x86/mm/pkeys: Optimize fault handling in access_error()

We might not strictly have to make modifictions to
access_error() to check the VMA here.

If we do not, we will do this:

 1. app sets VMA pkey to K
 2. app touches a !present page
 3. do_page_fault(), allocates and maps page, sets pte.pkey=K
 4. return to userspace
 5. touch instruction reexecutes, but triggers PF_PK
 6. do PKEY signal

What happens with this patch applied:

 1. app sets VMA pkey to K
 2. app touches a !present page
 3. do_page_fault() notices that K is inaccessible
 4. do PKEY signal

We basically skip the fault that does an allocation.

So what this lets us do is protect areas from even being
*populated* unless it is accessible according to protection
keys.  That seems handy to me and makes protection keys work
more like an mprotect()'d mapping.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20160212210222.EBB63D8C@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/fault.c

@@ -900,10 +900,16 @@ bad_area(struct pt_regs *regs, unsigned long error_code, unsigned long address)
 static inline bool bad_area_access_from_pkeys(unsigned long error_code,
 		struct vm_area_struct *vma)
 {
+	/* This code is always called on the current mm */
+	bool foreign = false;
+
 	if (!boot_cpu_has(X86_FEATURE_OSPKE))
 		return false;
 	if (error_code & PF_PK)
 		return true;
+	/* this checks permission keys on the VMA: */
+	if (!arch_vma_access_permitted(vma, (error_code & PF_WRITE), foreign))
+		return true;
 	return false;
 }
 
@@ -1091,6 +1097,8 @@ int show_unhandled_signals = 1;
 static inline int
 access_error(unsigned long error_code, struct vm_area_struct *vma)
 {
+	/* This is only called for the current mm, so: */
+	bool foreign = false;
 	/*
 	 * Access or read was blocked by protection keys. We do
 	 * this check before any others because we do not want
@@ -1099,6 +1107,13 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
 	 */
 	if (error_code & PF_PK)
 		return 1;
+	/*
+	 * Make sure to check the VMA so that we do not perform
+	 * faults just to hit a PF_PK as soon as we fill in a
+	 * page.
+	 */
+	if (!arch_vma_access_permitted(vma, (error_code & PF_WRITE), foreign))
+		return 1;
 
 	if (error_code & PF_WRITE) {
 		/* write, present and write, not present: */