[PATCH 26/34] mm: implement new mprotect_key() system call

[Dave Hansen <dave@sr71.net>]

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

mprotect_key() is just like mprotect, except it also takes a
protection key as an argument.  On systems that do not support
protection keys, it still works, but requires that key=0.
Otherwise it does exactly what mprotect does.

I expect it to get used like this, if you want to guarantee that
any mapping you create can *never* be accessed without the right
protection keys set up.

	pkey_deny_access(11); // random pkey
	int real_prot = PROT_READ|PROT_WRITE;
	ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
	ret = mprotect_key(ptr, PAGE_SIZE, real_prot, 11);

This way, there is *no* window where the mapping is accessible
since it was always either PROT_NONE or had a protection key set.

We settled on 'unsigned long' for the type of the key here.  We
only need 4 bits on x86 today, but I figured that other
architectures might need some more space.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-api@vger.kernel.org
---

 b/arch/x86/include/asm/mmu_context.h |   10 +++++++--
 b/include/linux/pkeys.h              |    7 +++++-
 b/mm/Kconfig                         |    7 ++++++
 b/mm/mprotect.c                      |   36 +++++++++++++++++++++++++++++------
 4 files changed, 51 insertions(+), 9 deletions(-)

diff -puN arch/x86/include/asm/mmu_context.h~pkeys-85-mprotect_pkey arch/x86/include/asm/mmu_context.h
--- a/arch/x86/include/asm/mmu_context.h~pkeys-85-mprotect_pkey	2015-12-03 16:21:30.181877894 -0800
+++ b/arch/x86/include/asm/mmu_context.h	2015-12-03 16:21:30.190878302 -0800
@@ -4,6 +4,7 @@
 #include <asm/desc.h>
 #include <linux/atomic.h>
 #include <linux/mm_types.h>
+#include <linux/pkeys.h>
 
 #include <trace/events/tlb.h>
 
@@ -243,10 +244,14 @@ static inline void arch_unmap(struct mm_
 		mpx_notify_unmap(mm, vma, start, end);
 }
 
+#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
+/*
+ * If the config option is off, we get the generic version from
+ * include/linux/pkeys.h.
+ */
 static inline int vma_pkey(struct vm_area_struct *vma)
 {
 	u16 pkey = 0;
-#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
 	unsigned long vma_pkey_mask = VM_PKEY_BIT0 | VM_PKEY_BIT1 |
 				      VM_PKEY_BIT2 | VM_PKEY_BIT3;
 	/*
@@ -259,9 +264,10 @@ static inline int vma_pkey(struct vm_are
 	 */
 	pkey = (vma->vm_flags >> vm_pkey_shift) &
 	       (vma_pkey_mask >> vm_pkey_shift);
-#endif
+
 	return pkey;
 }
+#endif
 
 static inline bool __pkru_allows_pkey(u16 pkey, bool write)
 {
diff -puN include/linux/pkeys.h~pkeys-85-mprotect_pkey include/linux/pkeys.h
--- a/include/linux/pkeys.h~pkeys-85-mprotect_pkey	2015-12-03 16:21:30.183877985 -0800
+++ b/include/linux/pkeys.h	2015-12-03 16:21:30.190878302 -0800
@@ -2,10 +2,10 @@
 #define _LINUX_PKEYS_H
 
 #include <linux/mm_types.h>
-#include <asm/mmu_context.h>
 
 #ifdef CONFIG_ARCH_HAS_PKEYS
 #include <asm/pkeys.h>
+#include <asm/mmu_context.h>
 #else /* ! CONFIG_ARCH_HAS_PKEYS */
 
 /*
@@ -17,6 +17,11 @@ static inline bool arch_validate_pkey(in
 {
 	return true;
 }
+
+static inline int vma_pkey(struct vm_area_struct *vma)
+{
+	return 0;
+}
 #endif /* ! CONFIG_ARCH_HAS_PKEYS */
 
 #endif /* _LINUX_PKEYS_H */
diff -puN mm/Kconfig~pkeys-85-mprotect_pkey mm/Kconfig
--- a/mm/Kconfig~pkeys-85-mprotect_pkey	2015-12-03 16:21:30.185878075 -0800
+++ b/mm/Kconfig	2015-12-03 16:21:30.190878302 -0800
@@ -673,3 +673,10 @@ config ARCH_USES_HIGH_VMA_FLAGS
 	bool
 config ARCH_HAS_PKEYS
 	bool
+
+config NR_PROTECTION_KEYS
+	int
+	# Everything supports a _single_ key, so allow folks to
+	# at least call APIs that take keys, but require that the
+	# key be 0.
+	default 1
diff -puN mm/mprotect.c~pkeys-85-mprotect_pkey mm/mprotect.c
--- a/mm/mprotect.c~pkeys-85-mprotect_pkey	2015-12-03 16:21:30.186878121 -0800
+++ b/mm/mprotect.c	2015-12-03 16:21:30.191878347 -0800
@@ -24,6 +24,7 @@
 #include <linux/migrate.h>
 #include <linux/perf_event.h>
 #include <linux/ksm.h>
+#include <linux/pkeys.h>
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
 #include <asm/cacheflush.h>
@@ -344,10 +345,13 @@ fail:
 	return error;
 }
 
-SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
-		unsigned long, prot)
+/*
+ * pkey=-1 when doing a legacy mprotect()
+ */
+static int do_mprotect_pkey(unsigned long start, size_t len,
+		unsigned long prot, int pkey)
 {
-	unsigned long vm_flags, nstart, end, tmp, reqprot;
+	unsigned long nstart, end, tmp, reqprot;
 	struct vm_area_struct *vma, *prev;
 	int error = -EINVAL;
 	const int grows = prot & (PROT_GROWSDOWN|PROT_GROWSUP);
@@ -373,8 +377,6 @@ SYSCALL_DEFINE3(mprotect, unsigned long,
 	if ((prot & PROT_READ) && (current->personality & READ_IMPLIES_EXEC))
 		prot |= PROT_EXEC;
 
-	vm_flags = calc_vm_prot_bits(prot, 0);
-
 	down_write(&current->mm->mmap_sem);
 
 	vma = find_vma(current->mm, start);
@@ -407,7 +409,14 @@ SYSCALL_DEFINE3(mprotect, unsigned long,
 
 		/* Here we know that vma->vm_start <= nstart < vma->vm_end. */
 
-		newflags = vm_flags;
+		/*
+		 * If this is a vanilla, non-pkey mprotect, inherit the
+		 * pkey from the VMA we are working on.
+		 */
+		if (pkey == -1)
+			newflags = calc_vm_prot_bits(prot, vma_pkey(vma));
+		else
+			newflags = calc_vm_prot_bits(prot, pkey);
 		newflags |= (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));
 
 		/* newflags >> 4 shift VM_MAY% in place of VM_% */
@@ -443,3 +452,18 @@ out:
 	up_write(&current->mm->mmap_sem);
 	return error;
 }
+
+SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
+		unsigned long, prot)
+{
+	return do_mprotect_pkey(start, len, prot, -1);
+}
+
+SYSCALL_DEFINE4(pkey_mprotect, unsigned long, start, size_t, len,
+		unsigned long, prot, int, pkey)
+{
+	if (!arch_validate_pkey(pkey))
+		return -EINVAL;
+
+	return do_mprotect_pkey(start, len, prot, pkey);
+}
_