[RFC][PATCH 2/7] mm: implement new pkey_mprotect() system call

From: Dave Hansen
Date: Mon Feb 22 2016 - 20:12:00 EST


From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

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.

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.
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.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: linux-api@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: x86@xxxxxxxxxx
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx
Cc: akpm@xxxxxxxxxxxxxxxxxxxx
---

b/arch/x86/include/asm/mmu_context.h | 15 ++++++++++-----
b/arch/x86/include/asm/pkeys.h | 11 +++++++++--
b/arch/x86/kernel/fpu/xstate.c | 15 ++++++++++++++-
b/arch/x86/mm/pkeys.c | 2 +-
b/mm/mprotect.c | 27 +++++++++++++++++++++++----
5 files changed, 57 insertions(+), 13 deletions(-)

diff -puN arch/x86/include/asm/mmu_context.h~pkeys-85-syscalls-mprotect_pkey arch/x86/include/asm/mmu_context.h
--- a/arch/x86/include/asm/mmu_context.h~pkeys-85-syscalls-mprotect_pkey 2016-02-22 17:09:23.217290781 -0800
+++ b/arch/x86/include/asm/mmu_context.h 2016-02-22 17:09:23.228291282 -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>

@@ -286,16 +287,20 @@ 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-85-syscalls-mprotect_pkey arch/x86/include/asm/pkeys.h
--- a/arch/x86/include/asm/pkeys.h~pkeys-85-syscalls-mprotect_pkey 2016-02-22 17:09:23.219290872 -0800
+++ b/arch/x86/include/asm/pkeys.h 2016-02-22 17:09:23.228291282 -0800
@@ -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 arch/x86/kernel/fpu/xstate.c~pkeys-85-syscalls-mprotect_pkey arch/x86/kernel/fpu/xstate.c
--- a/arch/x86/kernel/fpu/xstate.c~pkeys-85-syscalls-mprotect_pkey 2016-02-22 17:09:23.221290963 -0800
+++ b/arch/x86/kernel/fpu/xstate.c 2016-02-22 17:09:23.228291282 -0800
@@ -868,7 +868,7 @@ out:
* not modfiy PKRU *itself* here, only the XSAVE state that will
* be restored in to PKRU when we return back to userspace.
*/
-int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
+int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
unsigned long init_val)
{
struct xregs_state *xsave = &tsk->thread.fpu.state.xsave;
@@ -927,3 +927,16 @@ int arch_set_user_pkey_access(struct tas

return 0;
}
+
+/*
+ * When setting a userspace-provided value, we need to ensure
+ * that it is valid. The __ version can get used by
+ * kernel-internal uses like the execute-only support.
+ */
+int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
+ unsigned long init_val)
+{
+ if (!validate_pkey(pkey))
+ return -EINVAL;
+ return __arch_set_user_pkey_access(tsk, pkey, init_val);
+}
diff -puN arch/x86/mm/pkeys.c~pkeys-85-syscalls-mprotect_pkey arch/x86/mm/pkeys.c
--- a/arch/x86/mm/pkeys.c~pkeys-85-syscalls-mprotect_pkey 2016-02-22 17:09:23.222291008 -0800
+++ b/arch/x86/mm/pkeys.c 2016-02-22 17:09:23.229291327 -0800
@@ -38,7 +38,7 @@ int __execute_only_pkey(struct mm_struct
return PKEY_DEDICATED_EXECUTE_ONLY;
}
preempt_enable();
- ret = arch_set_user_pkey_access(current, PKEY_DEDICATED_EXECUTE_ONLY,
+ ret = __arch_set_user_pkey_access(current, PKEY_DEDICATED_EXECUTE_ONLY,
PKEY_DISABLE_ACCESS);
/*
* If the PKRU-set operation failed somehow, just return
diff -puN mm/mprotect.c~pkeys-85-syscalls-mprotect_pkey mm/mprotect.c
--- a/mm/mprotect.c~pkeys-85-syscalls-mprotect_pkey 2016-02-22 17:09:23.224291100 -0800
+++ b/mm/mprotect.c 2016-02-22 17:09:23.229291327 -0800
@@ -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;
@@ -410,11 +413,12 @@ SYSCALL_DEFINE3(mprotect, unsigned long,

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

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

- newflags = calc_vm_prot_bits(prot, pkey);
+ vma_pkey = arch_override_mprotect_pkey(vma, prot, pkey);
+ newflags = calc_vm_prot_bits(prot, vma_pkey);
newflags |= (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));

/* newflags >> 4 shift VM_MAY% in place of VM_% */
@@ -450,3 +454,18 @@ 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 (!validate_pkey(pkey))
+ return -EINVAL;
+
+ return do_mprotect_pkey(start, len, prot, pkey);
+}
_