[PATCH] X86/mm limit mmap of /dev/mem to valid addrs.

[Craig Bergstrom <craigb@google.com>]

Currently, it is possible to mmap any offset from /dev/mem.  If a
program mmaps() /dev/mem offsets outside of the addressable limits
of a system, the page table is corrupted by setting some reserved
bits.

If you mmap offset 0x0001000000000000 of /dev/mem on an x86_64 with
a 48-bit bus, the page fault handler will be called with error_code
set to RSVD.  The kernel then crashes with a page table corruption
error.

This change prevents this page table corruption on x86 by refusing
to mmap offsets higher than the highest valid address in the system.

Signed-off-by: Craig Bergstrom <craigb@google.com>
---
 arch/x86/include/asm/io.h |  4 ++++
 arch/x86/mm/mmap.c        | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index c40a95c33bb8..322d25ae23ab 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -110,6 +110,10 @@ build_mmio_write(__writeq, "q", unsigned long, "r", )
 
 #endif
 
+#define ARCH_HAS_VALID_PHYS_ADDR_RANGE
+extern int valid_phys_addr_range(phys_addr_t addr, size_t size);
+extern int valid_mmap_phys_addr_range(unsigned long pfn, size_t size);
+
 /**
  *	virt_to_phys	-	map virtual addresses to physical
  *	@address: address to remap
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index a99679826846..320c6237e1d1 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -174,3 +174,15 @@ const char *arch_vma_name(struct vm_area_struct *vma)
 		return "[mpx]";
 	return NULL;
 }
+
+int valid_phys_addr_range(phys_addr_t addr, size_t count)
+{
+	return addr + count <= __pa(high_memory);
+}
+
+int valid_mmap_phys_addr_range(unsigned long pfn, size_t count)
+{
+	phys_addr_t addr = (phys_addr_t)pfn << PAGE_SHIFT;
+
+	return valid_phys_addr_range(addr, count);
+}
-- 
2.15.0.rc1.287.g2b38de12cc-goog