CVE-2026-97943: x86/mm/pat: Acquire init_mm write lock on collapse to avoid UAF

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

x86/mm/pat: Acquire initmm write lock on collapse to avoid UAF

x86 implements page attribute modification using its Change Page Attributes (CPA) mechanism.

This tracks properties of ranges such as cache mode through x86 page attributes, and as part of that logic manipulates kernel page tables.

Since commit:

41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")

ranges of kernel page table entries can be collapsed into huge page table entries as part of this logic.

As part of this collapse, it frees the page tables which the collapsed entries previously pointed to, and it does so without any relevant locks being held to preclude concurrent kernel page table walkers.

The only way this code can be reached is if CPACOLLAPSE is specified, and this is only set in setmemoryrox() via:

setmemoryrox() -> changepageattrsetclr() -> cpaflush() -> cpacollapselargepages()

Notable users of this are execmem and BPF when manipulating executable mappings.

However, this is problematic for ptdump as it walks ranges it does not own and thus runs the risk of a use-after-free on page tables freed underneath it.

In addition, concurrent CPA collapse operations are possible which can also cause races.

Resolve the issue by acquiring the mmap write lock on initmm across the whole operation.

It is safe to acquire a sleeping lock as all the callers invoke setmemoryrox() from process context and in any case, changepageattrsetclr() calls vmunmapalias() which ultimately takes a mutex, disallowing atomic context here.