crossgcc: Enable GCC to get asan shadow offset at runtime

Unlike Linux kernel which has a static shadow region layout, we have
multiple stages in coreboot and thus require a different shadow offset
address. Unfortunately, GCC currently only supports adding a static
shadow offset at compile time using -fasan-shadow-offset flag.

For this reason, we enable GCC to determine asan shadow offset address
at runtime using a callback function named __asan_shadow_offset().
This supersedes the need to specify this address at compile time. GCC
then makes use of this shadow offset to protect stack buffers by
inserting red zones around them.

Some other benefits of having this GCC patch are:
a. We can place the shadow region in a separate linker section with
   all its advantages like automatic fit insurance. This ensures if
   a platform doesn't have enough memory space to hold shadow region,
   the build will fail. (However, if we use a fixed shadow offset on a
   platform that actually doesn't have enough memory, it may still
   build without any errors.)
b. We don't modify the memory layout compared to the current one, as
   we are placing the shadow region at the end of the space already
   occupied by the program.
c. We can be much more flexible later if needed (thinking of other
   stages like bootblock).
d. Since we are appending the shadow buffer to the region already
   occupied, we make efficient use of the limited memory available
   which is highly beneficial when using cache as ram.

Further, we have made sure that if you compile you tree with ASan
enabled but missed this patch, it will end up in the following
compilation error:
"invalid --param name 'asan-use-shadow-offset-callback'"
So, you cannot accidentally enable the feature without having your
compiler patched.

Change-Id: I401631938532a406a6d41e77c6c9716b6b2bf48d
Signed-off-by: Harshit Sharma <harshitsharmajs@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42794
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

