Linux Kernel CVE-2025-40124

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

sparc: fix accurate exception reporting in copy_{from_to}_user for UltraSPARC III

Anthony Yznaga tracked down that a BUG_ON in ext4 code with large folios enabled resulted from copy_from_user() returning impossibly large values greater than the size to be copied. This lead to __copy_from_iter() returning impossible values instead of the actual number of bytes it was able to copy.

The BUG_ON has been reported in https://lore.kernel.org/r/b14f55642207e63e907965e209f6323a0df6dcee.camel@physik.fu-berlin.de

The referenced commit introduced exception handlers on user-space memory references in copy_from_user and copy_to_user. These handlers return from the respective function and calculate the remaining bytes left to copy using the current register contents. The exception handlers expect that %o2 has already been masked during the bulk copy loop, but the masking was performed after that loop. This will fix the return value of copy_from_user and copy_to_user in the faulting case. The behaviour of memcpy stays unchanged.

The Linux kernel on SPARC64 / UltraSPARC III (Cheetah-class) processors is affected; the defect is in the architecture-specific copy_from_user/copy_to_user uaccess routines. Exact vulnerable version ranges are not enumerated in the provided NVD data (CPE strings were not supplied), but fixes were backported across multiple stable trees as shown by the eight git.kernel.org stable commits (e.g. https://git.kernel.org/stable/c/91eda032eb16e5d2be27c95584665bc555bb5a90 and https://git.kernel.org/stable/c/1198077606aeffb102587c6ea079ce99641c99d4). Distribution coverage is confirmed for Ubuntu via advisories USN-8126-1 (https://ubuntu.com/security/notices/USN-8126-1) and USN-8125-1 (https://ubuntu.com/security/notices/USN-8125-1). Non-SPARC architectures are not affected.