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

Dave Hansen
2020-09-14T15:06:29Z

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.