Re: [RFC PATCH v1 15/28] riscv/mm: Implement map_shadow_stack() syscall

From: Deepak Gupta <debug@rivosinc.com>
To: Mark Brown <broonie@kernel.org>
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Subject: Re: [RFC PATCH v1 15/28] riscv/mm: Implement map_shadow_stack() syscall
Date: Wed, 21 Feb 2024 16:47:11 -0800	[thread overview]
Message-ID: <ZdaZj0pqaVJiNOUg@debug.ba.rivosinc.com> (raw)
In-Reply-To: <ZcJX2IJb0hOM5RF5@finisterre.sirena.org.uk>

On Tue, Feb 06, 2024 at 04:01:28PM +0000, Mark Brown wrote:
>On Wed, Jan 24, 2024 at 10:21:40PM -0800, debug@rivosinc.com wrote:
>
>> As discussed extensively in the changelog for the addition of this
>> syscall on x86 ("x86/shstk: Introduce map_shadow_stack syscall") the
>> existing mmap() and madvise() syscalls do not map entirely well onto the
>> security requirements for guarded control stacks since they lead to
>> windows where memory is allocated but not yet protected or stacks which
>> are not properly and safely initialised. Instead a new syscall
>> map_shadow_stack() has been defined which allocates and initialises a
>> shadow stack page.
>
>While I agree that this is very well written you probably want to update
>the references to guarded control stacks to whatever the RISC-V term is :P

Noted. I'll do that in next patchset.

>
>> --- a/include/uapi/asm-generic/mman.h
>> +++ b/include/uapi/asm-generic/mman.h
>> @@ -19,4 +19,5 @@
>>  #define MCL_FUTURE	2		/* lock all future mappings */
>>  #define MCL_ONFAULT	4		/* lock all pages that are faulted in */
>>
>> +#define SHADOW_STACK_SET_TOKEN (1ULL << 0)     /* Set up a restore token in the shadow stack */
>>  #endif /* __ASM_GENERIC_MMAN_H */
>
>For arm64 I also added a SHADOW_STACK_SET_MARKER for adding a top of
>stack marker, did you have any thoughts on that for RISC-V?  I think x86
>were considering adding it too, it'd be good if we could get things
>consistent.

Please correct me on this. A token at the top which can't be consumed to restore
but *just* purely as marker, right?
It's a good design basic with not a lot of cost.

I think risc-v should be able to converge on that.