ARM: move vector stubs

Move the machine vector stubs into the page above the vector page,
which we can prevent from being visible to userspace.  Also move
the reset stub, and place the swi vector at a location that the
'ldr' can get to it.

This hides pointers into the kernel which could give valuable
information to attackers, and reduces the number of exploitable
instructions at a fixed address.

Cc: <stable@vger.kernel.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

4 files changed, 37 insertions, 30 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ba412e02ec0c..123b7924904f 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -218,7 +218,8 @@ config VECTORS_BASE
         default DRAM_BASE if REMAP_VECTORS_TO_RAM
         default 0x00000000
         help
-          The base address of exception vectors.
+          The base address of exception vectors.  This must be two pages
+          in size.
 
 config ARM_PATCH_PHYS_VIRT
         bool "Patch physical to virtual translations at runtime" if EMBEDDED
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 02437345289a..79a41fa978f5 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -944,9 +944,9 @@ __kuser_helper_end:
 /*
  * Vector stubs.
  *
- * This code is copied to 0xffff0200 so we can use branches in the
- * vectors, rather than ldr's.  Note that this code must not
- * exceed 0x300 bytes.
+ * This code is copied to 0xffff1000 so we can use branches in the
+ * vectors, rather than ldr's.  Note that this code must not exceed
+ * a page size.
  *
  * Common stub entry macro:
  *   Enter in IRQ mode, spsr = SVC/USR CPSR, lr = SVC/USR PC
@@ -995,6 +995,15 @@ ENDPROC(vector_\name)
 
         .globl  __stubs_start
 __stubs_start:
+        @ This must be the first word
+        .word   vector_swi
+
+vector_rst:
+ ARM(   swi     SYS_ERROR0      )
+ THUMB( svc     #0              )
+ THUMB( nop                     )
+        b       vector_und
+
 /*
  * Interrupt dispatcher
  */
@@ -1089,6 +1098,16 @@ __stubs_start:
         .align  5
 
 /*=============================================================================
+ * Address exception handler
+ *-----------------------------------------------------------------------------
+ * These aren't too critical.
+ * (they're not supposed to happen, and won't happen in 32-bit data mode).
+ */
+
+vector_addrexcptn:
+        b       vector_addrexcptn
+
+/*=============================================================================
  * Undefined FIQs
  *-----------------------------------------------------------------------------
  * Enter in FIQ mode, spsr = ANY CPSR, lr = ANY PC
@@ -1101,35 +1120,14 @@ __stubs_start:
 vector_fiq:
         subs    pc, lr, #4
 
-/*=============================================================================
- * Address exception handler
- *-----------------------------------------------------------------------------
- * These aren't too critical.
- * (they're not supposed to happen, and won't happen in 32-bit data mode).
- */
-
-vector_addrexcptn:
-        b       vector_addrexcptn
-
-/*
- * We group all the following data together to optimise
- * for CPUs with separate I & D caches.
- */
-        .align  5
-
-.LCvswi:
-        .word   vector_swi
-
         .globl  __stubs_end
 __stubs_end:
 
-        .equ    stubs_offset, __vectors_start + 0x200 - __stubs_start
+        .equ    stubs_offset, __vectors_start + 0x1000 - __stubs_start
 
         .globl  __vectors_start
 __vectors_start:
- ARM(   swi     SYS_ERROR0      )
- THUMB( svc     #0              )
- THUMB( nop                     )
+        W(b)    vector_rst + stubs_offset
         W(b)    vector_und + stubs_offset
         W(ldr)  pc, .LCvswi + stubs_offset
         W(b)    vector_pabt + stubs_offset
diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index 9433e8a12b5e..2c8c7fa78b8c 100644
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -837,7 +837,7 @@ void __init early_trap_init(void *vectors_base)
          * are visible to the instruction stream.
          */
         memcpy((void *)vectors, __vectors_start, __vectors_end - __vectors_start);
-        memcpy((void *)vectors + 0x200, __stubs_start, __stubs_end - __stubs_start);
+        memcpy((void *)vectors + 0x1000, __stubs_start, __stubs_end - __stubs_start);
         memcpy((void *)vectors + 0x1000 - kuser_sz, __kuser_helper_start, kuser_sz);
 
         /*
@@ -852,7 +852,7 @@ void __init early_trap_init(void *vectors_base)
         memcpy((void *)(vectors + KERN_SIGRETURN_CODE - CONFIG_VECTORS_BASE),
                sigreturn_codes, sizeof(sigreturn_codes));
 
-        flush_icache_range(vectors, vectors + PAGE_SIZE);
+        flush_icache_range(vectors, vectors + PAGE_SIZE * 2);
         modify_domain(DOMAIN_USER, DOMAIN_CLIENT);
 #else /* ifndef CONFIG_CPU_V7M */
         /*
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 4f56617a2392..9ea274d1af69 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -1160,7 +1160,7 @@ static void __init devicemaps_init(struct machine_desc *mdesc)
         /*
          * Allocate the vector page early.
          */
-        vectors = early_alloc(PAGE_SIZE);
+        vectors = early_alloc(PAGE_SIZE * 2);
 
         early_trap_init(vectors);
 
@@ -1210,10 +1210,18 @@ static void __init devicemaps_init(struct machine_desc *mdesc)
 
         if (!vectors_high()) {
                 map.virtual = 0;
+                map.length = PAGE_SIZE * 2;
                 map.type = MT_LOW_VECTORS;
                 create_mapping(&map);
         }
 
+        /* Now create a kernel read-only mapping */
+        map.pfn += 1;
+        map.virtual = 0xffff0000 + PAGE_SIZE;
+        map.length = PAGE_SIZE;
+        map.type = MT_LOW_VECTORS;
+        create_mapping(&map);
+
         /*
          * Ask the machine support to map in the statically mapped devices.
          */