[PATCH 4/6] x86_64/entry: Mark all syscalls __noendbr

[Peter Zijlstra]

Since system call dispatch is now strictly through direct calls, there is no
reason for the syscalls to be called indirectly. Mark them all __noendbr.

Notably, the sys_call_table[] is purely a lookup for tracing and the like,
nobody actually (should) dispatch anything through it, and with this we make
sure nobody *can*.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/entry/syscall_32.c            |    5 +++--
 arch/x86/entry/syscall_64.c            |    7 ++++---
 arch/x86/include/asm/syscall_wrapper.h |   23 ++++++++++++-----------
 tools/objtool/check.c                  |    9 ++++++++-
 4 files changed, 27 insertions(+), 17 deletions(-)

--- a/arch/x86/entry/syscall_32.c
+++ b/arch/x86/entry/syscall_32.c
@@ -12,6 +12,7 @@
 #include <asm/traps.h>
 #include <asm/cpufeature.h>
 #include <asm/syscall.h>
+#include <asm/ibt.h>
 
 #ifdef CONFIG_IA32_EMULATION
 #define __SYSCALL_WITH_COMPAT(nr, native, compat)	__SYSCALL(nr, compat)
@@ -19,8 +20,8 @@
 #define __SYSCALL_WITH_COMPAT(nr, native, compat)	__SYSCALL(nr, native)
 #endif
 
-#define __SYSCALL(nr, sym) extern long __ia32_##sym(const struct pt_regs *);
-#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __ia32_##sym(const struct pt_regs *);
+#define __SYSCALL(nr, sym) extern __noendbr long __ia32_##sym(const struct pt_regs *);
+#define __SYSCALL_NORETURN(nr, sym) extern __noendbr long __noreturn __ia32_##sym(const struct pt_regs *);
 #include <asm/syscalls_32.h>
 #undef  __SYSCALL
 
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -8,9 +8,10 @@
 #include <linux/entry-common.h>
 #include <linux/nospec.h>
 #include <asm/syscall.h>
+#include <asm/ibt.h>
 
-#define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
-#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __x64_##sym(const struct pt_regs *);
+#define __SYSCALL(nr, sym) extern __noendbr long __x64_##sym(const struct pt_regs *);
+#define __SYSCALL_NORETURN(nr, sym) extern __noendbr long __noreturn __x64_##sym(const struct pt_regs *);
 #include <asm/syscalls_64.h>
 #ifdef CONFIG_X86_X32_ABI
 #include <asm/syscalls_x32.h>
@@ -25,7 +26,7 @@
  * kernel/trace/trace_syscalls.c still wants to know the system
  * call address.
  */
-#define __SYSCALL(nr, sym) __x64_##sym,
+#define __SYSCALL(nr, sym) (void *)&__x64_##sym,
 const sys_call_ptr_t sys_call_table[] = {
 #include <asm/syscalls_64.h>
 };
--- a/arch/x86/include/asm/syscall_wrapper.h
+++ b/arch/x86/include/asm/syscall_wrapper.h
@@ -7,9 +7,10 @@
 #define _ASM_X86_SYSCALL_WRAPPER_H
 
 #include <asm/ptrace.h>
+#include <asm/ibt.h>
 
-extern long __x64_sys_ni_syscall(const struct pt_regs *regs);
-extern long __ia32_sys_ni_syscall(const struct pt_regs *regs);
+extern __noendbr long __x64_sys_ni_syscall(const struct pt_regs *regs);
+extern __noendbr long __ia32_sys_ni_syscall(const struct pt_regs *regs);
 
 /*
  * Instead of the generic __SYSCALL_DEFINEx() definition, the x86 version takes
@@ -83,22 +84,22 @@ extern long __ia32_sys_ni_syscall(const
 			__MAP(x, __SC_TYPE, __VA_ARGS__))		\
 
 #define __SYS_STUB0(abi, name)						\
-	long __##abi##_##name(const struct pt_regs *regs);		\
+	long __noendbr __##abi##_##name(const struct pt_regs *regs);	\
 	ALLOW_ERROR_INJECTION(__##abi##_##name, ERRNO);			\
-	long __##abi##_##name(const struct pt_regs *regs)		\
+	long __noendbr __##abi##_##name(const struct pt_regs *regs)	\
 		__alias(__do_##name);
 
 #define __SYS_STUBx(abi, name, ...)					\
-	long __##abi##_##name(const struct pt_regs *regs);		\
+	long __noendbr __##abi##_##name(const struct pt_regs *regs);	\
 	ALLOW_ERROR_INJECTION(__##abi##_##name, ERRNO);			\
-	long __##abi##_##name(const struct pt_regs *regs)		\
+	long __noendbr __##abi##_##name(const struct pt_regs *regs)	\
 	{								\
 		return __se_##name(__VA_ARGS__);			\
 	}
 
 #define __COND_SYSCALL(abi, name)					\
-	__weak long __##abi##_##name(const struct pt_regs *__unused);	\
-	__weak long __##abi##_##name(const struct pt_regs *__unused)	\
+	__weak long __noendbr __##abi##_##name(const struct pt_regs *__unused);	\
+	__weak long __noendbr __##abi##_##name(const struct pt_regs *__unused)	\
 	{								\
 		return sys_ni_syscall();				\
 	}
@@ -257,8 +258,8 @@ extern long __ia32_sys_ni_syscall(const
  * For VSYSCALLS, we need to declare these three syscalls with the new
  * pt_regs-based calling convention for in-kernel use.
  */
-long __x64_sys_getcpu(const struct pt_regs *regs);
-long __x64_sys_gettimeofday(const struct pt_regs *regs);
-long __x64_sys_time(const struct pt_regs *regs);
+long __noendbr __x64_sys_getcpu(const struct pt_regs *regs);
+long __noendbr __x64_sys_gettimeofday(const struct pt_regs *regs);
+long __noendbr __x64_sys_time(const struct pt_regs *regs);
 
 #endif /* _ASM_X86_SYSCALL_WRAPPER_H */
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -4672,8 +4672,15 @@ static int validate_ibt(struct objtool_f
 		    !strcmp(sec->name, "__patchable_function_entries"))
 			continue;
 
-		for_each_reloc(sec->rsec, reloc)
+		for_each_reloc(sec->rsec, reloc) {
+			struct symbol *sym = find_symbol_containing(reloc->sec->base,
+								    reloc_offset(reloc));
+
+			if (sym && !strcmp(sym->name, "sys_call_table"))
+				continue;
+
 			warnings += validate_ibt_data_reloc(file, reloc);
+		}
 	}
 
 	return warnings;