Patch "arm64: kernel: restrict /dev/mem read() calls to linear region" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

arm64: kernel: restrict /dev/mem read() calls to linear region

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm64-kernel-restrict-dev-mem-read-calls-to-linear-region.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ard Biesheuvel <[email protected]>
Date: Fri, 19 May 2017 16:42:00 +0100
Subject: arm64: kernel: restrict /dev/mem read() calls to linear region

From: Ard Biesheuvel <[email protected]>

[ Upstream commit 1151f838cb626005f4d69bf675dacaaa5ea909d6 ]

When running lscpu on an AArch64 system that has SMBIOS version 2.0 tables, it will segfault in the following way:

  Unable to handle kernel paging request at virtual address ffff8000bfff0000
  pgd = ffff8000f9615000
  [ffff8000bfff0000] *pgd=0000000000000000
  Internal error: Oops: 96000007 [#1] PREEMPT SMP
  Modules linked in:
  CPU: 0 PID: 1284 Comm: lscpu Not tainted 4.11.0-rc3+ #103
  Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
  task: ffff8000fa78e800 task.stack: ffff8000f9780000
  PC is at __arch_copy_to_user+0x90/0x220
  LR is at read_mem+0xcc/0x140

This is caused by the fact that lspci issues a read() on /dev/mem at the offset where it expects to find the SMBIOS structure array. However, this region is classified as EFI_RUNTIME_SERVICE_DATA (as per the UEFI spec), and so it is omitted from the linear mapping.

So let's restrict /dev/mem read/write access to those areas that are covered by the linear region.

---
 arch/arm64/mm/mmap.c |   19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

@@ -18,6 +18,7 @@
 
 #include <linux/elf.h>
 #include <linux/fs.h>
+#include <linux/memblock.h>
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/export.h>

@@ -102,12 +103,18 @@ void arch_pick_mmap_layout(struct mm_str
  */
 int valid_phys_addr_range(phys_addr_t addr, size_t size)
 {
-	if (addr < PHYS_OFFSET)
-		return 0;
-	if (addr + size > __pa(high_memory - 1) + 1)
-		return 0;
-
-	return 1;
+	/*
+	 * Check whether addr is covered by a memory region without the
+	 * MEMBLOCK_NOMAP attribute, and whether that region covers the
+	 * entire range. In theory, this could lead to false negatives
+	 * if the range is covered by distinct but adjacent memory regions
+	 * that only differ in other attributes. However, few of such
+	 * attributes have been defined, and it is debatable whether it
+	 * follows that /dev/mem read() calls should be able traverse
+	 * such boundaries.
+	 */
+	return memblock_is_region_memory(addr, size) &&
+	       memblock_is_map_memory(addr);
 }
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/x86-efi-disable-runtime-services-on-kexec-kernel-if-booted-with-efi-old_map.patch queue-4.9/arm64-kernel-restrict-dev-mem-read-calls-to-linear-region.patch