Re: [PATCH] mm/sparsemem: fix race in accessing memory_section->usage

[Marco Elver]

On Mon, 15 Jan 2024 at 19:44, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: kasan-dev@xxxxxxxxxxxxxxxx
> Cc: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> Cc: Nicholas Miehlbradt <nicholas@xxxxxxxxxxxxx>
>
> Hi folks,
>
> (adding KMSAN reviewers and IBM people who are currently porting KMSAN to other
> architectures, plus Paul for his opinion on refactoring RCU)
>
> this patch broke x86 KMSAN in a subtle way.
>
> 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.
>
> I am currently looking into inlining __rcu_read_lock()/__rcu_read_unlock(), into
> KMSAN code to prevent it from being instrumented, but that might require factoring
> out parts of kernel/rcu/tree_plugin.h into a non-private header. Do you think this
> is feasible?

__rcu_read_lock/unlock() is only outlined in PREEMPT_RCU. Not sure that helps.

Otherwise, there is rcu_read_lock_sched_notrace() which does the bare
minimum and is static inline.

Does that help?

> Another option is to cut some edges in the code calling virt_to_page(). First,
> my observation is that virt_addr_valid() is quite rare in the kernel code, i.e.
> not all cases of calling virt_to_page() are covered with it. Second, every
> memory access to KMSAN metadata residing in virt_to_page(kaddr)->shadow always
> accompanies an access to `kaddr` itself, so if there is a race on a PFN then
> the access to `kaddr` will probably also trigger a fault. Third, KMSAN metadata
> accesses are inherently non-atomic, and even if we ensure pfn_valid() is
> returning a consistent value for a single memory access, calling it twice may
> already return different results.
>
> Considering the above, how bad would it be to drop synchronization for KMSAN's
> version of pfn_valid() called from kmsan_virt_addr_valid()?