[PATCH v2 08/17] mm/x86: Allow special mappings with user access cleared

[Peter Zijlstra]

From: Peter Zijstra <peterz@xxxxxxxxxxxxx>

In order to create VMAs that are not accessible to userspace create a new
VM_NOUSER flag. This can be used in conjunction with
install_special_mapping() to inject 'kernel' data into the userspace map.

Similar to how arch_vm_get_page_prot() allows adding _PAGE_flags to
pgprot_t, introduce arch_vm_get_page_prot_excl() which masks
_PAGE_flags from pgprot_t and use this to implement VM_NOUSER for x86.

get_user_page() will allow things like FOLL_POPULATE but will fail
FOLL_GET (with or without FOLL_WRITE).

Signed-off-by: Peter Zijstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 arch/x86/include/uapi/asm/mman.h |    4 ++++
 include/linux/mm.h               |    2 ++
 include/linux/mman.h             |    4 ++++
 mm/mmap.c                        |   12 ++++++++++--
 4 files changed, 20 insertions(+), 2 deletions(-)

--- a/arch/x86/include/uapi/asm/mman.h
+++ b/arch/x86/include/uapi/asm/mman.h
@@ -26,6 +26,10 @@
 		((key) & 0x8 ? VM_PKEY_BIT3 : 0))
 #endif
 
+#define arch_vm_get_page_prot_excl(vm_flags) __pgprot(		\
+		((vm_flags) & VM_NOUSER ? _PAGE_USER : 0)	\
+		)
+
 #include <asm-generic/mman.h>
 
 #endif /* _ASM_X86_MMAN_H */
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -193,6 +193,7 @@ extern unsigned int kobjsize(const void
 #define VM_ARCH_1	0x01000000	/* Architecture-specific flag */
 #define VM_WIPEONFORK	0x02000000	/* Wipe VMA contents in child. */
 #define VM_DONTDUMP	0x04000000	/* Do not include in the core dump */
+#define VM_ARCH_0	0x08000000	/* Architecture-specific flag */
 
 #define VM_MIXEDMAP	0x10000000	/* Can contain "struct page" and pure PFN pages */
 #define VM_HUGEPAGE	0x20000000	/* MADV_HUGEPAGE marked this vma */
@@ -224,6 +225,7 @@ extern unsigned int kobjsize(const void
 #endif
 
 #if defined(CONFIG_X86)
+# define VM_NOUSER	VM_ARCH_0	/* Not accessible by userspace */
 # define VM_PAT		VM_ARCH_1	/* PAT reserves whole VMA at once (x86) */
 #if defined (CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS)
 # define VM_PKEY_SHIFT	VM_HIGH_ARCH_BIT_0
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -43,6 +43,10 @@ static inline void vm_unacct_memory(long
 #define arch_vm_get_page_prot(vm_flags) __pgprot(0)
 #endif
 
+#ifndef arch_vm_get_page_prot_excl
+#define arch_vm_get_page_prot_excl(vm_flags) __pgprot(0)
+#endif
+
 #ifndef arch_validate_prot
 /*
  * This is called from mprotect().  PROT_GROWSDOWN and PROT_GROWSUP have
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -102,9 +102,17 @@ pgprot_t protection_map[16] __ro_after_i
 
 pgprot_t vm_get_page_prot(unsigned long vm_flags)
 {
-	return __pgprot(pgprot_val(protection_map[vm_flags &
-				(VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]) |
+	pgprot_t prot;
+
+	prot = protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)];
+
+	prot = __pgprot(pgprot_val(prot) |
 			pgprot_val(arch_vm_get_page_prot(vm_flags)));
+
+	prot = __pgprot(pgprot_val(prot) &
+			~pgprot_val(arch_vm_get_page_prot_excl(vm_flags)));
+
+	return prot;
 }
 EXPORT_SYMBOL(vm_get_page_prot);