diff mbox series

[committed] aarch64: Clear canary value after stack_protect_test [PR96191]

Message ID mpteeol41lf.fsf@arm.com
State New
Headers show
Series [committed] aarch64: Clear canary value after stack_protect_test [PR96191] | expand

Commit Message

Richard Sandiford Aug. 5, 2020, 2:19 p.m. UTC
The stack_protect_test patterns were leaving the canary value in the
temporary register, meaning that it was often still in registers on
return from the function.  An attacker might therefore have been
able to use it to defeat stack-smash protection for a later function.

Tested on aarch64-linux-gnu and aarch64_be-elf, committed.
I'll backport to branches over the next few days.  I'm about
to post a patch for arm too.

This is needed to make aarch64 conform to GCC's current -fstack-protector
implementation.  However, I think we should reconsider whether the
zeroing is actually necessary and what it's actually protecting against.
I'll send a separate message about that to gcc@.

Richard


gcc/
	PR target/96191
	* config/aarch64/aarch64.md (stack_protect_test_<mode>): Set the
	CC register directly, instead of a GPR.  Replace the original GPR
	destination with an extra scratch register.  Zero out operand 3
	after use.
	(stack_protect_test): Update accordingly.

gcc/testsuite/
	PR target/96191
	* gcc.target/aarch64/stack-protector-1.c: New test.
	* gcc.target/aarch64/stack-protector-2.c: Likewise.
---
 gcc/config/aarch64/aarch64.md                 | 34 ++++---
 .../gcc.target/aarch64/stack-protector-1.c    | 89 +++++++++++++++++++
 .../gcc.target/aarch64/stack-protector-2.c    |  6 ++
 3 files changed, 110 insertions(+), 19 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/aarch64/stack-protector-1.c
 create mode 100644 gcc/testsuite/gcc.target/aarch64/stack-protector-2.c
diff mbox series

Patch

diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index 25d77256b96..9b20dd0b1a0 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -7217,10 +7217,8 @@  (define_expand "stack_protect_test"
    (match_operand 2)]
   ""
 {
-  rtx result;
   machine_mode mode = GET_MODE (operands[0]);
 
-  result = gen_reg_rtx(mode);
   if (aarch64_stack_protector_guard != SSP_GLOBAL)
   {
     /* Generate access through the system register. The
@@ -7245,29 +7243,27 @@  (define_expand "stack_protect_test"
     operands[1] = gen_rtx_MEM (mode, tmp_reg);
   }
   emit_insn ((mode == DImode
-		  ? gen_stack_protect_test_di
-		  : gen_stack_protect_test_si) (result,
-					        operands[0],
-					        operands[1]));
-
-  if (mode == DImode)
-    emit_jump_insn (gen_cbranchdi4 (gen_rtx_EQ (VOIDmode, result, const0_rtx),
-				    result, const0_rtx, operands[2]));
-  else
-    emit_jump_insn (gen_cbranchsi4 (gen_rtx_EQ (VOIDmode, result, const0_rtx),
-				    result, const0_rtx, operands[2]));
+	     ? gen_stack_protect_test_di
+	     : gen_stack_protect_test_si) (operands[0], operands[1]));
+
+  rtx cc_reg = gen_rtx_REG (CCmode, CC_REGNUM);
+  emit_jump_insn (gen_condjump (gen_rtx_EQ (VOIDmode, cc_reg, const0_rtx),
+				cc_reg, operands[2]));
   DONE;
 })
 
+;; DO NOT SPLIT THIS PATTERN.  It is important for security reasons that the
+;; canary value does not live beyond the end of this sequence.
 (define_insn "stack_protect_test_<mode>"
-  [(set (match_operand:PTR 0 "register_operand" "=r")
-	(unspec:PTR [(match_operand:PTR 1 "memory_operand" "m")
-		     (match_operand:PTR 2 "memory_operand" "m")]
-	 UNSPEC_SP_TEST))
+  [(set (reg:CC CC_REGNUM)
+	(unspec:CC [(match_operand:PTR 0 "memory_operand" "m")
+		    (match_operand:PTR 1 "memory_operand" "m")]
+		   UNSPEC_SP_TEST))
+   (clobber (match_scratch:PTR 2 "=&r"))
    (clobber (match_scratch:PTR 3 "=&r"))]
   ""
-  "ldr\t%<w>3, %1\;ldr\t%<w>0, %2\;eor\t%<w>0, %<w>3, %<w>0"
-  [(set_attr "length" "12")
+  "ldr\t%<w>2, %0\;ldr\t%<w>3, %1\;subs\t%<w>2, %<w>2, %<w>3\;mov\t%3, 0"
+  [(set_attr "length" "16")
    (set_attr "type" "multiple")])
 
