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

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Date:   Mon, 15 Jan 2018 12:06:17 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Dave P Martin <Dave.Martin@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <Will.Deacon@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        James Morse <James.Morse@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        zijun_hu <zijun_hu@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Windsor <dave@...lcore.net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Christoph Lameter <cl@...ux.com>,
        "David S. Miller" <davem@...emloft.net>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <Mark.Rutland@....com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        Jan Kara <jack@...e.cz>,
        Luis de Bethencourt <luisbg@...nel.org>,
        Marc Zyngier <Marc.Zyngier@....com>,
        Rik van Riel <riel@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 33/38] arm64: Implement thread_struct whitelist for
 hardened usercopy

On Mon, Jan 15, 2018 at 4:24 AM, Dave P Martin <Dave.Martin@....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@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Cc: Christian Borntraeger <borntraeger@...ibm.com>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: James Morse <james.morse@....com>
>> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
>> Cc: Dave Martin <Dave.Martin@....com>
>> Cc: zijun_hu <zijun_hu@....com>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Signed-off-by: Kees Cook <keescook@...omium.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