[PATCH] ext4: Fix possible use-after-free in ext4_find_extent

[Tudor Ambarus]

syzbot reported a use-after-free Read in ext4_find_extent that is hit when
using a corrupted file system. The bug was reported on Android 5.15, but
using the same reproducer triggers the bug on v6.2-rc1 as well.

Fix the use-after-free by checking the extent header magic. An alternative
would be to check the values of EXT4_{FIRST,LAST}_{EXTENT,INDEX} used in
ext4_ext_binsearch() and ext4_ext_binsearch_idx(), so that we make sure
that pointers returned by EXT4_{FIRST,LAST}_{EXTENT,INDEX} don't exceed the
bounds of the extent tree node. But this alternative will not squash
the bug for the cases where eh->eh_entries fit into eh->eh_max. We could
also try to check the sanity of the path, but costs more than checking just
the header magic, so stick to the header magic sanity check.

Link: https://syzkaller.appspot.com/bug?id=be6e90ce70987950e6deb3bac8418344ca8b96cd
Reported-by: syzbot+0827b4b52b5ebf65f219@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
---
 fs/ext4/extents.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 9de1c9d1a13d..18367767afd7 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -894,6 +894,12 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block,
 		gfp_flags |= __GFP_NOFAIL;
 
 	eh = ext_inode_hdr(inode);
+	if (le16_to_cpu(eh->eh_magic) != EXT4_EXT_MAGIC) {
+		EXT4_ERROR_INODE(inode, "Extent header has invalid magic.");
+		ret = -EFSCORRUPTED;
+		goto err;
+	}
+
 	depth = ext_depth(inode);
 	if (depth < 0 || depth > EXT4_MAX_EXTENT_DEPTH) {
 		EXT4_ERROR_INODE(inode, "inode has invalid extent depth: %d",
-- 
2.34.1