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Subject[PATCH 02/10] mm: implement new pkey_mprotect() system call
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From: Dave Hansen <dave.hansen@linux.intel.com>

pkey_mprotect() 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.

int real_prot = PROT_READ|PROT_WRITE;
pkey = pkey_alloc(0, PKEY_DENY_ACCESS);
ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey);

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

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.

Semantically, we have a bit of a problem if we combine this
syscall with our previously-introduced execute-only support:
What do we do when we mix execute-only pkey use with
pkey_mprotect() use? For instance:

pkey_mprotect(ptr, PAGE_SIZE, PROT_WRITE, 6); // set pkey=6
mprotect(ptr, PAGE_SIZE, PROT_EXEC); // set pkey=X_ONLY_PKEY?
mprotect(ptr, PAGE_SIZE, PROT_WRITE); // is pkey=6 again?

To solve that, we make the plain-mprotect()-initiated execute-only
support only apply to VMAs that have the default protection key (0)
set on them.

Proposed semantics:
1. protection key 0 is special and represents the default,
"unassigned" protection key. It is always allocated.
2. mprotect() never affects a mapping's pkey_mprotect()-assigned
protection key. A protection key of 0 (even if set explicitly)
represents an unassigned protection key.
2a. mprotect(PROT_EXEC) on a mapping with an assigned protection
key may or may not result in a mapping with execute-only
properties. pkey_mprotect() plus pkey_set() on all threads
should be used to _guarantee_ execute-only semantics if this
is not a strong enough semantic.
3. mprotect(PROT_EXEC) may result in an "execute-only" mapping. The
kernel will internally attempt to allocate and dedicate a
protection key for the purpose of execute-only mappings. This
may not be possible in cases where there are no free protection
keys available. It can also happen, of course, in situations
where there is no hardware support for protection keys.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: linux-api@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: x86@kernel.org
Cc: torvalds@linux-foundation.org
Cc: akpm@linux-foundation.org
Cc: Arnd Bergmann <arnd@arndb.de>
---

b/arch/x86/include/asm/mmu_context.h | 15 ++++++++++-----
b/arch/x86/include/asm/pkeys.h | 11 +++++++++--
b/include/linux/pkeys.h | 12 ------------
b/mm/mprotect.c | 30 ++++++++++++++++++++++++++----
4 files changed, 45 insertions(+), 23 deletions(-)

diff -puN arch/x86/include/asm/mmu_context.h~pkeys-110-syscalls-mprotect_pkey arch/x86/include/asm/mmu_context.h
--- a/arch/x86/include/asm/mmu_context.h~pkeys-110-syscalls-mprotect_pkey 2016-07-29 09:18:56.332467653 -0700
+++ b/arch/x86/include/asm/mmu_context.h 2016-07-29 09:18:56.341468060 -0700
@@ -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>

@@ -195,16 +196,20 @@ static inline void arch_unmap(struct mm_
mpx_notify_unmap(mm, vma, start, end);
}

+#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
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;
- pkey = (vma->vm_flags & vma_pkey_mask) >> VM_PKEY_SHIFT;
-#endif
- return pkey;
+
+ return (vma->vm_flags & vma_pkey_mask) >> VM_PKEY_SHIFT;
+}
+#else
+static inline int vma_pkey(struct vm_area_struct *vma)
+{
+ return 0;
}
+#endif

static inline bool __pkru_allows_pkey(u16 pkey, bool write)
{
diff -puN arch/x86/include/asm/pkeys.h~pkeys-110-syscalls-mprotect_pkey arch/x86/include/asm/pkeys.h
--- a/arch/x86/include/asm/pkeys.h~pkeys-110-syscalls-mprotect_pkey 2016-07-29 09:18:56.334467743 -0700
+++ b/arch/x86/include/asm/pkeys.h 2016-07-29 09:18:56.342468105 -0700
@@ -1,7 +1,12 @@
#ifndef _ASM_X86_PKEYS_H
#define _ASM_X86_PKEYS_H

