Affected: Linux

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

mm: prevent poison consumption when splitting THP

When performing memory error injection on a THP (Transparent Huge Page) mapped to userspace on an x86 server, the kernel panics with the following trace. The expected behavior is to terminate the affected process instead of panicking the kernel, as the x86 Machine Check code can recover from an in-userspace #MC.

mce: [Hardware Error]: CPU 0: Machine Check Exception: f Bank 3: bd80000000070134 mce: [Hardware Error]: RIP 10:<ffffffff8372f8bc> {memchr_inv+0x4c/0xf0} mce: [Hardware Error]: TSC afff7bbff88a ADDR 1d301b000 MISC 80 PPIN 1e741e77539027db mce: [Hardware Error]: PROCESSOR 0:d06d0 TIME 1758093249 SOCKET 0 APIC 0 microcode 80000320 mce: [Hardware Error]: Run the above through 'mcelog --ascii' mce: [Hardware Error]: Machine check: Data load in unrecoverable area of kernel Kernel panic - not syncing: Fatal local machine check

The root cause of this panic is that handling a memory failure triggered by an in-userspace #MC necessitates splitting the THP. The splitting process employs a mechanism, implemented in try_to_map_unused_to_zeropage(), which reads the pages in the THP to identify zero-filled pages. However, reading the pages in the THP results in a second in-kernel #MC, occurring before the initial memory_failure() completes, ultimately leading to a kernel panic. See the kernel panic call trace on the two #MCs.

First Machine C…