[tip:x86/urgent] x86/pkeys: Override pkey when moving away from PROT_EXEC

[tip-bot for Dave Hansen]

Commit-ID:  0a0b152083cfc44ec1bb599b57b7aab41327f998
Gitweb:     https://git.kernel.org/tip/0a0b152083cfc44ec1bb599b57b7aab41327f998
Author:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate: Wed, 9 May 2018 10:13:51 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 14 May 2018 11:14:45 +0200

x86/pkeys: Override pkey when moving away from PROT_EXEC

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.

Reported-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Michael Ellermen <mpe@xxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ram Pai <linuxram@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 62b5f7d013f ("mm/core, x86/mm/pkeys: Add execute-only protection keys support")
Link: http://lkml.kernel.org/r/20180509171351.084C5A71@xxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/include/asm/pkeys.h | 12 +++++++++++-
 arch/x86/mm/pkeys.c          | 21 +++++++++++----------
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
index a0ba1ffda0df..39cd292805a9 100644
--- a/arch/x86/include/asm/pkeys.h
+++ b/arch/x86/include/asm/pkeys.h
@@ -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_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_struct *mm, int pkey)
 		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 --git a/arch/x86/mm/pkeys.c b/arch/x86/mm/pkeys.c
index d7bc0eea20a5..6e98e0a7c923 100644
--- a/arch/x86/mm/pkeys.c
+++ b/arch/x86/mm/pkeys.c
@@ -94,26 +94,27 @@ int __arch_override_mprotect_pkey(struct vm_area_struct *vma, int prot, int pkey
 	 */
 	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