Implement AArch64-specific KCFI backend.
- Trap debugging through ESR (Exception Syndrome Register) encoding
in BRK instruction immediate values.
- Scratch register allocation using w16/w17 (x16/x17) following
AArch64 procedure call standard for intra-procedure-call registers.
Assembly Code Pattern for AArch64:
ldur w16, [target, #-4] ; Load actual type ID from preamble
mov w17, #type_id_low ; Load expected type (lower 16 bits)
movk w17, #type_id_high, lsl #16 ; Load upper 16 bits if needed
cmp w16, w17 ; Compare type IDs directly
b.eq .Lpass ; Branch if types match
.Ltrap: brk #esr_value ; Enhanced trap with register info
.Lpass: blr/br target ; Execute validated indirect transfer
ESR (Exception Syndrome Register) Integration:
- BRK instruction immediate encoding format:
0x8000 | ((TypeIndex & 31) << 5) | (AddrIndex & 31)
- TypeIndex indicates which W register contains expected type (W17 = 17)
- AddrIndex indicates which X register contains target address (0-30)
- Example: brk #33313 (0x8221) = expected type in W17, target address in X1
Build and run tested with Linux kernel ARCH=arm64.
gcc/ChangeLog:
config/aarch64/aarch64-protos.h: Declare aarch64_indirect_branch_asm,
and KCFI helpers.
config/aarch64/aarch64.cc (aarch64_expand_call): Wrap CALLs in
KCFI, with clobbers.
(aarch64_indirect_branch_asm): New function, extract common
logic for branch asm, like existing call asm helper.
(aarch64_output_kcfi_insn): Emit KCFI assembly.
config/aarch64/aarch64.md: Add KCFI RTL patterns and replace
open-coded branch emission with aarch64_indirect_branch_asm.
doc/invoke.texi: Document aarch64 nuances.
Signed-off-by: Kees Cook <k...@kernel.org>
---
gcc/config/aarch64/aarch64-protos.h | 5 ++
gcc/config/aarch64/aarch64.cc | 116 ++++++++++++++++++++++++++++
gcc/config/aarch64/aarch64.md | 64 +++++++++++++--
gcc/doc/invoke.texi | 14 ++++
4 files changed, 191 insertions(+), 8 deletions(-)
diff --git a/gcc/config/aarch64/aarch64-protos.h
b/gcc/config/aarch64/aarch64-protos.h
index 56efcf2c7f2c..c91fdcc80ea3 100644
--- a/gcc/config/aarch64/aarch64-protos.h
+++ b/gcc/config/aarch64/aarch64-protos.h
@@ -1261,6 +1261,7 @@ tree aarch64_resolve_overloaded_builtin_general
(location_t, tree, void *);
const char *aarch64_sls_barrier (int);
const char *aarch64_indirect_call_asm (rtx);
+const char *aarch64_indirect_branch_asm (rtx);
extern bool aarch64_harden_sls_retbr_p (void);
extern bool aarch64_harden_sls_blr_p (void);
@@ -1284,4 +1285,8 @@ extern unsigned aarch64_stack_alignment (const_tree exp,
unsigned align);
extern rtx aarch64_gen_compare_zero_and_branch (rtx_code code, rtx x,
rtx_code_label *label);
+/* KCFI support. */
+extern void kcfi_emit_trap_with_section (FILE *file, rtx trap_label_rtx);
+extern const char *aarch64_output_kcfi_insn (rtx_insn *insn, rtx *operands);
+
#endif /* GCC_AARCH64_PROTOS_H */
diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index fb8311b655d7..a7d17f18b72e 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -83,6 +83,7 @@
#include "rtlanal.h"
#include "tree-dfa.h"
#include "asan.h"
+#include "kcfi.h"
#include "aarch64-elf-metadata.h"
