Advisory [CVE-2026-64074] fix slab out-of-bounds write in statmount_mnt_idmap

In the Linux kernel, the following vulnerability has been resolved: fs/statmount: fix slab out-of-bounds write in statmount_mnt_idmap statmount_mnt_idmap() writes one mapping with seq_printf() and then manually advances seq->count to include the NUL separator. If seq_printf() overflows, seq_set_overflow() sets seq->count to seq->size.

The manual seq->count++ changes this to seq->size + 1. seq_has_overflowed() then no longer detects the overflow. The corrupted count returns to statmount_string(), which later executes: seq->buf[seq->count++] = '\0'; This causes a 1-byte NULL out-of-bounds write on the dynamically allocated seq buffer.

Fix this by checking for overflow immediately after seq_printf(). When processing mount ID mappings, an internal buffer can be incorrectly accessed due to an unchecked overflow, leading to a single-byte overwrite.

This memory corruption can cause system instability or a denial of service. Red Hat severity: not rated.

Weakness: CWE-787. Red Hat lists Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7; Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9 as not affected.