CVE-2023-52786

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

ext4: fix racy may inline data check in dio write

syzbot reports that the following warning from ext4_iomap_begin() triggers as of the commit referenced below:

    if (WARN_ON_ONCE(ext4_has_inline_data(inode)))
            return -ERANGE;

This occurs during a dio write, which is never expected to encounter an inode with inline data. To enforce this behavior, ext4_dio_write_iter() checks the current inline state of the inode and clears the MAY_INLINE_DATA state flag to either fall back to buffered writes, or enforce that any other writers in progress on the inode are not allowed to create inline data.

The problem is that the check for existing inline data and the state flag can span a lock cycle. For example, if the ilock is originally locked shared and subsequently upgraded to exclusive, another writer may have reacquired the lock and created inline data before the dio write task acquires the lock and proceeds.

The commit referenced below loosens the lock requirements to allow some forms of unaligned dio writes to occur under shared lock, but AFAICT the inline data check was technically already racy for any dio write that would have involved a lock cycle. Regardless, lift clearing of the state bit to the same lock critical section that checks for preexisting inline data on the inode to close the race.

Weakness

The product does not properly acquire or release a lock on a resource, leading to unexpected resource state changes and behaviors.

Affected Software

Extended Description

Locking is a type of synchronization behavior that ensures that multiple independently-operating processes or threads do not interfere with each other when accessing the same resource. All processes/threads are expected to follow the same steps for locking. If these steps are not followed precisely - or if no locking is done at all - then another process/thread could modify the shared resource in a way that is not visible or predictable to the original process. This can lead to data or memory corruption, denial of service, etc.

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/25.html
• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/27.html

References

• https://git.kernel.org/stable/c/7343c23ebcadbedc23a7063d1e24d976eccb0d0d
• https://git.kernel.org/stable/c/ce56d21355cd6f6937aca32f1f44ca749d1e4808
• https://git.kernel.org/stable/c/e3b83d87c93eb6fc96a80b5e8527f7dc9f5a11bc
• https://git.kernel.org/stable/c/7343c23ebcadbedc23a7063d1e24d976eccb0d0d
• https://git.kernel.org/stable/c/ce56d21355cd6f6937aca32f1f44ca749d1e4808
• https://git.kernel.org/stable/c/e3b83d87c93eb6fc96a80b5e8527f7dc9f5a11bc