x86/mm: Limit mmap() of /dev/mem to valid physical addresses

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 can be corrupted by setting reserved bits.

For example if you mmap() offset 0x0001000000000000 of /dev/mem on an
x86_64 system 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>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Toshi Kani <toshi.kani@hp.com>
Cc: dsafonov@virtuozzo.com
Cc: kirill.shutemov@linux.intel.com
Cc: mhocko@suse.com
Cc: oleg@redhat.com
Link: http://lkml.kernel.org/r/20171019192856.39672-1-craigb@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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

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);
+}