title: mm, kmsan: fix infinite recursion due to RCU critical section
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."
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.
affected:
{
product: Linux
vendor: Linux
defaultStatus: unaffected
repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
versions:
version: 68ed9e333240, lessThan: dc904345e377, status: affected, versionType: git
version: 70064241f222, lessThan: 6335c0cdb2ea, status: affected, versionType: git
version: 3a01daace71b, lessThan: 5a33420599fa, status: affected, versionType: git
version: 5ec8e8ea8b77, lessThan: f6564fce256a, status: affected, versionType: git
}
{
product: Linux
vendor: Linux
defaultStatus: unaffected
repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
versions:
version: 6.1.76, lessThan: 6.1.77, status: affected, versionType: custom
version: 6.6.15, lessThan: 6.6.16, status: affected, versionType: custom
version: 6.7.3, lessThan: 6.7.4, status: affected, versionType: custom
}
