[PATCH 09/13] x86/pkeys: Override pkey when moving away from PROT_EXEC

[PATCH 09/13] x86/pkeys: Override pkey when moving away from PROT_EXEC

[Dave Hansen]

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

I got a bug report that the following code (roughly) was
causing a SIGSEGV:

	mprotect(ptr, size, PROT_EXEC);
	mprotect(ptr, size, PROT_NONE);
	mprotect(ptr, size, PROT_READ);
	*ptr = 100;

The problem is hit when the mprotect(PROT_EXEC)
is implicitly assigned a protection key to the VMA, and made
that key ACCESS_DENY|WRITE_DENY.  The PROT_NONE mprotect()
failed to remove the protection key, and the PROT_NONE->
PROT_READ left the PTE usable, but the pkey still in place
and left the memory inaccessible.

To fix this, we ensure that we always "override" the pkee
at mprotect() if the VMA does not have execute-only
permissions, but the VMA has the execute-only pkey.

We had a check for PROT_READ/WRITE, but it did not work
for PROT_NONE.  This entirely removes the PROT_* checks,
which ensures that PROT_NONE now works.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Fixes: 62b5f7d013f ("mm/core, x86/mm/pkeys: Add execute-only protection keys support")
Reported-by: Shakeel Butt <shakeelb@google.com>
Cc: stable@vger.kernel.org
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Michael Ellermen <mpe@ellerman.id.au>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
---

 b/arch/x86/include/asm/pkeys.h |   12 +++++++++++-
 b/arch/x86/mm/pkeys.c          |   21 +++++++++++----------
 2 files changed, 22 insertions(+), 11 deletions(-)

diff -puN arch/x86/include/asm/pkeys.h~pkeys-abandon-exec-only-pkey-more-aggressively arch/x86/include/asm/pkeys.h
--- a/arch/x86/include/asm/pkeys.h~pkeys-abandon-exec-only-pkey-more-aggressively	2018-05-09 09:20:22.295698398 -0700
+++ b/arch/x86/include/asm/pkeys.h	2018-05-09 09:20:22.300698398 -0700
@@ -2,6 +2,8 @@
 #ifndef _ASM_X86_PKEYS_H
 #define _ASM_X86_PKEYS_H
 
+#define ARCH_DEFAULT_PKEY	0
+
 #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,
@@ -15,7 +17,7 @@ extern int __execute_only_pkey(struct mm
 static inline int execute_only_pkey(struct mm_struct *mm)
 {
 	if (!boot_cpu_has(X86_FEATURE_OSPKE))
-		return 0;
+		return ARCH_DEFAULT_PKEY;
 
 	return __execute_only_pkey(mm);
 }
@@ -56,6 +58,14 @@ bool mm_pkey_is_allocated(struct mm_stru
 		return false;
 	if (pkey >= arch_max_pkey())
 		return false;
+	/*
+	 * The exec-only pkey is set in the allocation map, but
+	 * is not available to any of the user interfaces like
+	 * mprotect_pkey().
+	 */
+	if (pkey == mm->context.execute_only_pkey)
+		return false;
+
 	return mm_pkey_allocation_map(mm) & (1U << pkey);
 }
 
diff -puN arch/x86/mm/pkeys.c~pkeys-abandon-exec-only-pkey-more-aggressively arch/x86/mm/pkeys.c
--- a/arch/x86/mm/pkeys.c~pkeys-abandon-exec-only-pkey-more-aggressively	2018-05-09 09:20:22.297698398 -0700
+++ b/arch/x86/mm/pkeys.c	2018-05-09 09:20:22.301698398 -0700
@@ -94,26 +94,27 @@ int __arch_override_mprotect_pkey(struct
 	 */
 	if (pkey != -1)
 		return pkey;
-	/*
-	 * Look for a protection-key-drive execute-only mapping
-	 * which is now being given permissions that are not
-	 * execute-only.  Move it back to the default pkey.
-	 */
-	if (vma_is_pkey_exec_only(vma) &&
-	    (prot & (PROT_READ|PROT_WRITE))) {
-		return 0;
-	}
+
 	/*
 	 * The mapping is execute-only.  Go try to get the
 	 * execute-only protection key.  If we fail to do that,
 	 * fall through as if we do not have execute-only
-	 * support.
+	 * support in this mm.
 	 */
 	if (prot == PROT_EXEC) {
 		pkey = execute_only_pkey(vma->vm_mm);
 		if (pkey > 0)
 			return pkey;
+	} else if (vma_is_pkey_exec_only(vma)) {
+		/*
+		 * Protections are *not* PROT_EXEC, but the mapping
+		 * is using the exec-only pkey.  This mapping was
+		 * PROT_EXEC and will no longer be.  Move back to
+		 * the default pkey.
+		 */
+		return ARCH_DEFAULT_PKEY;
 	}
+
 	/*
 	 * This is a vanilla, non-pkey mprotect (or we failed to
 	 * setup execute-only), inherit the pkey from the VMA we
_

