[PATCH 03/10] x86/pkeys: Make mprotect_key() mask off additional vm_flags

[Tim Gardner]

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

BugLink: http://bugs.launchpad.net/bugs/1591804

Today, mprotect() takes 4 bits of data: PROT_READ/WRITE/EXEC/NONE.
Three of those bits: READ/WRITE/EXEC get translated directly in to
vma->vm_flags by calc_vm_prot_bits().  If a bit is unset in
mprotect()'s 'prot' argument then it must be cleared in vma->vm_flags
during the mprotect() call.

We do this clearing today by first calculating the VMA flags we
want set, then clearing the ones we do not want to inherit from
the original VMA:

	vm_flags = calc_vm_prot_bits(prot, key);
	...
	newflags = vm_flags;
	newflags |= (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));

However, we *also* want to mask off the original VMA's vm_flags in
which we store the protection key.

To do that, this patch adds a new macro:

	ARCH_VM_PKEY_FLAGS

which allows the architecture to specify additional bits that it would
like cleared.  We use that to ensure that the VM_PKEY_BIT* bits get
cleared.

Signed-off-by: Dave Hansen <dave.hansen at linux.intel.com>
Acked-by: Mel Gorman <mgorman at techsingularity.net>
Reviewed-by: Thomas Gleixner <tglx at linutronix.de>
Cc: linux-arch at vger.kernel.org
Cc: Dave Hansen <dave at sr71.net>
Cc: arnd at arndb.de
Cc: linux-api at vger.kernel.org
Cc: linux-mm at kvack.org
Cc: luto at kernel.org
Cc: akpm at linux-foundation.org
Cc: torvalds at linux-foundation.org
Link: http://lkml.kernel.org/r/20160729163013.E48D6981@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>

(cherry picked from commit a8502b67d739c1d7a4542c1da0a5d98a6a58c177)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 arch/x86/include/asm/pkeys.h |  2 ++
 include/linux/pkeys.h        |  1 +
 mm/mprotect.c                | 11 ++++++++++-
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
index 33777c2..666ffc8 100644
--- a/arch/x86/include/asm/pkeys.h
+++ b/arch/x86/include/asm/pkeys.h
@@ -38,4 +38,6 @@ static inline int arch_override_mprotect_pkey(struct vm_area_struct *vma,
 extern int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
 		unsigned long init_val);
 
+#define ARCH_VM_PKEY_FLAGS (VM_PKEY_BIT0 | VM_PKEY_BIT1 | VM_PKEY_BIT2 | VM_PKEY_BIT3)
+
 #endif /*_ASM_X86_PKEYS_H */
diff --git a/include/linux/pkeys.h b/include/linux/pkeys.h
index 0030b40..6899b0b 100644
--- a/include/linux/pkeys.h
+++ b/include/linux/pkeys.h
@@ -16,6 +16,7 @@
 #define execute_only_pkey(mm) (0)
 #define arch_override_mprotect_pkey(vma, prot, pkey) (0)
 #define PKEY_DEDICATED_EXECUTE_ONLY 0
+#define ARCH_VM_PKEY_FLAGS 0
 #endif /* ! CONFIG_ARCH_HAS_PKEYS */
 
 #endif /* _LINUX_PKEYS_H */
diff --git a/mm/mprotect.c b/mm/mprotect.c
index dd3f40a..abd9c82 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -417,6 +417,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 		prev = vma;
 
 	for (nstart = start ; ; ) {
+		unsigned long mask_off_old_flags;
 		unsigned long newflags;
 		int new_vma_pkey;
 
@@ -426,9 +427,17 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 		if (rier && (vma->vm_flags & VM_MAYEXEC))
 			prot |= PROT_EXEC;
 
+		/*
+		 * Each mprotect() call explicitly passes r/w/x permissions.
+		 * If a permission is not passed to mprotect(), it must be
+		 * cleared from the VMA.
+		 */
+		mask_off_old_flags = VM_READ | VM_WRITE | VM_EXEC |
+					ARCH_VM_PKEY_FLAGS;
+
 		new_vma_pkey = arch_override_mprotect_pkey(vma, prot, pkey);
 		newflags = calc_vm_prot_bits(prot, new_vma_pkey);
-		newflags |= (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));
+		newflags |= (vma->vm_flags & ~mask_off_old_flags);
 
 		/* newflags >> 4 shift VM_MAY% in place of VM_% */
 		if ((newflags & ~(newflags >> 4)) & (VM_READ | VM_WRITE | VM_EXEC)) {
-- 
2.7.4