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

One thing /dev/mem access APIs should verify is that there's no way
that excessively large pfn's can leak into the high bits of the
page table entry.

In particular, if people can use "very large physical page addresses"
through /dev/mem to set the bits past bit 58 - SOFTW4 and permission
key bits and NX bit, that could *really* confuse the kernel.

We had an earlier attempt:

  ce56a86e2ade ("x86/mm: Limit mmap() of /dev/mem to valid physical addresses")

... which turned out to be too restrictive (breaking mem=... bootups for example) and
had to be reverted in:

  90edaac62729 ("Revert "x86/mm: Limit mmap() of /dev/mem to valid physical addresses"")

This v2 attempt modifies the original patch and makes sure that mmap(/dev/mem)
limits the pfns so that it at least fits in the actual pteval_t architecturally:

 - Make sure mmap_mem() actually validates that the offset fits in phys_addr_t

    ( This may be indirectly true due to some other check, but it's not
      entirely obvious. )

 - Change valid_mmap_phys_addr_range() to just use phys_addr_valid()
   on the top byte

    ( Top byte is sufficient, because mmap_mem() has already checked that
      it cannot wrap. )

 - Add a few comments about what the valid_phys_addr_range() vs.
   valid_mmap_phys_addr_range() difference is.

Signed-off-by: Craig Bergstrom <>
[ Fixed the checks and added comments. ]
Signed-off-by: Linus Torvalds <>
[ Collected the discussion and patches into a commit. ]
Cc: Boris Ostrovsky <>
Cc: Fengguang Wu <>
Cc: Greg Kroah-Hartman <>
Cc: Hans Verkuil <>
Cc: Mauro Carvalho Chehab <>
Cc: Peter Zijlstra <>
Cc: Sander Eikelenboom <>
Cc: Sean Young <>
Cc: Thomas Gleixner <>
Signed-off-by: Ingo Molnar <>
(cherry picked from commit be62a32044061cb4a3b70a10598e093f1319102e)
Signed-off-by: Daniel Kurtz <>
[djkurtz: Ignoring context diff due to chromeos-4.14 lacking:
 1e0f25dbf246 x86/mm: Prevent non-MAP_FIXED mapping across DEFAULT_MAP_WINDOW border

TEST=pagemash (from b:66966804)

Change-Id: I43ead5c50a154c5430736a43b1cb04461dda75be
Commit-Ready: Daniel Kurtz <>
Tested-by: Daniel Kurtz <>
Reviewed-by: Craig Bergstrom <>
diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index 11398d5..efb6c17 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -111,6 +111,10 @@
+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 a996798..269369d 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -33,6 +33,8 @@
 #include <linux/compat.h>
 #include <asm/elf.h>
+#include "physaddr.h"
 struct va_alignment __read_mostly va_align = {
 	.flags = -1,
@@ -174,3 +176,17 @@
 		return "[mpx]";
 	return NULL;
+/* Can we access it for direct reading/writing? Must be RAM: */
+int valid_phys_addr_range(phys_addr_t addr, size_t count)
+	return addr + count <= __pa(high_memory);
+/* Can we access it through mmap? Must be a valid physical address: */
+int valid_mmap_phys_addr_range(unsigned long pfn, size_t count)
+	phys_addr_t addr = (phys_addr_t)pfn << PAGE_SHIFT;
+	return phys_addr_valid(addr + count - 1);
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index cc855d3..3475eac 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -344,6 +344,10 @@
 	size_t size = vma->vm_end - vma->vm_start;
 	phys_addr_t offset = (phys_addr_t)vma->vm_pgoff << PAGE_SHIFT;
+	/* Does it even fit in phys_addr_t? */
+	if (offset >> PAGE_SHIFT != vma->vm_pgoff)
+		return -EINVAL;
 	/* It's illegal to wrap around the end of the physical address space. */
 	if (offset + (phys_addr_t)size - 1 < offset)
 		return -EINVAL;