linux-kernel - Re: [PATCH] x86/uaccess: instrument copy_from_user

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Date:   Thu, 27 Oct 2022 11:26:50 -0700
From:   Alexander Potapenko <glider@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>, x86@...nel.org
Subject: Re: [PATCH] x86/uaccess: instrument copy_from_user_nmi()

On Thu, Oct 27, 2022 at 1:05 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Oct 26, 2022 at 11:38:53AM -0700, Alexander Potapenko wrote:
> > A bigger issue from the NMI perspective is probably
> > having __msan_poison_alloca() inserted in every non-noinstr kernel
> > function, because that hook may acquire the stackdepot lock.
>
> *urgghhh* that's broken, that must not be. There is a *TON* of NMI
> functions that are non-noinstr.

__msan_poison_alloca() is guarded by kmsan_in_runtime(), which is
currently implemented as:

  static __always_inline bool kmsan_in_runtime(void)
  {
          if ((hardirq_count() >> HARDIRQ_SHIFT) > 1)
                  return true;
          return kmsan_get_context()->kmsan_in_runtime;
  }

I think the easiest way to fix the NMI situation would be adding "if
in_nmi() return true"?

Currently that will render kmsan_unpoison_memory() useless in NMI
context, but I think we don't need a check for kmsan_in_runtime()
there, because unpoisoning is self-contained and normally does not
recurse (guess we can tolerate a pr_err() on the rare assertion
violation path?)

> What's worse, it seems to do a memory allocation as well, and that's out
> the window with PREEMPT_RT where you can't do even GFP_ATOMIC from
> regular IRQ context.

Yes, there's a lazy call to alloc_pages() in lib/stackdepot.c that is
done when we run out of storage space.
It would be a pity to ignore everything that is happening inside
regular IRQs (by making kmsan_in_runtime() bail out on in_irq()) - I
think we occasionally see errors from there, e.g.
https://syzkaller.appspot.com/bug?id=233563e79a8e00f86412eb3d2fb4eb1f425e70c3
We could make stackdepot avoid allocating memory in IRQs/NMIs and hope
that calls to __msan_poison_alloca() from regular contexts keep up
with draining the storage from interrupts.
Another option would be to preallocate a very big chunk of memory for
stackdepot and never do allocations again.

These tricks won't however save us from acquiring depot_lock from
lib/stackdepot.c every time we want to create a new origin.
But that should not be a problem by itself, because we always do
kmsan_enter_runtime() before accessing the stack depot - i.e. it won't
be taken recursively.

Given that PREEMPT_RT is not the default at the moment, shall we make
KMSAN incompatible with it for the time being?

> That function is wholly unacceptable to be added to every kernel
> function.
>

-- 
Alexander Potapenko
Software Engineer

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