[PATCH 19/19] aarch64: Make stack smash canary protect saved registers

[Richard Sandiford <richard.sandiford@arm.com>]

AArch64 normally puts the saved registers near the bottom of the frame,
immediately above any dynamic allocations.  But this means that a
stack-smash attack on those dynamic allocations could overwrite the
saved registers without needing to reach as far as the stack smash
canary.

The same thing could also happen for variable-sized arguments that are
passed by value, since those are allocated before a call and popped on
return.

This patch avoids that by putting the locals (and thus the canary) below
the saved registers when stack smash protection is active.

The patch fixes CVE-2023-4039.

gcc/
	* config/aarch64/aarch64.cc (aarch64_save_regs_above_locals_p):
	New function.
	(aarch64_layout_frame): Use it to decide whether locals should
	go above or below the saved registers.
	(aarch64_expand_prologue): Update stack layout comment.
	Emit a stack tie after the final adjustment.

gcc/testsuite/
	* gcc.target/aarch64/stack-protector-8.c: New test.
	* gcc.target/aarch64/stack-protector-9.c: Likewise.
---
 gcc/config/aarch64/aarch64.cc                 | 46 +++++++--
 .../gcc.target/aarch64/stack-protector-8.c    | 95 +++++++++++++++++++
 .../gcc.target/aarch64/stack-protector-9.c    | 33 +++++++
 3 files changed, 168 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/aarch64/stack-protector-8.c
 create mode 100644 gcc/testsuite/gcc.target/aarch64/stack-protector-9.c

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 51e57370807..3739a44bfd9 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -8433,6 +8433,20 @@ aarch64_needs_frame_chain (void)
   return aarch64_use_frame_pointer;
 }
 
+/* Return true if the current function should save registers above
+   the locals area, rather than below it.  */
+
+static bool
+aarch64_save_regs_above_locals_p ()
+{
+  /* When using stack smash protection, make sure that the canary slot
+     comes between the locals and the saved registers.  Otherwise,
+     it would be possible for a carefully sized smash attack to change
+     the saved registers (particularly LR and FP) without reaching the
+     canary.  */
+  return crtl->stack_protect_guard;
+}
+
 /* Mark the registers that need to be saved by the callee and calculate
    the size of the callee-saved registers area and frame record (both FP
    and LR may be omitted).  */
@@ -8444,6 +8458,7 @@ aarch64_layout_frame (void)
   poly_int64 vector_save_size = GET_MODE_SIZE (vector_save_mode);
   bool frame_related_fp_reg_p = false;
   aarch64_frame &frame = cfun->machine->frame;
+  poly_int64 top_of_locals = -1;
 
   frame.emit_frame_chain = aarch64_needs_frame_chain ();
 
@@ -8510,9 +8525,16 @@ aarch64_layout_frame (void)
 	&& !crtl->abi->clobbers_full_reg_p (regno))
       frame.reg_offset[regno] = SLOT_REQUIRED;
 
+  bool regs_at_top_p = aarch64_save_regs_above_locals_p ();
 
   poly_int64 offset = crtl->outgoing_args_size;
   gcc_assert (multiple_p (offset, STACK_BOUNDARY / BITS_PER_UNIT));
+  if (regs_at_top_p)
+    {
+      offset += get_frame_size ();
+      offset = aligned_upper_bound (offset, STACK_BOUNDARY / BITS_PER_UNIT);
+      top_of_locals = offset;
+    }
   frame.bytes_below_saved_regs = offset;
   frame.sve_save_and_probe = INVALID_REGNUM;
 
@@ -8652,15 +8674,18 @@ aarch64_layout_frame (void)
      at expand_prologue.  */
   gcc_assert (crtl->is_leaf || maybe_ne (saved_regs_size, 0));
 
-  offset += get_frame_size ();
-  offset = aligned_upper_bound (offset, STACK_BOUNDARY / BITS_PER_UNIT);
-  auto top_of_locals = offset;
-
+  if (!regs_at_top_p)
+    {
+      offset += get_frame_size ();
+      offset = aligned_upper_bound (offset, STACK_BOUNDARY / BITS_PER_UNIT);
+      top_of_locals = offset;
+    }
   offset += frame.saved_varargs_size;
   gcc_assert (multiple_p (offset, STACK_BOUNDARY / BITS_PER_UNIT));
   frame.frame_size = offset;
 
   frame.bytes_above_hard_fp = frame.frame_size - frame.bytes_below_hard_fp;
+  gcc_assert (known_ge (top_of_locals, 0));
   frame.bytes_above_locals = frame.frame_size - top_of_locals;
 
   frame.initial_adjust = 0;
@@ -9979,10 +10004,10 @@ aarch64_epilogue_uses (int regno)
 	|  for register varargs         |
 	|                               |
 	+-------------------------------+
-	|  local variables              | <-- frame_pointer_rtx
+	|  local variables (1)          | <-- frame_pointer_rtx
 	|                               |
 	+-------------------------------+
-	|  padding                      |
+	|  padding (1)                  |
 	+-------------------------------+
 	|  callee-saved registers       |
 	+-------------------------------+
@@ -9994,6 +10019,10 @@ aarch64_epilogue_uses (int regno)
 	+-------------------------------+
 	|  SVE predicate registers      |
 	+-------------------------------+
+	|  local variables (2)          |
+	+-------------------------------+
+	|  padding (2)                  |
+	+-------------------------------+
 	|  dynamic allocation           |
 	+-------------------------------+
 	|  padding                      |
@@ -10003,6 +10032,9 @@ aarch64_epilogue_uses (int regno)
 	+-------------------------------+
 	|                               | <-- stack_pointer_rtx (aligned)
 
