[RFC/RFT,V2 3/3] [PR102768] aarch64: Add support for Kernel Control Flow Integrity

Dan Li
2023-03-25T08:12:13Z

In the AArch64 platform, typeid can be directly inserted in front
of the function header (offset is patch_area_entry + 4), it should
be assumed that patch_area_entry is the same for all functions.

For all functions that will not be called indirectly, insert the
reserved RESERVED_CFI_TYPEID (0x0) as typeid in front of them. If
not, the attacker may use the instruction/data before the function
as typeid to bypass CFI.

All typeids ignore some bits (& AARCH64_UNALLOCATED_INSN_MASK) to
avoid conflicts with the AArch64 instruction set (see AAPCS64 for
details).

Signed-off-by: Dan Li <ashimida.1990@gmail.com>

gcc/ChangeLog:

* config/aarch64/aarch64.cc (RESERVED_CFI_TYPEID): Macro definition.
(DEFAULT_CFI_TYPEID): Likewise.
(AARCH64_UNALLOCATED_INSN_MASK): Likewise.
(aarch64_calc_func_cfi_typeid): Platform-dependent CFI function.
(cgraph_indirectly_callable): Determine whether a funtion may
be called indirectly.
(aarch64_output_func_kcfi_typeid): Platform-dependent CFI function.
(aarch64_output_icall_kcfi_check): Likewise.
(TARGET_HAVE_KCFI): New hook.
(TARGET_CALC_FUNC_CFI_TYPEID): Likewise.
(TARGET_ASM_OUTPUT_FUNC_KCFI_TYPEID): Likewise.
(TARGET_ASM_OUTPUT_ICALL_KCFI_CHECK): Likewise.
* doc/invoke.texi: Document -fsanitize=kcfi.
---
gcc/config/aarch64/aarch64.cc | 166 ++++++++++++++++++++++++++++++++++
gcc/doc/invoke.texi | 36 ++++++++
2 files changed, 202 insertions(+)

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 5c9e7791a12..5b55541d437 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -5450,6 +5450,160 @@ aarch64_output_sve_addvl_addpl (rtx offset)
return buffer;
}

