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SubjectRe: [PATCH 03/10] x86/cet: Signal handling for shadow stack
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On 06/07/2018 08:30 PM, Andy Lutomirski wrote:
> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>
>> Set and restore shadow stack pointer for signals.
>
> How does this interact with siglongjmp()?

We plan to use some unused signal mask bits in the jump buffer (we have
a lot of those in glibc for some reason) to store the shadow stack pointer.

> This patch makes me extremely nervous due to the possibility of ABI
> issues and CRIU breakage.
>
>> diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
>> index 844d60eb1882..6c8997a0156a 100644
>> --- a/arch/x86/include/uapi/asm/sigcontext.h
>> +++ b/arch/x86/include/uapi/asm/sigcontext.h
>> @@ -230,6 +230,7 @@ struct sigcontext_32 {
>> __u32 fpstate; /* Zero when no FPU/extended context */
>> __u32 oldmask;
>> __u32 cr2;
>> + __u32 ssp;
>> };
>>
>> /*
>> @@ -262,6 +263,7 @@ struct sigcontext_64 {
>> __u64 trapno;
>> __u64 oldmask;
>> __u64 cr2;
>> + __u64 ssp;
>>
>> /*
>> * fpstate is really (struct _fpstate *) or (struct _xstate *)
>> @@ -320,6 +322,7 @@ struct sigcontext {
>> struct _fpstate __user *fpstate;
>> __u32 oldmask;
>> __u32 cr2;
>> + __u32 ssp;
>
> Is it actually okay to modify these structures like this? They're
> part of the user ABI, and I don't know whether any user code relies on
> the size being constant.

Probably not. Historically, these things have been tacked at the end of
the floating point state, see struct _xstate:

/* New processor state extensions go here: */

However, I'm not sure if this is really ideal because I doubt that
everyone who needs the shadow stack pointer also wants to sacrifice
space for the AVX-512 save area (which is already a backwards
compatibility hazard). Other architectures have variable offsets and
some TLV-style setup here.

Thanks,
Florian

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