Subject: Re: pkeys: Reserve PKEY_DISABLE_READ

From: Florian Weimer <[email-masked]>
Date: Thu, 29 Nov 2018 12:37:15 +0100

* Dave Hansen:

> On 11/27/18 3:57 AM, Florian Weimer wrote:
>> I would have expected something that translates PKEY_DISABLE_WRITE |
>> PKEY_DISABLE_READ into PKEY_DISABLE_ACCESS, and also accepts
>> PKEY_DISABLE_ACCESS | PKEY_DISABLE_READ, for consistency with POWER.
>> 
>> (My understanding is that PKEY_DISABLE_ACCESS does not disable all
>> access, but produces execute-only memory.)
>
> Correct, it disables all data access, but not execution.

So I would expect something like this (completely untested, I did not
even compile this):

diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
index 20ebf153c871..bed23f9e8336 100644
--- a/arch/powerpc/include/asm/pkeys.h
+++ b/arch/powerpc/include/asm/pkeys.h
@@ -199,6 +199,11 @@ static inline bool arch_pkeys_enabled(void)
 	return !static_branch_likely(&pkey_disabled);
 }
 
+static inline bool arch_pkey_access_rights_valid(unsigned long rights)
+{
+	return (rights & ~(unsigned long)PKEY_ACCESS_MASK) == 0;
+}
+
 extern void pkey_mm_init(struct mm_struct *mm);
 extern bool arch_supports_pkeys(int cap);
 extern unsigned int arch_usable_pkeys(void);
diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
index 19b137f1b3be..e3e1d5a316e8 100644
--- a/arch/x86/include/asm/pkeys.h
+++ b/arch/x86/include/asm/pkeys.h
@@ -14,6 +14,17 @@ static inline bool arch_pkeys_enabled(void)
 	return boot_cpu_has(X86_FEATURE_OSPKE);
 }
 
+static inline bool arch_pkey_access_rights_valid(unsigned long rights)
+{
+	if (rights & ~(unsigned long)PKEY_ACCESS_MASK)
+		return false;
+	if (rights & PKEY_DISABLE_READ) {
+		/* x86 can only disable read access along with write access. */
+		return rights & (PKEY_DISABLE_WRITE | PKEY_DISABLE_ACCESS);
+	}
+	return true;
+}
+
 /*
  * Try to dedicate one of the protection keys to be used as an
  * execute-only protection key.
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 87a57b7642d3..b9b78145017f 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -928,7 +928,13 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
 		return -EINVAL;
 
 	/* Set the bits we need in PKRU:  */
-	if (init_val & PKEY_DISABLE_ACCESS)
+	if (init_val & (PKEY_DISABLE_ACCESS | PKEY_DISABLE_READ))
+		/*
+		 * arch_pkey_access_rights_valid checked that
+		 * PKEY_DISABLE_READ is actually representable on x86
+		 * (that is, it comes with PKEY_DISABLE_ACCESS or
+		 * PKEY_DISABLE_WRITE).
+		 */
 		new_pkru_bits |= PKRU_AD_BIT;
 	if (init_val & PKEY_DISABLE_WRITE)
 		new_pkru_bits |= PKRU_WD_BIT;
diff --git a/include/linux/pkeys.h b/include/linux/pkeys.h
index 2955ba976048..2c330fabbe55 100644
--- a/include/linux/pkeys.h
+++ b/include/linux/pkeys.h
@@ -48,6 +48,11 @@ static inline void copy_init_pkru_to_fpregs(void)
 {
 }
 
+static inline bool arch_pkey_access_rights_valid(unsigned long rights)
+{
+	return false;
+}
+
 #endif /* ! CONFIG_ARCH_HAS_PKEYS */
 
 #endif /* _LINUX_PKEYS_H */
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 6d331620b9e5..f4cefc3540df 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -597,7 +597,7 @@ SYSCALL_DEFINE2(pkey_alloc, unsigned long, flags, unsigned long, init_val)
 	if (flags)
 		return -EINVAL;
 	/* check for unsupported init values */
-	if (init_val & ~PKEY_ACCESS_MASK)
+	if (!arch_pkey_access_rights_valid(init_val))
 		return -EINVAL;
 
 	down_write(&current->mm->mmap_sem);

Thanks,
Florian