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 p4doffsetlockless() will return a 'p4dt *' corresponding to the 'pgdt' allocated on the stack of the caller, gupfastpgdrange(). This is normally fine, but when the fourth level of page-table is folded at runtime, pudoffsetlockless() will offset from the address of the 'p4dt' 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 p4doffsetlockless() when CONFIGPGTABLELEVELS <= 4 which returns the real page-table pointer rather than the address of the local stack variable.