#include "aarch64-feature-deps.h"
#include "config/arm/aarch-common.h"
@@ -11848,6 +11849,16 @@ aarch64_expand_call (rtx result, rtx mem, rtx cookie,
bool sibcall)
call = gen_rtx_CALL (VOIDmode, mem, const0_rtx);
+ /* Only indirect calls need KCFI instrumentation. */
+ bool is_direct_call = SYMBOL_REF_P (XEXP (mem, 0));
+ rtx kcfi_type_rtx = is_direct_call ? NULL_RTX
+ : kcfi_get_type_id_for_expanding_gimple_call ();
+ if (kcfi_type_rtx)
+ {
+ /* Wrap call in KCFI. */
+ call = gen_rtx_KCFI (VOIDmode, call, kcfi_type_rtx);
+ }
+
if (result != NULL_RTX)
call = gen_rtx_SET (result, call);
@@ -11864,6 +11875,16 @@ aarch64_expand_call (rtx result, rtx mem, rtx cookie,
bool sibcall)
auto call_insn = aarch64_emit_call_insn (call);
+ /* Add KCFI clobbers for indirect calls. */
+ if (kcfi_type_rtx)
+ {
+ rtx usage = CALL_INSN_FUNCTION_USAGE (call_insn);
+ /* Add X16 and X17 clobbers for AArch64 KCFI scratch registers. */
+ clobber_reg (&usage, gen_rtx_REG (DImode, 16));
+ clobber_reg (&usage, gen_rtx_REG (DImode, 17));
+ CALL_INSN_FUNCTION_USAGE (call_insn) = usage;
+ }
+
/* Check whether the call requires a change to PSTATE.SM. We can't
emit the instructions to change PSTATE.SM yet, since they involve
a change in vector length and a change in instruction set, which
@@ -30630,6 +30651,14 @@ aarch64_indirect_call_asm (rtx addr)
return "";
}
+const char *
+aarch64_indirect_branch_asm (rtx addr)
+{
+ gcc_assert (REG_P (addr));
+ output_asm_insn ("br\t%0", &addr);
+ return aarch64_sls_barrier (aarch64_harden_sls_retbr_p ());
+}
+
/* Emit the assembly instruction to load the thread pointer into DEST.
Select between different tpidr_elN registers depending on -mtp= setting. */
@@ -32823,6 +32852,93 @@ aarch64_libgcc_floating_mode_supported_p
#undef TARGET_DOCUMENTATION_NAME
#define TARGET_DOCUMENTATION_NAME "AArch64"
+/* Output the assembly for a KCFI checked call instruction. */
+
+const char *
+aarch64_output_kcfi_insn (rtx_insn *insn, rtx *operands)
+{
+ /* KCFI is only supported in LP64 mode. */
+ if (TARGET_ILP32)
+ {
+ sorry ("%<-fsanitize=kcfi%> is not supported for %<-mabi=ilp32%>");
+ return "";
+ }
+ /* Target register is operands[0]. */
+ rtx target_reg = operands[0];
+ gcc_assert (REG_P (target_reg));
+
+ /* Get KCFI type ID from operand[3]. */
+ uint32_t type_id = (uint32_t) INTVAL (operands[3]);
+
+ /* Calculate typeid offset from call target. */
+ HOST_WIDE_INT offset = -(4 + kcfi_patchable_entry_prefix_nops);
+
+ /* Generate labels internally. */
+ rtx trap_label = gen_label_rtx ();
+ rtx call_label = gen_label_rtx ();
+
+ /* Get label numbers for custom naming. */
+ int trap_labelno = CODE_LABEL_NUMBER (trap_label);
+ int call_labelno = CODE_LABEL_NUMBER (call_label);
+
+ /* Generate custom label names. */
+ char trap_name[32];
+ char call_name[32];
+ ASM_GENERATE_INTERNAL_LABEL (trap_name, "Lkcfi_trap", trap_labelno);
+ ASM_GENERATE_INTERNAL_LABEL (call_name, "Lkcfi_call", call_labelno);
+
+ rtx temp_operands[3];
+
+ /* Load actual type into w16 from memory at offset using ldur. */
+ temp_operands[0] = gen_rtx_REG (SImode, R16_REGNUM);
