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Re: [PATCH v5 15/30] arm64/sve: Signal handling support
- From: Will Deacon <will dot deacon at arm dot com>
- To: Kees Cook <keescook at chromium dot org>
- Cc: Dave Martin <Dave dot Martin at arm dot com>, linux-arm-kernel at lists dot infradead dot org, linux-arch <linux-arch at vger dot kernel dot org>, Okamoto Takayuki <tokamoto at jp dot fujitsu dot com>, libc-alpha <libc-alpha at sourceware dot org>, Ard Biesheuvel <ard dot biesheuvel at linaro dot org>, Szabolcs Nagy <szabolcs dot nagy at arm dot com>, Catalin Marinas <catalin dot marinas at arm dot com>, Alex Bennée <alex dot bennee at linaro dot org>, kvmarm at lists dot cs dot columbia dot edu
- Date: Thu, 7 Dec 2017 10:49:48 +0000
- Subject: Re: [PATCH v5 15/30] arm64/sve: Signal handling support
- Authentication-results: sourceware.org; auth=none
- References: <1509465082-30427-1-git-send-email-Dave.Martin@arm.com> <1509465082-30427-16-git-send-email-Dave.Martin@arm.com> <CAGXu5jJgsAg1VBMbx=mV3ep4hzs+1G46Sow4eeFqCK31b_sORA@mail.gmail.com>
On Wed, Dec 06, 2017 at 11:56:50AM -0800, Kees Cook wrote:
> On Tue, Oct 31, 2017 at 8:51 AM, Dave Martin <Dave.Martin@arm.com> wrote:
> > Miscellaneous:
> > * Change inconsistent copy_to_user() calls to __copy_to_user() in
> > preserve_sve_context().
> > There are already __put_user_error() calls here.
> > The whole extended signal frame is already checked for
> > access_ok(VERIFY_WRITE) in get_sigframe().
> Verifying all these __copy_to/from_user() calls is rather non-trivial.
> For example, I had to understand that the access_ok() check actually
> spans memory that both user->sigframe and user->next_frame point into.
I don't think that's particularly difficult -- you just have to read the
four lines preceding the access_ok.
> And it isn't clear to me that all users of apply_user_offset() are
> within this range too, along with other manually calculated offsets in
The offsets passed into apply_user_offset are calculated by
setup_sigframe_layout as the stack is allocated, so they're correct by
construction. We could add a size check in apply_user_offset if you like?
> And it's not clear if parse_user_sigframe() is safe either. Are
> user->fpsimd and user->sve checked somewhere? It seems like it's
> safely contained by in sf->uc.uc_mcontext.__reserved, but it's hard to
> read, though I do see access_ok() checks against __reserved at the end
> of the while loop.
This one is certainly more difficult to follow, mainly because it's spread
about a bit and we have to check the extra context separately. However, the
main part of the frame is checked in sys_rt_sigreturn before calling
restore_sigframe, and the extra context is checked in parse_user_sigframe
if we find it.
Dave, any thoughts on making this easier to understand?