linux-ext4 - [Bug 199183] Invalid pointer dereference in ext4_xattr_inode_hash when mounting and later operating on a crafted image

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Message-ID: <bug-199183-13602-wQlsF2YpBh@https.bugzilla.kernel.org/>
Date:   Sun, 25 Mar 2018 17:23:18 +0000
From:   bugzilla-daemon@...zilla.kernel.org
To:     linux-ext4@...nel.org
Subject: [Bug 199183] Invalid pointer dereference in ext4_xattr_inode_hash
 when mounting and later operating on a crafted image

https://bugzilla.kernel.org/show_bug.cgi?id=199183

--- Comment #3 from Wen Xu (wen.xu@...ech.edu) ---
Sorry, it was my mistake. 

After debugging the kernel again, I found that it crashes here:
https://elixir.bootlin.com/linux/v4.15/source/fs/ext4/ext4.h#L2005

static inline u32 ext4_chksum(struct ext4_sb_info *sbi, u32 crc,
                              const void *address, unsigned int length)
{
        struct {
                struct shash_desc shash;
                char ctx[4];
        } desc;
        int err;

        BUG_ON(crypto_shash_descsize(sbi->s_chksum_driver)!=sizeof(desc.ctx));
<-

sbi->s_chksum_driver may not always be set up and can be NULL, while 

static inline unsigned int crypto_shash_descsize(struct crypto_shash *tfm)
{
        return tfm->descsize;
}

the function crypto_shash_descsize() directly dereferences the pointer. 

By the way, I reproduce it on Linux 4.15.0-12-generic used by latest Ubuntu
18.04.

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