+/* Reserved for all functions that cannot be called indirectly. */
+#define RESERVED_CFI_TYPEID 0x0U
+
+/* If the typeid of a function that can be called indirectly is equal to
+ RESERVED_CFI_TYPEID, change it to DEFAULT_CFI_TYPEID. */
+#define DEFAULT_CFI_TYPEID 0x00000ADAU
+
+/* Mask of reserved and unallocated instructions in AArch64 platform. */
+#define AARCH64_UNALLOCATED_INSN_MASK 0xE7FFFFFFU
+
+static unsigned int
+aarch64_calc_func_cfi_typeid (const_tree fntype)
+{
+ unsigned int hash;
+
+ /* The value of typeid has a probability of being the same as the encoding
+ of an instruction. If the attacker can find the same encoding as the
+ typeid in the assembly code, then he has found a usable jump location.
+ So here, a platform-related mask is used when generating a typeid to
+ avoid such conflicts as much as possible. */
+ hash = unified_type_hash (fntype) & AARCH64_UNALLOCATED_INSN_MASK;
+
+ /* RESERVED_CFI_TYPEID is reserved for functions that cannot
+ be called indirectly. */
+ if (hash == RESERVED_CFI_TYPEID)
+ hash = DEFAULT_CFI_TYPEID;
+
+ return hash;
+}
+
+static bool
+cgraph_indirectly_callable (struct cgraph_node *node,
+ void *data ATTRIBUTE_UNUSED)
+{
+ if (node->externally_visible || node->address_taken)
+ return true;
+
+ return false;
+}
+
+static void
+aarch64_output_func_kcfi_typeid (FILE * stream, tree decl)
+{
+ struct cgraph_node *node;
+ unsigned int cur_func_typeid;
+
+ node = cgraph_node::get (decl);
+
+ if (!node->call_for_symbol_thunks_and_aliases (cgraph_indirectly_callable,
+ NULL, true))
+ /* CFI's typeid check always considers that there is a typeid before the
+ target function, so it is also necessary to output typeid for functions
+ that cannot be called indirectly to prevent attackers from bypassing
+ CFI by using instructions/data before those functions.
+ The typeid inserted before such a function is RESERVED_CFI_TYPEID,
+ and the calculation of the typeid must ensure that this value is always
+ reserved. */
+ cur_func_typeid = RESERVED_CFI_TYPEID;
+ else
+ cur_func_typeid = aarch64_calc_func_cfi_typeid (TREE_TYPE (decl));
+
+ fprintf (stream, "__kcfi_%s:\n", get_name (decl));
+ fprintf (stream, "\t.4byte %#010x\n", cur_func_typeid);
+}
+
+/* This function outputs assembly instructions to check cfi typeid before
+ indirect call (blr Xn), which may destroy x16, x17, x9 registers (according
+ to the AAPCS64 specification, these registers do not need to be restored
+ after the function call).
+ The assembly code output by this function is as follows:
+ ldur w16, [x1, #-4]
+ movk w17, #13570
+ movk w17, #17309, lsl #16
+ cmp w16, w17
+ b.eq .Lkcfi8
+ brk #0x8221
+.Lkcfi8:
+ blr x1
+ */
+
+static void
+aarch64_output_icall_kcfi_check (rtx reg, unsigned int value)
+{
+ unsigned int addr_reg, scratch_reg1, scratch_reg2;
+ unsigned int esr, addr_index, type_index;
+ char label_buf[256];
+ const char *label_ptr;
+ unsigned HOST_WIDE_INT patch_area_entry = crtl->patch_area_entry;
+ rtx_code_label * tmp_label = gen_label_rtx ();
+
+ gcc_assert (GET_CODE (reg) == REG);
+
+ addr_reg = REGNO (reg);
+
+ /* The typeid read from the front of the callee is saved in the
+ register specified by scratch_reg1, the default is R16_REGNUM. */
+ scratch_reg1 = R16_REGNUM;
+
+ /* The expected typeid of the caller is saved in the register
+ specified by scratch_reg2, which defaults to R17_REGNUM. */
+ scratch_reg2 = R17_REGNUM;
+
+ gcc_assert (GP_REGNUM_P (addr_reg));
+
+ /* If one of the scratch registers is used for the call target,
+ we can clobber another caller-saved temporary register instead
+ (in this case, R9_REGNUM) as the check is immediately followed
+ by the call instruction. */
+ if (addr_reg == R16_REGNUM)
+ {
+ scratch_reg1 = R9_REGNUM;
+ }
+ else if (addr_reg == R17_REGNUM)
+ {
+ scratch_reg2 = R9_REGNUM;
+ }
+
+ gcc_assert ((scratch_reg1 != addr_reg) && (scratch_reg2 != addr_reg));
+
+ ASM_GENERATE_INTERNAL_LABEL (label_buf, "Lkcfi",
+ CODE_LABEL_NUMBER (tmp_label));
+ label_ptr = targetm.strip_name_encoding (label_buf);
+
+ /* The offset of callee's typeid needs to be adjusted according to
+ patch_area_entry. This assumes that patch_area_entry is the
+ same for all functions. */
+ fprintf (asm_out_file, "\tldur\tw%d, [x%d, #-%ld]\n",
+ scratch_reg1, addr_reg, patch_area_entry * 4 + 4);
+
+ fprintf (asm_out_file, "\tmovk\tw%d, #%d\n", scratch_reg2, value & 0xFFFF);
+
+ fprintf (asm_out_file, "\tmovk\tw%d, #%d, lsl #16\n",
+ scratch_reg2, (value >> 16) & 0xFFFF);
+
+ fprintf (asm_out_file, "\tcmp\tw%d, w%d\n", scratch_reg1, scratch_reg2);
+
+ fprintf (asm_out_file, "\tb.eq\t%s\n", label_ptr);
+
+ /* The base ESR for brk is 0x8000 and the register information is
+ encoded in bits 0-9 as follows:
+ - 0-4: n, where the register Xn contains the callee address
+ - 5-9: m, where the register Wm contains the expected typeid
+ Where n, m are in[0,30].
+ */
+ addr_index = addr_reg - R0_REGNUM;
+ type_index = scratch_reg2 - R0_REGNUM;
+ esr = 0x8000 | ((type_index & 31) << 5) | (addr_index & 31);
+ fprintf (asm_out_file, "\tbrk\t#0x%x\n", esr);
+
+ fprintf (asm_out_file, "%s:\n", label_ptr);
+
+ return;
+}
+
/* Return true if X is a valid immediate for an SVE vector INC or DEC
instruction. If it is, store the number of elements in each vector
quadword in *NELTS_PER_VQ_OUT (if nonnull) and store the multiplication
@@ -27823,6 +27977,18 @@ aarch64_libgcc_floating_mode_supported_p
#undef TARGET_HAVE_SHADOW_CALL_STACK
#define TARGET_HAVE_SHADOW_CALL_STACK true