 ;; Write into the Floating-point Status or Control Register.
diff --git a/gcc/testsuite/gcc.target/aarch64/stack-protector-1.c b/gcc/testsuite/gcc.target/aarch64/stack-protector-1.c
new file mode 100644
index 00000000000..73e83bc413f
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/stack-protector-1.c
@@ -0,0 +1,89 @@ 
+/* { dg-do run } */
+/* { dg-require-effective-target fstack_protector } */
+/* { dg-options "-fstack-protector-all -O2" } */
+
+extern volatile long *stack_chk_guard_ptr;
+
+volatile long *
+get_ptr (void)
+{
+  return stack_chk_guard_ptr;
+}
+
+void __attribute__ ((noipa))
+f (void)
+{
+  volatile int x;
+  x = 1;
+  x += 1;
+}
+
+#define CHECK(REG) "\tcmp\tx0, " #REG "\n\tbeq\t1f\n"
+
+asm (
+"	.pushsection .data\n"
+"	.align	3\n"
+"	.globl	stack_chk_guard_ptr\n"
+"stack_chk_guard_ptr:\n"
+#if __ILP32__
+"	.word	__stack_chk_guard\n"
+#else
+"	.xword	__stack_chk_guard\n"
+#endif
+"	.weak	__stack_chk_guard\n"
+"__stack_chk_guard:\n"
+"	.word	0xdead4321\n"
+"	.word	0xbeef8765\n"
+"	.text\n"
+"	.globl	main\n"
+"	.type	main, %function\n"
+"main:\n"
+"	bl	get_ptr\n"
+"	str	x0, [sp, #-16]!\n"
+"	bl	f\n"
+"	str	x0, [sp, #8]\n"
+"	ldr	x0, [sp]\n"
+#if __ILP32__
+"	ldr     w0, [x0]\n"
+#else
+"	ldr     x0, [x0]\n"
+#endif
+	CHECK (x1)
+	CHECK (x2)
+	CHECK (x3)
+	CHECK (x4)
+	CHECK (x5)
+	CHECK (x6)
+	CHECK (x7)
+	CHECK (x8)
+	CHECK (x9)
+	CHECK (x10)
+	CHECK (x11)
+	CHECK (x12)
+	CHECK (x13)
+	CHECK (x14)
+	CHECK (x15)
+	CHECK (x16)
+	CHECK (x17)
+	CHECK (x18)
+	CHECK (x19)
+	CHECK (x20)
+	CHECK (x21)
+	CHECK (x22)
+	CHECK (x23)
+	CHECK (x24)
+	CHECK (x25)
+	CHECK (x26)
+	CHECK (x27)
+	CHECK (x28)
+	CHECK (x29)
+	CHECK (x30)
+"	ldr	x1, [sp]\n"
+	CHECK (x1)
+"	mov	x0, #0\n"
+"	b	exit\n"
+"1:\n"
+"	b	abort\n"
+"	.size	main, .-main\n"
+"	.popsection"
+);
diff --git a/gcc/testsuite/gcc.target/aarch64/stack-protector-2.c b/gcc/testsuite/gcc.target/aarch64/stack-protector-2.c
new file mode 100644
index 00000000000..266c36fdbc6
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/stack-protector-2.c
@@ -0,0 +1,6 @@ 
+/* { dg-do run } */
+/* { dg-require-effective-target fstack_protector } */
+/* { dg-require-effective-target fpic } */
+/* { dg-options "-fstack-protector-all -O2 -fpic" } */
+
+#include "stack-protector-1.c"