CVE-2024-42293

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

arm64: mm: Fix lockless walks with static and dynamic page-table folding

Lina reports random oopsen originating from the fast GUP code when

16K pages are used with 4-level page-tables, the fourth level being

folded at runtime due to lack of LPA2.

In this configuration, the generic implementation of

p4d_offset_lockless() will return a 'p4d_t *' corresponding to the

'pgd_t' allocated on the stack of the caller, gup_fast_pgd_range().

This is normally fine, but when the fourth level of page-table is folded

at runtime, pud_offset_lockless() will offset from the address of the

'p4d_t' to calculate the address of the PUD in the same page-table page.

This results in a stray stack read when the 'p4d_t' has been allocated

on the stack and can send the walker into the weeds.

Fix the problem by providing our own definition of p4d_offset_lockless()

when CONFIG_PGTABLE_LEVELS <= 4 which returns the real page-table

pointer rather than the address of the local stack variable.