+#undef TARGET_HAVE_KCFI
+#define TARGET_HAVE_KCFI true
+
+#undef TARGET_CALC_FUNC_CFI_TYPEID
+#define TARGET_CALC_FUNC_CFI_TYPEID aarch64_calc_func_cfi_typeid
+
+#undef TARGET_ASM_OUTPUT_FUNC_KCFI_TYPEID
+#define TARGET_ASM_OUTPUT_FUNC_KCFI_TYPEID aarch64_output_func_kcfi_typeid
+
+#undef TARGET_ASM_OUTPUT_ICALL_KCFI_CHECK
+#define TARGET_ASM_OUTPUT_ICALL_KCFI_CHECK aarch64_output_icall_kcfi_check
+
struct gcc_target targetm = TARGET_INITIALIZER;

#include "gt-aarch64.h"
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index ff6c338bedb..1b2ba7a0f29 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -15736,6 +15736,42 @@ to turn off exceptions.
See @uref{https://clang.llvm.org/docs/ShadowCallStack.html} for more
details.

+@item -fsanitize=kcfi
+@opindex fsanitize=kcfi
+The KCFI sanitizer, enabled with @option{-fsanitize=kcfi}, implements a
+forward-edge control flow integrity scheme for indirect calls. It
+attaches a type identifier (@code{typeid}) for each function and injects
+verification code before indirect calls.
+
+A @code{typeid} is a 32-bit constant, its value is mainly related to the
+return value type and all parameter types of the function, and is invariant
+for each compilation. Since the value of @code{typeid} may conflict with
+the instruction set encoding of the current platform, some bits may be
+ignored on different platforms.
+
+At compile time, the compiler inserts checking code on all indirect calls,
+and at run time, before any indirect calls occur, the code checks that
+the @code{typeid} before the callee function matches the @code{typeid}
+requested by the caller. If the match fails, an exception instruction
+will be triggered, such as a @code{brk} in aarch64. This mechanism is
+mainly designed for low-level codes, such as operating systems, and the
+system needs to handle those exceptions by itself.
+
+If a program contains indirect calls to assembly functions, they must be
+manually annotated with the expected type identifiers to prevent errors.
+To make this easier, CFI generates a weak SHN_ABS
+@code{__kcfi_typeid_<function>} symbol for each address-taken function
+declaration, which can be used to annotate functions in assembly as long
+as at least one C translation unit linked into the program takes the
+function address.
+
+Currently this feature only supports the aarch64 platform, mainly for
+the linux kernel. Users who want to use this feature in other system
+need to provide their own support for the exception handling.
+
+See @uref{https://clang.llvm.org/docs/ControlFlowIntegrity.html} for
+more details.
+
@item -fsanitize=thread
@opindex fsanitize=thread
Enable ThreadSanitizer, a fast data race detector.
--
2.17.1