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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 25 Aug 2018 21:21:22 -0700
Message-ID: <CALCETrXPaX-+R6Z9LqZp0uOVmq-TUX_ksPbUL7mnfbdqo6z2AA@mail.gmail.com>
Subject: Re: TLB flushes on fixmap changes
To:     Masami Hiramatsu <mhiramat@kernel.org>,
        Kees Cook <keescook@chromium.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Nadav Amit <nadav.amit@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jiri Kosina <jkosina@suse.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
        Benjamin Herrenschmidt <benh@au1.ibm.com>,
        Nick Piggin <npiggin@gmail.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Rik van Riel <riel@surriel.com>,
        Jann Horn <jannh@google.com>,
        Adin Scannell <ascannell@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        David Miller <davem@davemloft.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>
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On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> On Fri, 24 Aug 2018 21:23:26 -0700
> Andy Lutomirski <luto@kernel.org> wrote:
>> Couldn't text_poke() use kmap_atomic()?  Or, even better, just change CR3?
>
> No, since kmap_atomic() is only for x86_32 and highmem support kernel.
> In x86-64, it seems that returns just a page address. That is not
> good for text_poke, since it needs to make a writable alias for RO
> code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses ioremap_cache?
>

I just re-read text_poke().  It's, um, horrible.  Not only is the
implementation overcomplicated and probably buggy, but it's SLOOOOOW.
It's totally the wrong API -- poking one instruction at a time
basically can't be efficient on x86.  The API should either poke lots
of instructions at once or should be text_poke_begin(); ...;
text_poke_end();.

Anyway, the attached patch seems to boot.  Linus, Kees, etc: is this
too scary of an approach?  With the patch applied, text_poke() is a
fantastic exploit target.  On the other hand, even without the patch
applied, text_poke() is every bit as juicy.

--Andy

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