+ temp_operands[1] = target_reg;
+ temp_operands[2] = GEN_INT (offset);
+ output_asm_insn ("ldur\t%w0, [%1, #%2]", temp_operands);
+
+ /* Load expected type low 16 bits into w17. */
+ temp_operands[0] = gen_rtx_REG (SImode, R17_REGNUM);
+ temp_operands[1] = GEN_INT (type_id & 0xFFFF);
+ output_asm_insn ("mov\t%w0, #%1", temp_operands);
+
+ /* Load expected type high 16 bits into w17. */
+ temp_operands[0] = gen_rtx_REG (SImode, R17_REGNUM);
+ temp_operands[1] = GEN_INT ((type_id >> 16) & 0xFFFF);
+ output_asm_insn ("movk\t%w0, #%1, lsl #16", temp_operands);
+
+ /* Compare types. */
+ temp_operands[0] = gen_rtx_REG (SImode, R16_REGNUM);
+ temp_operands[1] = gen_rtx_REG (SImode, R17_REGNUM);
+ output_asm_insn ("cmp\t%w0, %w1", temp_operands);
+
+ /* Output conditional branch to call label. */
+ fputs ("\tb.eq\t", asm_out_file);
+ assemble_name (asm_out_file, call_name);
+ fputc ('\n', asm_out_file);
+
+ /* Output trap label and BRK instruction. */
+ ASM_OUTPUT_LABEL (asm_out_file, trap_name);
+
+ /* Calculate and emit BRK with ESR encoding. */
+ unsigned type_index = R17_REGNUM;
+ unsigned addr_index = REGNO (operands[0]) - R0_REGNUM;
+ unsigned esr_value = 0x8000 | ((type_index & 31) << 5) | (addr_index & 31);
+
+ temp_operands[0] = GEN_INT (esr_value);
+ output_asm_insn ("brk\t#%0", temp_operands);
+
+ /* Output call label. */
+ ASM_OUTPUT_LABEL (asm_out_file, call_name);
+
+ /* Return appropriate call instruction based on SIBLING_CALL_P. */
+ if (SIBLING_CALL_P (insn))
+ return aarch64_indirect_branch_asm (operands[0]);
+ else
+ return aarch64_indirect_call_asm (operands[0]);
+}
+
+#undef TARGET_KCFI_SUPPORTED
+#define TARGET_KCFI_SUPPORTED hook_bool_void_true
+
struct gcc_target targetm = TARGET_INITIALIZER;
#include "gt-aarch64.h"
diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index fedbd4026a06..1a5abc142f50 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -1483,6 +1483,19 @@
}"
)
+;; KCFI indirect call
+(define_insn "*call_insn"
+ [(kcfi (call (mem:DI (match_operand:DI 0 "aarch64_call_insn_operand" "Ucr"))
+ (match_operand 1 "" ""))
+ (match_operand 3 "const_int_operand"))
+ (unspec:DI [(match_operand:DI 2 "const_int_operand")] UNSPEC_CALLEE_ABI)
+ (clobber (reg:DI LR_REGNUM))]
+ "!SIBLING_CALL_P (insn)"
+{
+ return aarch64_output_kcfi_insn (insn, operands);
+}
+ [(set_attr "type" "call")])
+
(define_insn "*call_insn"
[(call (mem:DI (match_operand:DI 0 "aarch64_call_insn_operand"))
(match_operand 1 "" ""))
@@ -1510,6 +1523,20 @@
}"
)
+;; KCFI call with return value
+(define_insn "*call_value_insn"
+ [(set (match_operand 0 "" "")
+ (kcfi (call (mem:DI (match_operand:DI 1 "aarch64_call_insn_operand"
"Ucr"))
+ (match_operand 2 "" ""))
+ (match_operand 4 "const_int_operand")))
+ (unspec:DI [(match_operand:DI 3 "const_int_operand")] UNSPEC_CALLEE_ABI)
+ (clobber (reg:DI LR_REGNUM))]
+ "!SIBLING_CALL_P (insn)"
+{
+ return aarch64_output_kcfi_insn (insn, &operands[1]);
+}
+ [(set_attr "type" "call")])
+
(define_insn "*call_value_insn"
[(set (match_operand 0 "" "")
(call (mem:DI (match_operand:DI 1 "aarch64_call_insn_operand"))
@@ -1550,6 +1577,19 @@
