selftests/vm/pkeys: refill shadow register after implicit kernel write

The pkey test code keeps a "shadow" of the pkey register around. This
ensures that any bugs which might write to the register can be caught more
quickly.

Generally, userspace has a good idea when the kernel is going to write to
the register. For instance, alloc_pkey() is passed a permission mask.
The caller of alloc_pkey() can update the shadow based on the return value
and the mask.

But, the kernel can also modify the pkey register in a more sneaky way.
For mprotect(PROT_EXEC) mappings, the kernel will allocate a pkey and
write the pkey register to create an execute-only mapping. The kernel
never tells userspace what key it uses for this.

This can cause the test to fail with messages like:

protection_keys_64.2: pkey-helpers.h:132: _read_pkey_reg: Assertion `pkey_reg == shadow_pkey_reg' failed.

because the shadow was not updated with the new kernel-set value.

Forcibly update the shadow value immediately after an mprotect().

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 6af17cf89e99 ("x86/pkeys/selftests: Add PROT_EXEC test")
Signed-off-by: Dave Hansen <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Tested-by: Aneesh Kumar K.V <[e-mail]>
Cc: Ram Pai <[e-mail]>
Cc: Sandipan Das <[e-mail]>
Cc: Florian Weimer <[e-mail]>
Cc: "Desnes A. Nunes do Rosario" <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Thiago Jung Bauermann <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Michal Suchanek <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/vm/protection_keys.c

···
 	ret = mprotect(p1, PAGE_SIZE, PROT_EXEC);
 	pkey_assert(!ret);
 
+	/*
+	 * Reset the shadow, assuming that the above mprotect()
+	 * correctly changed PKRU, but to an unknown value since
+	 * the actual alllocated pkey is unknown.
+	 */
+	shadow_pkey_reg = __read_pkey_reg();
+
 	dprintf2("pkey_reg: %016llx\n", read_pkey_reg());
 
 	/* Make sure this is an *instruction* fault */