CVE-2024-26956 nilfs2: fix failure to detect DAT corruption in btree and direct mappings

What is CVE-2024-26956?

CVE-2024-26956 is a medium-severity denial of service vulnerability in Debian Debian Linux, published on 1 May 2024. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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

nilfs2: fix failure to detect DAT corruption in btree and direct mappings

Patch series "nilfs2: fix kernel bug at submit_bh_wbc()".

This resolves a kernel BUG reported by syzbot. Since there are two flaws involved, I've made each one a separate patch.

The first patch alone resolves the syzbot-reported bug, but I think both fixes should be sent to stable, so I've tagged them as such.

This patch (of 2):

Syzbot has reported a kernel bug in submit_bh_wbc() when writing file data to a nilfs2 file system whose metadata is corrupted.

There are two flaws involved in this issue.

The first flaw is that when nilfs_get_block() locates a data block using btree or direct mapping, if the disk address translation routine nilfs_dat_translate() fails with internal code -ENOENT due to DAT metadata corruption, it can be passed back to nilfs_get_block(). This causes nilfs_get_block() to misidentify an existing block as non-existent, causing both data block lookup and insertion to fail inconsistently.

The second flaw is that nilfs_get_block() returns a successful status in this inconsistent state. This causes the caller __block_write_begin_int() or others to request a read even though the buffer is not mapped, resulting in a BUG_ON check for the BH_Mapped flag in submit_bh_wbc() failing.

This fixes the first issue by changing the return value to code -EINVAL when a conversion using DAT fails with code -ENOENT, avoiding the conflicting condition that leads to the kernel bug described above. Here, code -EINVAL indicates that metadata corruption was detected during the block lookup, which will be properly handled as a file system error and converted to -EIO when passing through the nilfs2 bmap layer.

Which versions are affected by CVE-2024-26956?

It affects 2 products.

Products include Debian Debian Linux, Linux kernel.

How do I fix CVE-2024-26956?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2024-26956 being exploited?

There is no known exploitation of CVE-2024-26956 in the wild and no public exploit code.

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Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BELL-CVE-2024-26956, DEBIAN-CVE-2024-26956, GHSA-qwvj-5fwc-qxf7, UBUNTU-CVE-2024-26956.