CVE-2024-26639 Information

Description

In the Linux kernel the following vulnerability has been resolved:

mm kmsan: fix infinite recursion due to RCU critical section

Alexander Potapenko writes in [1]: \For every memory access in the code instrumented by KMSAN we call kmsan_get_metadata() to obtain the metadata for the memory being accessed. For virtual memory the metadata pointers are stored in the corresponding struct page therefore we need to call virt_to_page() to get them.

According to the comment in arch/x86/include/asm/page.h virt_to_page(kaddr) returns a valid pointer iff virt_addr_valid(kaddr) is true so KMSAN needs to call virt_addr_valid() as well.

To avoid recursion kmsan_get_metadata() must not call instrumented code therefore ./arch/x86/include/asm/kmsan.h forks parts of arch/x86/mm/physaddr.c to check whether a virtual address is valid or not.

But the introduction of rcu_read_lock() to pfn_valid() added instrumented RCU API calls to virt_to_page_or_null() which is called by kmsan_get_metadata() so there is an infinite recursion now. I do not think it is correct to stop that recursion by doing kmsan_enter_runtime()/kmsan_exit_runtime() in kmsan_get_metadata(): that would prevent instrumented functions called from within the runtime from tracking the shadow values which might introduce false positives.\n Fix the issue by switching pfn_valid() to the _sched() variant of rcu_read_lock/unlock() which does not require calling into RCU. Given the critical section in pfn_valid() is very small this is a reasonable trade-off (with preemptible RCU).

KMSAN further needs to be careful to suppress calls into the scheduler which would be another source of recursion. This can be done by wrapping the call to pfn_valid() into preempt_disable/enable_no_resched(). The downside is that this sacrifices breaking scheduling guarantees; however a kernel compiled with KMSAN has already given up any performance guarantees due to being heavily instrumented.

Note KMSAN code already disables tracing via Makefile and since mmzone.h is included it is not necessary to use the notrace variant which is generally preferred in all other cases.

Reference

https://git.kernel.org/stable/c/dc904345e3771aa01d0b8358b550802fdc6fe00b https://git.kernel.org/stable/c/6335c0cdb2ea0ea02c999e04d34fd84f69fb27ff https://git.kernel.org/stable/c/5a33420599fa0288792537e6872fd19cc8607ea6