[Bug 82201] ext4 crash in ext4_superblock_csum

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=82201

--- Comment #4 from Dmitry Monakhov <dmonakhov@openvz.org> ---
AFAIU I've accidentally directly write some crap to SB and later 
ext4_superblock_csum_set() performs check s_es which was crewed-up,
and it try to recalculate csum, but sbi->s_chksum_driver == NULL
So it looks like we have to guard all direct checks of 
EXT4_HAS_RO_COMPAT_FEATURE(sb,EXT4_FEATURE_RO_COMPAT_METADATA_CSUM))
with extra check that csum_context was created on mount.
Likely that other places where we make similar assumptions.

Patch likely will be simple but large, so we should figure out quick crunch for
stable releases, how about this ugly one?
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index b0c225c..d4f0dd1 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1749,6 +1749,10 @@ static inline u32 ext4_chksum(struct ext4_sb_info *sbi,
u32 crc,
        } desc;
        int err;

+       if (unlikely(!sbi->s_chksum_driver)) {
+               WARN_ON_ONCE(1);
+               return 0xDEADBEEF;
+       }
        BUG_ON(crypto_shash_descsize(sbi->s_chksum_driver)!=sizeof(desc.ctx));

        desc.shash.tfm = sbi->s_chksum_driver;

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