linux-kernel - Re: [PATCH v2] x86/syscall: make sys

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Message-ID: <CAMzpN2gwvSasZgTGPon1gwYwBMykHSn44vQts0jfLx+SmwmifQ@mail.gmail.com>
Date: Sat, 20 Dec 2025 08:50:31 -0500
From: Brian Gerst <brgerst@...il.com>
To: ellyndra <ellyesparza8@...il.com>
Cc: linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] x86/syscall: make sys_call helpers static

On Thu, Dec 18, 2025 at 10:01 AM ellyndra <ellyesparza8@...il.com> wrote:
>
> 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@...radead.org>
> Signed-off-by: Elly I Esparza <ellyesparza8@...il.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
>
>

The main benefit of having the switch in a discrete function is that
it allows the compiler to do tail-call optimizations, reducing text
size.  Isn't it possible to blacklist functions from kprobes?


Brian Gerst