Re: [PATCH 17/34] x86, pkeys: check VMAs and PTEs for protection keys

[Thomas Gleixner]

On Thu, 3 Dec 2015, Dave Hansen wrote:
> Today, for normal faults and page table walks, we check the VMA
> and/or PTE to ensure that it is compatible with the action.  For
> instance, if we get a write fault on a non-writeable VMA, we
> SIGSEGV.
> 
> We try to do the same thing for protection keys.  Basically, we
> try to make sure that if a user does this:
> 
> 	mprotect(ptr, size, PROT_NONE);
> 	*ptr = foo;
> 
> they see the same effects with protection keys when they do this:
> 
> 	mprotect(ptr, size, PROT_READ|PROT_WRITE);
> 	set_pkey(ptr, size, 4);
> 	wrpkru(0xffffff3f); // access disable pkey 4
> 	*ptr = foo;
> 
> The state to do that checking is in the VMA, but we also
> sometimes have to do it on the page tables only, like when doing
> a get_user_pages_fast() where we have no VMA.
> 
> We add two functions and expose them to generic code:
> 
> 	arch_pte_access_permitted(pte_flags, write)
> 	arch_vma_access_permitted(vma, write)
> 
> These are, of course, backed up in x86 arch code with checks
> against the PTE or VMA's protection key.
> 
> But, there are also cases where we do not want to respect
> protection keys.  When we ptrace(), for instance, we do not want
> to apply the tracer's PKRU permissions to the PTEs from the
> process being traced.
> 
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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