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().