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

Manoj Iyer manoj.iyer at canonical.com
Thu Sep 14 19:24:00 UTC 2017

From: Ard Biesheuvel <ard.biesheuvel at linaro.org>

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.

Reported-by: Alexander Graf <agraf at suse.de>
Fixes: 4dffbfc48d65 ("arm64/efi: mark UEFI reserved regions as MEMBLOCK_NOMAP")

BugLink: https://launchpad.net/bugs/1716483

Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
Signed-off-by: Will Deacon <will.deacon at arm.com>
(cherry picked from commit 1151f838cb626005f4d69bf675dacaaa5ea909d6)
Signed-off-by: Manoj Iyer <manoj.iyer at canonical.com>
 arch/arm64/mm/mmap.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 01c171723bb3..caf75abf6ae7 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -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_struct *mm)
 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);

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