compel: gcs: initial GCS support for signal frames

Add basic prerequisites for Guarded Control Stack (GCS) state on AArch64.

This adds a gcs_context to the signal frame and extends user_fpregs_struct_t to
carry GCS metadata, preparing the groundwork for GCS in the parasite.

For now, the GCS fields are zeroed during compel_get_task_regs(), technically
ignoring GCS since it does not reach the control logic yet; that will be
introduced in the next commit.

The code path is gated and does not affect normal tests. Can be explicitly
enabled and tested via:

    make -C infect GCS_ENABLE=1 && make -C infect run

Signed-off-by: Igor Svilenkov Bozic <svilenkov@gmail.com>
[ alex: clean up fixes ]
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Acked-by: Mike Rapoport <rppt@kernel.org>

compel/arch/aarch64/src/lib/include/uapi/asm/infect-types.h

@@ -2,6 +2,7 @@
 #define UAPI_COMPEL_ASM_TYPES_H__
 
 #include <stdint.h>
+#include <stdbool.h>
 #include <signal.h>
 #include <sys/mman.h>
 #include <asm/ptrace.h>
@@ -17,8 +18,18 @@
 
 typedef struct user_pt_regs user_regs_struct_t;
 
+/*
+ * GCS (Guarded Control Stack)
+ */
+struct user_gcs {
+	__u64 features_enabled;
+	__u64 features_locked;
+	__u64 gcspr_el0;
+};
+
 struct user_fpregs_struct {
 	struct user_fpsimd_state fpstate;
+	struct user_gcs gcs;
 };
 typedef struct user_fpregs_struct user_fpregs_struct_t;
 
@@ -43,4 +54,7 @@ typedef struct user_fpregs_struct user_fpregs_struct_t;
 		__NR_##syscall; \
 	})
 
+extern bool __compel_host_supports_gcs(void);
+#define compel_host_supports_gcs __compel_host_supports_gcs
+
 #endif /* UAPI_COMPEL_ASM_TYPES_H__ */

compel/arch/aarch64/src/lib/include/uapi/asm/sigframe.h

@@ -10,11 +10,20 @@
 /* Copied from the kernel header arch/arm64/include/uapi/asm/sigcontext.h */
 
 #define FPSIMD_MAGIC 0x46508001
+#define GCS_MAGIC    0x47435300
 
 typedef struct fpsimd_context fpu_state_t;
 
+struct gcs_context {
+	struct _aarch64_ctx head;
+	__u64 gcspr;
+	__u64 features_enabled;
+	__u64 reserved;
+};
+
 struct aux_context {
 	struct fpsimd_context fpsimd;
+	struct gcs_context gcs;
 	/* additional context to be added before "end" */
 	struct _aarch64_ctx end;
 };
@@ -63,6 +72,7 @@ struct cr_sigcontext {
 #define RT_SIGFRAME_AUX_CONTEXT(rt_sigframe) ((struct aux_context *)&(RT_SIGFRAME_SIGCONTEXT(rt_sigframe)->__reserved))
 #define RT_SIGFRAME_FPU(rt_sigframe)	     (&RT_SIGFRAME_AUX_CONTEXT(rt_sigframe)->fpsimd)
 #define RT_SIGFRAME_OFFSET(rt_sigframe)	     0
+#define RT_SIGFRAME_GCS(rt_sigframe)	     (&RT_SIGFRAME_AUX_CONTEXT(rt_sigframe)->gcs)
 
 #define rt_sigframe_erase_sigset(sigframe)	memset(&sigframe->uc.uc_sigmask, 0, sizeof(k_rtsigset_t))
 #define rt_sigframe_copy_sigset(sigframe, from) memcpy(&sigframe->uc.uc_sigmask, from, sizeof(k_rtsigset_t))

compel/arch/aarch64/src/lib/infect.c

@@ -2,8 +2,8 @@
 #include <sys/ptrace.h>
 #include <sys/types.h>
 #include <sys/uio.h>
+#include <sys/auxv.h>
 #include <asm/ptrace.h>
-#include <linux/elf.h>
 
