Re: [PATCH RESEND] kprobes: Blacklist risky functions from being probed

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Dec 04, 2025 at 03:50:13PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 04, 2025 at 11:41:41AM -0300, ellyndra wrote:
> > From: Elly I Esparza <ellyesparza8@gmail.com>
> > 
> > Blacklist 'x64_sys_call()' from being kprobed to prevent syscall hooking
> > techniques that overwrite the content of a 'case' block inside the main
> > syscall dispatch switch statement.
> > 
> > Also blacklist 'kallsyms_lookup_name()' to prevent a potential bypass
> > of the blacklist, since this function can be used to discover and target
> > arbitrary kernel symbols.
> > 
> > Add a Kconfig option under security/ to enable or disable this feature.
> > 
> > Signed-off-by: Elly I Esparza <ellyesparza8@gmail.com>
> > ---
> 
> I'd be okay doing this unconditionally. Pretty much everything else
> until that point lives in noinstr which is already excluded from probes.

How about something like this (completely untested) instead?

---
 arch/x86/entry/syscall_32.c    |  6 ++++--
 arch/x86/entry/syscall_64.c    | 10 ++++++----
 arch/x86/include/asm/syscall.h |  8 --------
 3 files changed, 10 insertions(+), 14 deletions(-)

diff --git a/arch/x86/entry/syscall_32.c b/arch/x86/entry/syscall_32.c
index a67a644d0cfe..6451550df027 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..ce9ded561354 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)
 {
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