diff --git a/arch/ia64/kernel/efi.c b/arch/ia64/kernel/efi.c
index f45f91d38cab..78d29b79947d 100644
--- a/arch/ia64/kernel/efi.c
+++ b/arch/ia64/kernel/efi.c
@@ -660,6 +660,29 @@ efi_memory_descriptor (unsigned long phys_addr)
 	return NULL;
 }
 
+static int
+efi_memmap_intersects (unsigned long phys_addr, unsigned long size)
+{
+	void *efi_map_start, *efi_map_end, *p;
+	efi_memory_desc_t *md;
+	u64 efi_desc_size;
+	unsigned long end;
+
+	efi_map_start = __va(ia64_boot_param->efi_memmap);
+	efi_map_end   = efi_map_start + ia64_boot_param->efi_memmap_size;
+	efi_desc_size = ia64_boot_param->efi_memdesc_size;
+
+	end = phys_addr + size;
+
+	for (p = efi_map_start; p < efi_map_end; p += efi_desc_size) {
+		md = p;
+
+		if (md->phys_addr < end && efi_md_end(md) > phys_addr)
+			return 1;
+	}
+	return 0;
+}
+
 u32
 efi_mem_type (unsigned long phys_addr)
 {
@@ -766,11 +789,28 @@ valid_phys_addr_range (unsigned long phys_addr, unsigned long size)
 int
 valid_mmap_phys_addr_range (unsigned long pfn, unsigned long size)
 {
+	unsigned long phys_addr = pfn << PAGE_SHIFT;
+	u64 attr;
+
+	attr = efi_mem_attribute(phys_addr, size);
+
 	/*
-	 * MMIO regions are often missing from the EFI memory map.
-	 * We must allow mmap of them for programs like X, so we
-	 * currently can't do any useful validation.
+	 * /dev/mem mmap uses normal user pages, so we don't need the entire
+	 * granule, but the entire region we're mapping must support the same
+	 * attribute.
 	 */
+	if (attr & EFI_MEMORY_WB || attr & EFI_MEMORY_UC)
+		return 1;
+
+	/*
+	 * Intel firmware doesn't tell us about all the MMIO regions, so
+	 * in general we have to allow mmap requests.  But if EFI *does*
+	 * tell us about anything inside this region, we should deny it.
+	 * The user can always map a smaller region to avoid the overlap.
+	 */
+	if (efi_memmap_intersects(phys_addr, size))
+		return 0;
+
 	return 1;
 }