 #include <compel/plugins/std/syscall-codes.h>
 #include "common/page.h"
@@ -13,6 +13,8 @@
 #include "infect.h"
 #include "infect-priv.h"
 #include "asm/breakpoints.h"
+#include "asm/gcs-types.h"
+#include <linux/prctl.h>
 
 unsigned __page_size = 0;
 unsigned __page_shift = 0;
@@ -33,12 +35,32 @@ static inline void __always_unused __check_code_syscall(void)
 	BUILD_BUG_ON(!is_log2(sizeof(code_syscall)));
 }
 
+bool __compel_host_supports_gcs(void)
+{
+	unsigned long hwcap = getauxval(AT_HWCAP);
+	return (hwcap & HWCAP_GCS) != 0;
+}
+
+static bool __compel_gcs_enabled(struct user_gcs *gcs)
+{
+	if (!compel_host_supports_gcs())
+		return false;
+
+	if (gcs->features_enabled & PR_SHADOW_STACK_ENABLE)
+		return true;
+
+	return false;
+}
+
 int sigreturn_prep_regs_plain(struct rt_sigframe *sigframe, user_regs_struct_t *regs, user_fpregs_struct_t *fpregs)
 {
 	struct fpsimd_context *fpsimd = RT_SIGFRAME_FPU(sigframe);
+	struct gcs_context *gcs = RT_SIGFRAME_GCS(sigframe);
 
 	memcpy(sigframe->uc.uc_mcontext.regs, regs->regs, sizeof(regs->regs));
 
+	pr_debug("sigreturn_prep_regs_plain: sp %lx pc %lx\n", (long)regs->sp, (long)regs->pc);
+
 	sigframe->uc.uc_mcontext.sp = regs->sp;
 	sigframe->uc.uc_mcontext.pc = regs->pc;
 	sigframe->uc.uc_mcontext.pstate = regs->pstate;
@@ -51,6 +73,19 @@ int sigreturn_prep_regs_plain(struct rt_sigframe *sigframe, user_regs_struct_t *
 	fpsimd->head.magic = FPSIMD_MAGIC;
 	fpsimd->head.size = sizeof(*fpsimd);
 
+	if (__compel_gcs_enabled(&fpregs->gcs)) {
+		gcs->head.magic = GCS_MAGIC;
+		gcs->head.size = sizeof(*gcs);
+		gcs->reserved = 0;
+		gcs->gcspr = fpregs->gcs.gcspr_el0 - 8;
+		gcs->features_enabled = fpregs->gcs.features_enabled;
+
+		pr_debug("sigframe gcspr=%llx features_enabled=%llx\n", fpregs->gcs.gcspr_el0 - 8, fpregs->gcs.features_enabled);
+	} else {
+		pr_debug("sigframe gcspr=[disabled]\n");
+		memset(gcs, 0, sizeof(*gcs));
+	}
+
 	return 0;
 }
 
@@ -81,6 +116,8 @@ int compel_get_task_regs(pid_t pid, user_regs_struct_t *regs, user_fpregs_struct
 		goto err;
 	}
 
+	memset(&ext_regs->gcs, 0, sizeof(ext_regs->gcs));
+
 	ret = save(pid, arg, regs, ext_regs);
 err:
 	return ret;

compel/include/uapi/infect.h

@@ -192,6 +192,14 @@ void compel_set_thread_ip(struct parasite_thread_ctl *tctl, uint64_t v);
 
 extern void compel_get_stack(struct parasite_ctl *ctl, void **rstack, void **r_thread_stack);
 
+#ifndef compel_host_supports_gcs
+static inline bool compel_host_supports_gcs(void)
+{
+	return false;
+}
+#define compel_host_supports_gcs
+#endif
+
 #ifndef compel_shstk_enabled
 static inline bool compel_shstk_enabled(user_fpregs_struct_t *ext_regs)
 {