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Date:   Thu, 10 Oct 2019 20:55:04 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to
 unsafe_put_user()
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On Mon, Oct 07, 2019 at 09:24:17PM -0700, Linus Torvalds wrote:
> On Mon, Oct 7, 2019 at 9:09 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Try the attached patch, and then count the number of "rorx"
> > instructions in the kernel. Hint: not many. On my personal config,
> > this triggers 15 times in the whole kernel build (not counting
> > modules).
> 
> .. and four of them are in perf_callchain_user(), and are due to those
> "__copy_from_user_nmi()" with either 4-byte or 8-byte copies.
> 
> It might as well just use __get_user() instead.
> 
> The point being that the silly code in the header files is just
> pointless. We shouldn't do it.

FWIW, the one that looks the most potentiall sensitive in that bunch is
arch/x86/kvm/paging_tmpl.h:388:         if (unlikely(__copy_from_user(&pte, ptep_user, sizeof(pte))))
in the bowels of KVM page fault handling.  I would be very surprised if
the rest would be detectable...

Anyway, another question you way: what do you think of try/catch approaches
to __get_user() blocks, like e.g. restore_sigcontext() is doing?

Should that be available outside of arch/*?  For that matter, would
it be a good idea to convert get_user_ex() users in arch/x86 to
unsafe_get_user()?