[PATCH v2] x86/syscall: make sys_call helpers static

[ellyndra <ellyesparza8@gmail.com>]

Make 'x64_sys_call()' 'x32_sys_call()' and 'ia32_sys_call()' static to
prevent them from being resolved via kallsyms and by extension kprobes.

These functions contain the final calls to the syscall handlers, having
access to their address can be used to hook this calls altering their
behavior. Preventing symbol visibility limits the area of such attacks.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Elly I Esparza <ellyesparza8@gmail.com>
---
 arch/x86/entry/syscall_32.c    |  6 ++++--
 arch/x86/entry/syscall_64.c    | 12 ++++++++----
 arch/x86/include/asm/syscall.h |  8 --------
 3 files changed, 12 insertions(+), 14 deletions(-)

diff --git a/arch/x86/entry/syscall_32.c b/arch/x86/entry/syscall_32.c
index 2b15ea17bb7c..28c9a99fb2de 100644
--- a/arch/x86/entry/syscall_32.c
+++ b/arch/x86/entry/syscall_32.c
@@ -41,7 +41,7 @@ const sys_call_ptr_t sys_call_table[] = {
 #endif
 
 #define __SYSCALL(nr, sym) case nr: return __ia32_##sym(regs);
-long ia32_sys_call(const struct pt_regs *regs, unsigned int nr)
+static __always_inline long ia32_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
 	switch (nr) {
 	#include <asm/syscalls_32.h>
@@ -70,7 +70,7 @@ early_param("ia32_emulation", ia32_emulation_override_cmdline);
 /*
  * Invoke a 32-bit syscall.  Called with IRQs on in CT_STATE_KERNEL.
  */
-static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, int nr)
+static noinstr void do_syscall_32_irqs_on(struct pt_regs *regs, int nr)
 {
 	/*
 	 * Convert negative numbers to very high and thus out of range
@@ -78,12 +78,14 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, int nr)
 	 */
 	unsigned int unr = nr;
 
+	instrumentation_begin();
 	if (likely(unr < IA32_NR_syscalls)) {
 		unr = array_index_nospec(unr, IA32_NR_syscalls);
 		regs->ax = ia32_sys_call(regs, unr);
 	} else if (nr != -1) {
 		regs->ax = __ia32_sys_ni_syscall(regs);
 	}
+	instrumentation_end();
 }
 
 #ifdef CONFIG_IA32_EMULATION
diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index b6e68ea98b83..55ad274f57f5 100644
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -32,7 +32,7 @@ const sys_call_ptr_t sys_call_table[] = {
 #undef  __SYSCALL
 
 #define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
-long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
+static __always_inline long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
 	switch (nr) {
 	#include <asm/syscalls_64.h>
@@ -40,15 +40,17 @@ long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
 	}
 }
 
-#ifdef CONFIG_X86_X32_ABI
-long x32_sys_call(const struct pt_regs *regs, unsigned int nr)
+static __always_inline long x32_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
+#ifdef CONFIG_X86_X32_ABI
 	switch (nr) {
 	#include <asm/syscalls_x32.h>
 	default: return __x64_sys_ni_syscall(regs);
 	}
-}
+#else
+	return __x64_sys_ni_syscall(regs);
 #endif
+}
 
 static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
 {
@@ -58,11 +60,13 @@ static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
 	 */
 	unsigned int unr = nr;
 
+	instrumentation_begin();
 	if (likely(unr < NR_syscalls)) {
 		unr = array_index_nospec(unr, NR_syscalls);
 		regs->ax = x64_sys_call(regs, unr);
 		return true;
 	}
+	instrumentation_end();
 	return false;
 }
 
diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index c10dbb74cd00..59a406074dc0 100644
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -20,14 +20,6 @@
 typedef long (*sys_call_ptr_t)(const struct pt_regs *);
 extern const sys_call_ptr_t sys_call_table[];
 
-/*
- * These may not exist, but still put the prototypes in so we
- * can use IS_ENABLED().
- */
-extern long ia32_sys_call(const struct pt_regs *, unsigned int nr);
-extern long x32_sys_call(const struct pt_regs *, unsigned int nr);
-extern long x64_sys_call(const struct pt_regs *, unsigned int nr);
-
 /*
  * Only the low 32 bits of orig_ax are meaningful, so we return int.
  * This importantly ignores the high bits on 64-bit, so comparisons
-- 
2.43.0