}
)
+;; KCFI sibling call
+(define_insn "*sibcall_insn"
+ [(kcfi (call (mem:DI (match_operand:DI 0 "aarch64_call_insn_operand" "Ucs"))
+ (match_operand 1 ""))
+ (match_operand 3 "const_int_operand"))
+ (unspec:DI [(match_operand:DI 2 "const_int_operand")] UNSPEC_CALLEE_ABI)
+ (return)]
+ "SIBLING_CALL_P (insn)"
+{
+ return aarch64_output_kcfi_insn (insn, operands);
+}
+ [(set_attr "type" "branch")])
+
(define_insn "*sibcall_insn"
[(call (mem:DI (match_operand:DI 0 "aarch64_call_insn_operand" "Ucs, Usf"))
(match_operand 1 ""))
@@ -1558,16 +1598,27 @@
"SIBLING_CALL_P (insn)"
{
if (which_alternative == 0)
- {
- output_asm_insn ("br\\t%0", operands);
- return aarch64_sls_barrier (aarch64_harden_sls_retbr_p ());
- }
+ return aarch64_indirect_branch_asm (operands[0]);
return "b\\t%c0";
}
[(set_attr "type" "branch, branch")
(set_attr "sls_length" "retbr,none")]
)
+;; KCFI sibling call with return value
+(define_insn "*sibcall_value_insn"
+ [(set (match_operand 0 "")
+ (kcfi (call (mem:DI (match_operand:DI 1 "aarch64_call_insn_operand"
"Ucs"))
+ (match_operand 2 ""))
+ (match_operand 4 "const_int_operand")))
+ (unspec:DI [(match_operand:DI 3 "const_int_operand")] UNSPEC_CALLEE_ABI)
+ (return)]
+ "SIBLING_CALL_P (insn)"
+{
+ return aarch64_output_kcfi_insn (insn, &operands[1]);
+}
+ [(set_attr "type" "branch")])
+
(define_insn "*sibcall_value_insn"
[(set (match_operand 0 "")
(call (mem:DI
@@ -1578,10 +1629,7 @@
"SIBLING_CALL_P (insn)"
{
if (which_alternative == 0)
- {
- output_asm_insn ("br\\t%1", operands);
- return aarch64_sls_barrier (aarch64_harden_sls_retbr_p ());
- }
+ return aarch64_indirect_branch_asm (operands[1]);
return "b\\t%c1";
}
[(set_attr "type" "branch, branch")
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index bd84b7dd903f..972e8e76494f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -18425,6 +18425,20 @@ check-call bundle are considered ABI, as the Linux
kernel may
optionally rewrite these areas at boot time to mitigate detected CPU
errata.
+On AArch64, KCFI type identifiers are emitted as a @code{.word ID}
+directive (a 32-bit constant) before the function entry. AArch64's
+natural 4-byte instruction alignment eliminates the need for additional
+alignment NOPs. When used with @option{-fpatchable-function-entry}, the
+type identifier is placed before any prefix NOPs. The runtime check
+uses @code{x16} and @code{x17} as scratch registers. Type mismatches
+trigger a @code{brk} instruction with an immediate value that encodes
+both the expected type register index and the target address register
+index in the format @code{0x8000 | (type_reg << 5) | addr_reg}. This
+encoding is captured in the ESR (Exception Syndrome Register) when the
+trap is taken, allowing the kernel to identify both the KCFI violation
+and the involved registers for detailed diagnostics (eliminating the need
+for a separate @code{.kcfi_traps} section as used on x86_64).
+
KCFI is intended primarily for kernel code and may not be suitable
for user-space applications that rely on techniques incompatible
with strict type checking of indirect calls.
--
2.34.1
