CVE-2022-49348 — ext4: filter out EXT4_FC_REPLAY from on-disk superblock field s_state

Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

ext4: filter out EXT4_FC_REPLAY from on-disk superblock field s_state

The EXT4_FC_REPLAY bit in sbi->s_mount_state is used to indicate that we are in the middle of replay the fast commit journal. This was actually a mistake, since the sbi->s_mount_info is initialized from es->s_state. Arguably s_mount_state is misleadingly named, but the name is historical --- s_mount_state and s_state dates back to ext2.

What should have been used is the ext4_{set,clear,test}_mount_flag() inline functions, which sets EXT4_MF_* bits in sbi->s_mount_flags.

The problem with using EXT4_FC_REPLAY is that a maliciously corrupted superblock could result in EXT4_FC_REPLAY getting set in s_mount_state. This bypasses some sanity checks, and this can trigger a BUG() in ext4_es_cache_extent(). As a easy-to-backport-fix, filter out the EXT4_FC_REPLAY bit for now. We should eventually transition away from EXT4_FC_REPLAY to something like EXT4_MF_REPLAY.

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CVE-2022-49348 — ext4: filter out EXT4_FC_REPLAY from on-disk superblock field s_state