CVE-2024-42104

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

nilfs2: add missing check for inode numbers on directory entries

Syzbot reported that mounting and unmounting a specific pattern of corrupted nilfs2 filesystem images causes a use-after-free of metadata file inodes, which triggers a kernel bug in lru_add_fn().

As Jan Kara pointed out, this is because the link count of a metadata file gets corrupted to 0, and nilfs_evict_inode(), which is called from iput(), tries to delete that inode (ifile inode in this case).

The inconsistency occurs because directories containing the inode numbers of these metadata files that should not be visible in the namespace are read without checking.

Fix this issue by treating the inode numbers of these internal files as errors in the sanity check helper when reading directory folios/pages.

Also thanks to Hillf Danton and Matthew Wilcox for their initial mm-layer analysis.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/07c176e7acc5579c133bb923ab21316d192d0a95
• https://git.kernel.org/stable/c/1b7d549ed2c1fa202c751b69423a0d3a6bd5a180
• https://git.kernel.org/stable/c/265fff1a01cdc083aeaf0d934c929db5cc64aebf
• https://git.kernel.org/stable/c/2f2fa9cf7c3537958a82fbe8c8595a5eb0861ad7
• https://git.kernel.org/stable/c/3ab40870edb883b9633dc5cd55f5a2a11afa618d
• https://git.kernel.org/stable/c/b11e8fb93ea5eefb2e4e719497ea177a58ff6131
• https://git.kernel.org/stable/c/bb76c6c274683c8570ad788f79d4b875bde0e458
• https://git.kernel.org/stable/c/c33c2b0d92aa1c2262d999b2598ad6fbd53bd479