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Subject[patch 05/16] mm: Allow special mappings with user access cleared
From: Peter Zijstra <peterz@infradead.org>

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.

Signed-off-by: Peter Zijstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
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);


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