f33ea40866 ]---

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.

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@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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;