Re: [PATCH 33/38] arm64: Implement thread_struct whitelist for hardened usercopy

[Dave Martin <Dave.Martin@arm.com>]

[Dropped most of the original Cc list, since most people are unlikely to
care about this thread archaeology.]

On Mon, Jan 15, 2018 at 12:06:17PM -0800, Kees Cook wrote:
> On Mon, Jan 15, 2018 at 4:24 AM, Dave P Martin <Dave.Martin@arm.com> wrote:
> > On Thu, Jan 11, 2018 at 02:03:05AM +0000, Kees Cook wrote:
> >> This whitelists the FPU register state portion of the thread_struct for
> >> copying to userspace, instead of the default entire structure.
> >>
> >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >> Cc: Will Deacon <will.deacon@arm.com>
> >> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> >> Cc: Ingo Molnar <mingo@kernel.org>
> >> Cc: James Morse <james.morse@arm.com>
> >> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
> >> Cc: Dave Martin <Dave.Martin@arm.com>
> >> Cc: zijun_hu <zijun_hu@htc.com>
> >> Cc: linux-arm-kernel@lists.infradead.org
> >> Signed-off-by: Kees Cook <keescook@chromium.org>
> >> ---
> >>  arch/arm64/Kconfig                 | 1 +
> >>  arch/arm64/include/asm/processor.h | 8 ++++++++
> >>  2 files changed, 9 insertions(+)
> >>
> >> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> >> index a93339f5178f..c84477e6a884 100644
> >> --- a/arch/arm64/Kconfig
> >> +++ b/arch/arm64/Kconfig
> >> @@ -90,6 +90,7 @@ config ARM64
> >>       select HAVE_ARCH_MMAP_RND_BITS
> >>       select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
> >>       select HAVE_ARCH_SECCOMP_FILTER
> >> +     select HAVE_ARCH_THREAD_STRUCT_WHITELIST
> >>       select HAVE_ARCH_TRACEHOOK
> >>       select HAVE_ARCH_TRANSPARENT_HUGEPAGE
> >>       select HAVE_ARCH_VMAP_STACK
> >> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> >> index 023cacb946c3..e58a5864ec89 100644
> >> --- a/arch/arm64/include/asm/processor.h
> >> +++ b/arch/arm64/include/asm/processor.h
> >> @@ -113,6 +113,14 @@ struct thread_struct {
> >>       struct debug_info       debug;          /* debugging */
> >>  };
> >>
> >> +/* Whitelist the fpsimd_state for copying to userspace. */
> >> +static inline void arch_thread_struct_whitelist(unsigned long *offset,
> >> +                                             unsigned long *size)
> >> +{
> >> +     *offset = offsetof(struct thread_struct, fpsimd_state);
> >> +     *size = sizeof(struct fpsimd_state);
> >
> > This should be fpsimd_state.user_fpsimd (fpsimd_state.cpu is important
> > for correctly context switching and not supposed to be user-accessible.
> > A user copy that encompasses that is definitely a bug).
> 
> So, I actually spent some more time looking at this due to the
> comments from rmk on arm32, and I don't think any whitelist is needed
> here at all. (i.e. it can be *offset = *size = 0) This is because all
> the usercopying I could find uses static sizes or bounce buffers, both
> of which bypass the dynamic-size hardened usercopy checks.
> 
> I've been running some arm64 builds now with this change, and I
> haven't tripped over any problems yet...

Hmmm, it looks like we may be hitting this with user_regset_copyout()
when reading the fp regs via ptrace.  This is maybe not surprising,
since the size comes from userspace for PTRACE_{GET,SET}REGSET.
Also, while we copy into a bounce buffer for SETREGSET here, we do copy
straight out of task_struct for GETREGSET here.

This suggests we should have:

	*offset = offsetof(struct thread_struct, fpsimd_state);
	*size = sizeof(struct user_fpsimd_state);

Thoughts?

I'm making some assumptions about how the usercopy hardening works.

Cheers
---Dave