[PATCH 13/13] x86/pkeys: Do not special case protection key 0

[Dave Hansen]

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

mm_pkey_is_allocated() treats pkey 0 as unallocated.  That is
inconsistent with the manpages, and also inconsistent with
mm->context.pkey_allocation_map.  Stop special casing it and only
disallow values that are actually bad (< 0).

The end-user visible effect of this is that you can now use
mprotect_pkey() to set pkey=0.

This is a bit nicer than what Ram proposed[1] because it is simpler
and removes special-casing for pkey 0.  On the other hand, it does
allow applications to pkey_free() pkey-0, but that's just a silly
thing to do, so we are not going to protect against it.

The scenario that could happen is similar to what happens if you free
any other pkey that is in use: it might get reallocated later and used
to protect some other data.  The most likely scenario is that pkey-0
comes back from pkey_alloc(), an access-disable or write-disable bit
is set in PKRU for it, and the next stack access will SIGSEGV.  It's
not horribly different from if you mprotect()'d your stack or heap to
be unreadable or unwritable, which is generally very foolish, but also
not explicitly prevented by the kernel.

1. http://lkml.kernel.org/r/1522112702-27853-1-git-send-email-linuxram@us.ibm.com

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Fixes: 58ab9a088dda ("x86/pkeys: Check against max pkey to avoid overflows")
Cc: stable@vger.kernel.org
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Michael Ellermen <mpe@ellerman.id.au>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>p
Cc: Shuah Khan <shuah@kernel.org>
---

 b/arch/x86/include/asm/mmu_context.h |    2 +-
 b/arch/x86/include/asm/pkeys.h       |    6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff -puN arch/x86/include/asm/mmu_context.h~x86-pkey-0-default-allocated arch/x86/include/asm/mmu_context.h
--- a/arch/x86/include/asm/mmu_context.h~x86-pkey-0-default-allocated	2018-05-09 09:20:24.362698393 -0700
+++ b/arch/x86/include/asm/mmu_context.h	2018-05-09 09:20:24.367698393 -0700
@@ -193,7 +193,7 @@ static inline int init_new_context(struc
 
 #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
 	if (cpu_feature_enabled(X86_FEATURE_OSPKE)) {
-		/* pkey 0 is the default and always allocated */
+		/* pkey 0 is the default and allocated implicitly */
 		mm->context.pkey_allocation_map = 0x1;
 		/* -1 means unallocated or invalid */
 		mm->context.execute_only_pkey = -1;
diff -puN arch/x86/include/asm/pkeys.h~x86-pkey-0-default-allocated arch/x86/include/asm/pkeys.h
--- a/arch/x86/include/asm/pkeys.h~x86-pkey-0-default-allocated	2018-05-09 09:20:24.364698393 -0700
+++ b/arch/x86/include/asm/pkeys.h	2018-05-09 09:20:24.367698393 -0700
@@ -51,10 +51,10 @@ bool mm_pkey_is_allocated(struct mm_stru
 {
 	/*
 	 * "Allocated" pkeys are those that have been returned
-	 * from pkey_alloc().  pkey 0 is special, and never
-	 * returned from pkey_alloc().
+	 * from pkey_alloc() or pkey 0 which is allocated
+	 * implicitly when the mm is created.
 	 */
-	if (pkey <= 0)
+	if (pkey < 0)
 		return false;
 	if (pkey >= arch_max_pkey())
 		return false;
_