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SubjectRe: [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@kernel.org> wrote:
> >
> > * Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >>
> >>
> >> > On Feb 5, 2018, at 3:42 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >> >
> >> >
> >> > * Dan Williams <dan.j.williams@intel.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

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