Re: [NEEDS-REVIEW] Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Dave Hansen <dave.hansen@intel.com>]

On 9/11/20 3:59 PM, Yu-cheng Yu wrote:
...
> Here are the changes if we take the mprotect(PROT_SHSTK) approach.
> Any comments/suggestions?

I still don't like it. :)

I'll also be much happier when there's a proper changelog to accompany
this which also spells out the alternatives any why they suck so much.

> diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
> index d4a8d0424bfb..024f006fcfe8 100644
> --- a/arch/x86/include/uapi/asm/mman.h
> +++ b/arch/x86/include/uapi/asm/mman.h
> @@ -4,6 +4,8 @@
>  
>  #define MAP_32BIT	0x40		/* only give out 32bit addresses */
>  
> +#define PROT_SHSTK	0x10		/* shadow stack pages */
> +
>  #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>  /*
>   * Take the 4 protection key bits out of the vma->vm_flags
> @@ -19,13 +21,35 @@
>  		((vm_flags) & VM_PKEY_BIT2 ? _PAGE_PKEY_BIT2 : 0) |	\
>  		((vm_flags) & VM_PKEY_BIT3 ? _PAGE_PKEY_BIT3 : 0))
>  
> -#define arch_calc_vm_prot_bits(prot, key) (		\
> +#define pkey_vm_prot_bits(prot, key) (			\
>  		((key) & 0x1 ? VM_PKEY_BIT0 : 0) |      \
>  		((key) & 0x2 ? VM_PKEY_BIT1 : 0) |      \
>  		((key) & 0x4 ? VM_PKEY_BIT2 : 0) |      \
>  		((key) & 0x8 ? VM_PKEY_BIT3 : 0))
> +#else
> +#define pkey_vm_prot_bits(prot, key)
>  #endif

My inner compiler doesn't think this will compile:

	( | shstk_vm_prot_bits(prot))


> +#define shstk_vm_prot_bits(prot) ( \
> +		(static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK)) ? \
> +		VM_SHSTK : 0)

Why do you need to filter PROT_SHSTK twice.  Won't the prot passed in
here be filtered by arch_validate_prot()?

> +#define arch_calc_vm_prot_bits(prot, key) \
> +		(pkey_vm_prot_bits(prot, key) | shstk_vm_prot_bits(prot))
> +

IMNHO, this is eminently more readable if you do:

#define arch_calc_vm_prot_bits(prot, key)	\
		(shstk_vm_prot_bits(prot))	\
		  pkey_vm_prot_bits(prot, key))

BTW, can these be static inlines?  I forget if I had a good reason for
making them #defines.

> +static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
> +{
> +	unsigned long supported = PROT_READ | PROT_EXEC | PROT_SEM;
> +
> +	if (static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK))
> +		supported |= PROT_SHSTK;
> +	else
> +		supported |= PROT_WRITE;

I generally like to make the common case dirt simple to understand.
That would probably be:

	unsigned long supported = PROT_READ | PROT_WRITE |
				  PROT_EXEC | PROT_SEM;

	if (static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK)) {
		supported |= PROT_SHSTK;
		// Comment about why SHSTK and WRITE
		// are mutually exclusive.

		supported &= ~PROT_WRITE;
	}

>  #endif /* _ASM_X86_MMAN_H */
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index a8edbcb3af99..520bd8caa005 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -571,6 +571,17 @@ static int do_mprotect_pkey(unsigned long start, size_t
> len,
>  				goto out;
>  		}
>  	}
> +
> +	/*
> +	 * Only anonymous mapping is suitable for shadow stack.
> +	 */

Why?

> +	if (prot & PROT_SHSTK) {
> +		if (vma->vm_file) {
> +			error = -EINVAL;
> +			goto out;
> +		}
> +	}

You can also save a couple of lines there.  The two conditions are
pretty small.