linux-kernel - Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

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Message-ID: <20180205172638.briyyjtbxxd7wyjh@treble>
Date:   Mon, 5 Feb 2018 11:26:38 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall
 arguments for 64bit kernels

On Mon, Feb 05, 2018 at 04:37:26PM +0000, Andy Lutomirski wrote:
> On Mon, Feb 5, 2018 at 4:30 PM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * Andy Lutomirski <luto@...capital.net> wrote:
> >
> >>
> >>
> >> > On Feb 5, 2018, at 3:42 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >> >
> >> >
> >> > * Dan Williams <dan.j.williams@...el.com> wrote:
> >> >
> >> >> +    /*
> >> >> +     * Sanitize extra registers of values that a speculation attack
> >> >> +     * might want to exploit. In the CONFIG_FRAME_POINTER=y case,
> >> >> +     * the expectation is that %ebp will be clobbered before it
> >> >> +     * could be used.
> >> >> +     */
> >> >> +    .macro CLEAR_EXTRA_REGS_NOSPEC
> >> >> +    xorq %r15, %r15
> >> >> +    xorq %r14, %r14
> >> >> +    xorq %r13, %r13
> >> >> +    xorq %r12, %r12
> >> >> +    xorl %ebx, %ebx
> >> >> +#ifndef CONFIG_FRAME_POINTER
> >> >> +    xorl %ebp, %ebp
> >> >> +#endif
> >> >> +    .endm
> >> >
> >> > Yeah, so this series look pretty good to me, but there's one small detail: I think
> >> > RBP should be cleared unconditionally here, even in the CONFIG_FRAME_POINTERS=y
> >> > case, because:
> >>
> >> ENCODE_FRAME_POINTER should take care of rbp, though.
> >
> > AFAICS there's various entry paths where it's not used I think: for example the
> > compat system calls in entry_64_compat.S don't seem to encode RBP in such a
> > fashion (unless I missed some macro side effect).
> 
> Then that's a separate bug that should be fixed.  Josh?

We don't encode the frame pointer on syscalls, because "fast path"
(though that's obviously no longer a consideration).

-- 
Josh