1 files changed, 109 insertions, 0 deletions

diff --git a/util/crossgcc/patches/gcc-8.3.0_asan_shadow_offset_callback.patch b/util/crossgcc/patches/gcc-8.3.0_asan_shadow_offset_callback.patch
new file mode 100644
index 0000000000..975cad8e1b
--- /dev/null
+++ b/util/crossgcc/patches/gcc-8.3.0_asan_shadow_offset_callback.patch
@@ -0,0 +1,109 @@
+From 41a82fb711f3637b4b7f57756492b628058f9d5f Mon Sep 17 00:00:00 2001
+From: Harshit Sharma <harshitsharmajs@gmail.com>
+Date: Fri, 10 Jul 2020 13:06:08 -0700
+Subject: [PATCH] crossgcc: Enable GCC to get asan shadow offset at runtime
+
+Unlike Linux kernel which has a static shadow region layout, we have multiple stages in
+coreboot and thus require a different shadow offset address. Unfortunately, GCC currently
+only supports adding a static shadow offset at compile time using -fasan-shadow-offset flag.
+
+For this reason, we enable GCC to determine asan shadow offset address at runtime using a
+callback function named __asan_shadow_offset(). This supersedes the need to specify this
+address at compile time. GCC then makes use of this shadow offset to protect stack buffers
+by inserting red zones around them.
+
+Some other benefits of having this GCC patch are:
+a. We can place the shadow region in a separate linker section with all its advantages like
+   automatic fit insurance. This ensures if a platform doesn't have enough memory space to
+   hold shadow region, the build will fail. (However, if we use a fixed shadow offset on a
+   platform that actually doesn't have enough memory, it may still build without any errors.)
+b. We don't modify the memory layout compared to the current one, as we are placing the
+   shadow region at the end of the space already occupied by the program.
+c. We can be much more flexible later if needed (thinking of other stages like bootblock).
+d. Since we are appending the shadow buffer to the region already occupied, we make efficient
+   use of the limited memory available which is highly beneficial when using cache as ram.
+
+Further, we have made sure that if you compile you tree with ASan enabled but missed this
+patch, it will end up in the following compilation error:
+"invalid --param name 'asan-use-shadow-offset-callback'"
+So, you cannot accidentally enable the feature without having your compiler patched.
+
+Signed-off-by: Harshit Sharma <harshitsharmajs@gmail.com>
+---
+ gcc/asan.c     | 29 ++++++++++++++++++++++-------
+ gcc/params.def |  6 ++++++
+ gcc/params.h   |  2 ++
+ 3 files changed, 30 insertions(+), 7 deletions(-)
+
+diff --git a/gcc/asan.c b/gcc/asan.c
+index 235e21947..713bf994d 100644
+--- a/gcc/asan.c
++++ b/gcc/asan.c
+@@ -1389,13 +1389,28 @@ asan_emit_stack_protection (rtx base, rtx pbase, unsigned int alignb,
+   TREE_ASM_WRITTEN (decl) = 1;
+   TREE_ASM_WRITTEN (id) = 1;
+   emit_move_insn (mem, expand_normal (build_fold_addr_expr (decl)));
+-  shadow_base = expand_binop (Pmode, lshr_optab, base,
+-			      gen_int_shift_amount (Pmode, ASAN_SHADOW_SHIFT),
+-			      NULL_RTX, 1, OPTAB_DIRECT);
+-  shadow_base
+-    = plus_constant (Pmode, shadow_base,
+-		     asan_shadow_offset ()
+-		     + (base_align_bias >> ASAN_SHADOW_SHIFT));
++  if (ASAN_USE_SHADOW_OFFSET_CALLBACK) {
++    rtx addr, shadow_offset_rtx;
++    ret = init_one_libfunc ("__asan_shadow_offset");
++    addr= convert_memory_address (ptr_mode, base);
++    ret = emit_library_call_value (ret, NULL_RTX, LCT_NORMAL, ptr_mode,
++                                  addr, ptr_mode);
++    shadow_offset_rtx = convert_memory_address (Pmode, ret);
++    shadow_base = expand_binop (Pmode, lshr_optab, base,
++			                          gen_int_shift_amount (Pmode, ASAN_SHADOW_SHIFT),
++			                          NULL_RTX, 1, OPTAB_DIRECT);
++    shadow_base = expand_binop (Pmode, add_optab, shadow_base,
++			                          shadow_offset_rtx, NULL_RTX, 1, OPTAB_LIB_WIDEN);
++    shadow_base = plus_constant (Pmode, shadow_base,
++		                            (base_align_bias >> ASAN_SHADOW_SHIFT));
++  } else {
++    shadow_base = expand_binop (Pmode, lshr_optab, base,
++			                          gen_int_shift_amount (Pmode, ASAN_SHADOW_SHIFT),
++			                          NULL_RTX, 1, OPTAB_DIRECT);
++    shadow_base = plus_constant (Pmode, shadow_base,
++		                            asan_shadow_offset ()
++		                            + (base_align_bias >> ASAN_SHADOW_SHIFT));
++  }
+   gcc_assert (asan_shadow_set != -1
+ 	      && (ASAN_RED_ZONE_SIZE >> ASAN_SHADOW_SHIFT) == 4);
+   shadow_mem = gen_rtx_MEM (SImode, shadow_base);
+diff --git a/gcc/params.def b/gcc/params.def
+index dad47ec2b..bfe6eaa0b 100644
+--- a/gcc/params.def
++++ b/gcc/params.def
+@@ -1203,6 +1203,12 @@ DEFPARAM (PARAM_ASAN_INSTRUMENTATION_WITH_CALL_THRESHOLD,
+          "in function becomes greater or equal to this number.",
+          7000, 0, INT_MAX)
+
++DEFPARAM (PARAM_ASAN_USE_SHADOW_OFFSET_CALLBACK,
++	 "asan-use-shadow-offset-callback",
++	 "Use shadow offset callback function at runtime instead of "
++	 "fixed value at compile time at the cost of runtime overhead.",
++	 0, 0, 1)
++
+ DEFPARAM (PARAM_USE_AFTER_SCOPE_DIRECT_EMISSION_THRESHOLD,
+ 	 "use-after-scope-direct-emission-threshold",
+ 	 "Use direct poisoning/unpoisoning instructions for variables "
+diff --git a/gcc/params.h b/gcc/params.h
+index 98249d2a1..d3bd6be38 100644
+--- a/gcc/params.h
++++ b/gcc/params.h
+@@ -246,6 +246,8 @@ extern void init_param_values (int *params);
+   PARAM_VALUE (PARAM_ASAN_USE_AFTER_RETURN)
+ #define ASAN_INSTRUMENTATION_WITH_CALL_THRESHOLD \
+   PARAM_VALUE (PARAM_ASAN_INSTRUMENTATION_WITH_CALL_THRESHOLD)
++#define ASAN_USE_SHADOW_OFFSET_CALLBACK \
++  PARAM_VALUE (PARAM_ASAN_USE_SHADOW_OFFSET_CALLBACK)
+ #define ASAN_PARAM_USE_AFTER_SCOPE_DIRECT_EMISSION_THRESHOLD \
+   ((unsigned) PARAM_VALUE (PARAM_USE_AFTER_SCOPE_DIRECT_EMISSION_THRESHOLD))
+
+--
+2.17.1