Re: [PATCH 33/33] x86, pkeys: execute-only support

On Feb 17, 2016 1:27 PM, "Kees Cook" [off-list ref] wrote:

On Fri, Feb 12, 2016 at 1:02 PM, Dave Hansen [off-list ref] wrote:

quoted

From: Dave Hansen <dave.hansen@linux.intel.com>

Protection keys provide new page-based protection in hardware.
But, they have an interesting attribute: they only affect data
accesses and never affect instruction fetches.  That means that
if we set up some memory which is set as "access-disabled" via
protection keys, we can still execute from it.

This patch uses protection keys to set up mappings to do just that.
If a user calls:

        mmap(..., PROT_EXEC);
or
        mprotect(ptr, sz, PROT_EXEC);

(note PROT_EXEC-only without PROT_READ/WRITE), the kernel will
notice this, and set a special protection key on the memory.  It
also sets the appropriate bits in the Protection Keys User Rights
(PKRU) register so that the memory becomes unreadable and
unwritable.

I haven't found any userspace that does this today.  With this
facility in place, we expect userspace to move to use it
eventually.  Userspace _could_ start doing this today.  Any
PROT_EXEC calls get converted to PROT_READ inside the kernel, and
would transparently be upgraded to "true" PROT_EXEC with this
code.  IOW, userspace never has to do any PROT_EXEC runtime
detection.

Random thought while skimming email:

Is there a way to detect this feature's availability without userspace
having to set up a segv handler and attempting to read a
PROT_EXEC-only region? (i.e. cpu flag for protection keys, or a way to
check the protection to see if PROT_READ got added automatically,
etc?)

We could add an HWCAP.

--Andy

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