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

Kees Cook
2018-01-15T20:06:17Z

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...

-Kees

--
Kees Cook
Pixel Security