Messages in this thread | | | Date | Mon, 5 Feb 2018 11:48:29 -0600 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels |
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On Mon, Feb 05, 2018 at 05:31:39PM +0000, Andy Lutomirski wrote: > On Mon, Feb 5, 2018 at 5:26 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote: > > 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). > > Should we start encoding the frame pointer?
Perhaps, but I should clarify it's not a bug. For syscalls, we instead just standardized the location of the last stack frame. But that's a bit fragile and I was never too happy with it.
Encoding the frame pointer would be a lot more straightforward, though we'd still need to figure out a way to detect the "end" of the stack for kthreads. We could probably encode that information as well: "here's the end, but there are no pt_regs". Could just be an "encoded" NULL.
-- Josh
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