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From:   Jann Horn <jannh@google.com>
Date:   Mon, 27 Aug 2018 00:48:13 +0200
Message-ID: <CAG48ez1cYsa+5Grbzp0oGTFyjqE4pR-Qe9gb=0TKc9Q5HuOpmA@mail.gmail.com>
Subject: Re: TLB flushes on fixmap changes
To:     Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@chromium.org>
Cc:     mhiramat@kernel.org, Nadav Amit <nadav.amit@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>, jkosina@suse.cz,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>, benh@au1.ibm.com,
        npiggin@gmail.com, "the arch/x86 maintainers" <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Rik van Riel <riel@surriel.com>,
        Adin Scannell <ascannell@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        "David S. Miller" <davem@davemloft.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>
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On Sun, Aug 26, 2018 at 6:21 AM Andy Lutomirski <luto@kernel.org> wrote:
>
> 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.

Twiddling CR0.WP is incompatible with Xen PV, right? It can't let you
do it because you're not actually running in ring 0 (but in ring 1 or
3), so CR0.WP has no influence on what you can access; and it must not
let you bypass write protection because you have read-only access to
host page tables. I think this code has to be compatible with Xen PV,
right?

In theory Xen PV could support this by emulating X86 instructions, but
I don't see anything related to CR0.WP in their emulation code. From
xen/arch/x86/pv/emul-priv-op.c:

    case 0: /* Write CR0 */
        if ( (val ^ read_cr0()) & ~X86_CR0_TS )
        {
            gdprintk(XENLOG_WARNING,
                     "Attempt to change unmodifiable CR0 flags\n");
            break;
        }
        do_fpu_taskswitch(!!(val & X86_CR0_TS));
        return X86EMUL_OKAY;

Having a special fallback path for "patch kernel code while running
under Xen PV" would be kinda ugly.