Re: [PATCH] mm, kmsan: fix infinite recursion due to RCU critical section

[Alexander Potapenko <glider@google.com>]

On Thu, Jan 18, 2024 at 12:00 PM Marco Elver <elver@google.com> wrote:
>
> 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.
>
> Link: https://lkml.kernel.org/r/20240115184430.2710652-1-glider@google.com [1]
> Reported-by: Alexander Potapenko <glider@google.com>
> Reported-by: syzbot+93a9e8a3dea8d6085e12@syzkaller.appspotmail.com
> Signed-off-by: Marco Elver <elver@google.com>
> Cc: Charan Teja Kalla <quic_charante@quicinc.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Tested-by: Alexander Potapenko <glider@google.com>