+   The regions marked (1) and (2) are mutually exclusive.  (2) is used
+   when aarch64_save_regs_above_locals_p is true.
+
    Dynamic stack allocations via alloca() decrease stack_pointer_rtx
    but leave frame_pointer_rtx and hard_frame_pointer_rtx
    unchanged.
@@ -10198,6 +10230,8 @@ aarch64_expand_prologue (void)
   gcc_assert (known_eq (bytes_below_sp, final_adjust));
   aarch64_allocate_and_probe_stack_space (tmp1_rtx, tmp0_rtx, final_adjust,
 					  !frame_pointer_needed, true);
+  if (emit_frame_chain && maybe_ne (final_adjust, 0))
+    aarch64_emit_stack_tie (hard_frame_pointer_rtx);
 }
 
 /* Return TRUE if we can use a simple_return insn.
diff --git a/gcc/testsuite/gcc.target/aarch64/stack-protector-8.c b/gcc/testsuite/gcc.target/aarch64/stack-protector-8.c
new file mode 100644
index 00000000000..e71d820e365
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/stack-protector-8.c
@@ -0,0 +1,95 @@
+/* { dg-options " -O -fstack-protector-strong -mstack-protector-guard=sysreg -mstack-protector-guard-reg=tpidr2_el0 -mstack-protector-guard-offset=16" } */
+/* { dg-final { check-function-bodies "**" "" } } */
+
+void g(void *);
+__SVBool_t *h(void *);
+
+/*
+** test1:
+**	sub	sp, sp, #288
+**	stp	x29, x30, \[sp, #?272\]
+**	add	x29, sp, #?272
+**	mrs	(x[0-9]+), tpidr2_el0
+**	ldr	(x[0-9]+), \[\1, #?16\]
+**	str	\2, \[sp, #?264\]
+**	mov	\2, #?0
+**	add	x0, sp, #?8
+**	bl	g
+**	...
+**	mrs	.*
+**	...
+**	bne	.*
+**	...
+**	ldp	x29, x30, \[sp, #?272\]
+**	add	sp, sp, #?288
+**	ret
+**	bl	__stack_chk_fail
+*/
+int test1() {
+  int y[0x40];
+  g(y);
+  return 1;
+}
+
+/*
+** test2:
+**	stp	x29, x30, \[sp, #?-16\]!
+**	mov	x29, sp
+**	sub	sp, sp, #1040
+**	mrs	(x[0-9]+), tpidr2_el0
+**	ldr	(x[0-9]+), \[\1, #?16\]
+**	str	\2, \[sp, #?1032\]
+**	mov	\2, #?0
+**	add	x0, sp, #?8
+**	bl	g
+**	...
+**	mrs	.*
+**	...
+**	bne	.*
+**	...
+**	add	sp, sp, #?1040
+**	ldp	x29, x30, \[sp\], #?16
+**	ret
+**	bl	__stack_chk_fail
+*/
+int test2() {
+  int y[0x100];
+  g(y);
+  return 1;
+}
+
+#pragma GCC target "+sve"
+
+/*
+** test3:
+**	stp	x29, x30, \[sp, #?-16\]!
+**	mov	x29, sp
+**	addvl	sp, sp, #-18
+**	...
+**	str	p4, \[sp\]
+**	...
+**	sub	sp, sp, #272
+**	mrs	(x[0-9]+), tpidr2_el0
+**	ldr	(x[0-9]+), \[\1, #?16\]
+**	str	\2, \[sp, #?264\]
+**	mov	\2, #?0
+**	add	x0, sp, #?8
+**	bl	h
+**	...
+**	mrs	.*
+**	...
+**	bne	.*
+**	...
+**	add	sp, sp, #?272
+**	...
+**	ldr	p4, \[sp\]
+**	...
+**	addvl	sp, sp, #18
+**	ldp	x29, x30, \[sp\], #?16
+**	ret
+**	bl	__stack_chk_fail
+*/
+__SVBool_t test3() {
+  int y[0x40];
+  return *h(y);
+}
diff --git a/gcc/testsuite/gcc.target/aarch64/stack-protector-9.c b/gcc/testsuite/gcc.target/aarch64/stack-protector-9.c
new file mode 100644
index 00000000000..58f322aa480
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/stack-protector-9.c
@@ -0,0 +1,33 @@
+/* { dg-options "-O2 -mcpu=neoverse-v1 -fstack-protector-all" } */
+/* { dg-final { check-function-bodies "**" "" } } */
+
+/*
+** main:
+**	...
+**	stp	x29, x30, \[sp, #?-[0-9]+\]!
+**	...
+**	sub	sp, sp, #[0-9]+
+**	...
+**	str	x[0-9]+, \[x29, #?-8\]
+**	...
+*/
+int f(const char *);
+void g(void *);
+int main(int argc, char* argv[])
+{
+  int a;
+  int b;
+  char c[2+f(argv[1])];
+  int d[0x100];
+  char y;
+
+  y=42; a=4; b=10;
+  c[0] = 'h'; c[1] = '\0';
+
+  c[f(argv[2])] = '\0';
+
+  __builtin_printf("%d %d\n%s\n", a, b, c);
+  g(d);
+
+  return 0;
+}
-- 
2.25.1