x86/pkeys: Add check for pkey "overflow"

Alex Shi reported the pkey macros above arch_set_user_pkey_access()
to be unused.  They are unused, and even refer to a nonexistent
CONFIG option.

But, they might have served a good use, which was to ensure that
the code does not try to set values that would not fit in the
PKRU register.  As it stands, a too-large 'pkey' value would
be likely to silently overflow the u32 new_pkru_bits.

Add a check to look for overflows.  Also add a comment to remind
any future developer to closely examine the types used to store
pkey values if arch_max_pkey() ever changes.

This boots and passes the x86 pkey selftests.
Reported-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200122165346.AD4DA150@viggo.jf.intel.com

arch/x86/include/asm/pkeys.h

@@ -4,6 +4,11 @@
 
 #define ARCH_DEFAULT_PKEY	0
 
+/*
+ * If more than 16 keys are ever supported, a thorough audit
+ * will be necessary to ensure that the types that store key
+ * numbers and masks have sufficient capacity.
+ */
 #define arch_max_pkey() (boot_cpu_has(X86_FEATURE_OSPKE) ? 16 : 1)
 
 extern int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,

arch/x86/kernel/fpu/xstate.c

@@ -895,8 +895,6 @@ const void *get_xsave_field_ptr(int xfeature_nr)
 
 #ifdef CONFIG_ARCH_HAS_PKEYS
 
-#define NR_VALID_PKRU_BITS (CONFIG_NR_PROTECTION_KEYS * 2)
-#define PKRU_VALID_MASK (NR_VALID_PKRU_BITS - 1)
 /*
  * This will go out and modify PKRU register to set the access
  * rights for @pkey to @init_val.
@@ -915,6 +913,13 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
 	if (!boot_cpu_has(X86_FEATURE_OSPKE))
 		return -EINVAL;
 
+	/*
+	 * This code should only be called with valid 'pkey'
+	 * values originating from in-kernel users.  Complain
+	 * if a bad value is observed.
+	 */
+	WARN_ON_ONCE(pkey >= arch_max_pkey());
+
 	/* Set the bits we need in PKRU:  */
 	if (init_val & PKEY_DISABLE_ACCESS)
 		new_pkru_bits |= PKRU_AD_BIT;