-#define arch_max_pkey() (boot_cpu_has(X86_FEATURE_OSPKE) ? 16 : 1)
+#define PKEY_DEDICATED_EXECUTE_ONLY 15
+/*
+ * Consider the PKEY_DEDICATED_EXECUTE_ONLY key unavailable.
+ */
+#define arch_max_pkey() (boot_cpu_has(X86_FEATURE_OSPKE) ? \
+ PKEY_DEDICATED_EXECUTE_ONLY : 1)

extern int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
unsigned long init_val);
@@ -10,7 +15,6 @@ extern int arch_set_user_pkey_access(str
* Try to dedicate one of the protection keys to be used as an
* execute-only protection key.
*/
-#define PKEY_DEDICATED_EXECUTE_ONLY 15
extern int __execute_only_pkey(struct mm_struct *mm);
static inline int execute_only_pkey(struct mm_struct *mm)
{
@@ -31,4 +35,7 @@ static inline int arch_override_mprotect
return __arch_override_mprotect_pkey(vma, prot, pkey);
}

+extern int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
+ unsigned long init_val);
+
#endif /*_ASM_X86_PKEYS_H */
diff -puN include/linux/pkeys.h~pkeys-110-syscalls-mprotect_pkey include/linux/pkeys.h
--- a/include/linux/pkeys.h~pkeys-110-syscalls-mprotect_pkey 2016-07-29 09:18:56.336467834 -0700
+++ b/include/linux/pkeys.h 2016-07-29 09:18:56.342468105 -0700
@@ -18,16 +18,4 @@
#define PKEY_DEDICATED_EXECUTE_ONLY 0
#endif /* ! CONFIG_ARCH_HAS_PKEYS */

-/*
- * This is called from mprotect_pkey().
- *
- * Returns true if the protection keys is valid.
- */
-static inline bool validate_pkey(int pkey)
-{
- if (pkey < 0)
- return false;
- return (pkey < arch_max_pkey());
-}
-
#endif /* _LINUX_PKEYS_H */
diff -puN mm/mprotect.c~pkeys-110-syscalls-mprotect_pkey mm/mprotect.c
--- a/mm/mprotect.c~pkeys-110-syscalls-mprotect_pkey 2016-07-29 09:18:56.338467924 -0700
+++ b/mm/mprotect.c 2016-07-29 09:18:56.342468105 -0700
@@ -352,8 +352,11 @@ 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 nstart, end, tmp, reqprot;
struct vm_area_struct *vma, *prev;
@@ -361,6 +364,12 @@ SYSCALL_DEFINE3(mprotect, unsigned long,
const int grows = prot & (PROT_GROWSDOWN|PROT_GROWSUP);
const bool rier = (current->personality & READ_IMPLIES_EXEC) &&
(prot & PROT_READ);
+ /*
+ * A temporary safety check since we are not validating
+ * the pkey before we introduce the allocation code.
+ */
+ if (pkey != -1)
+ return -EINVAL;

prot &= ~(PROT_GROWSDOWN|PROT_GROWSUP);
if (grows == (PROT_GROWSDOWN|PROT_GROWSUP)) /* can't be both */
@@ -409,7 +418,7 @@ SYSCALL_DEFINE3(mprotect, unsigned long,

for (nstart = start ; ; ) {
unsigned long newflags;
- int pkey = arch_override_mprotect_pkey(vma, prot, -1);
+ int new_vma_pkey;

/* Here we know that vma->vm_start <= nstart < vma->vm_end. */

@@ -417,7 +426,8 @@ SYSCALL_DEFINE3(mprotect, unsigned long,
if (rier && (vma->vm_flags & VM_MAYEXEC))
prot |= PROT_EXEC;

- newflags = calc_vm_prot_bits(prot, pkey);
+ 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 >> 4 shift VM_MAY% in place of VM_% */
@@ -454,3 +464,15 @@ 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)
+{
+ return do_mprotect_pkey(start, len, prot, pkey);
+}
_
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