Commit e057b9477232 ("arm64: syscall: Ensure saved x0 is kept in-sync
with tracer updates") attempted to resolve a long-standing issue with
syscall entry tracing, where a tracer is able to manipulate the first
syscall argument without being subjected to seccomp or audit checking.

Unfortunately, that fix was incomplete [1], as it failed to update
'orig_x0' between a tracer updating x0 during a seccomp ptrace exit
(SECCOMP_RET_TRACE) and the seccomp filter being re-evaluated.

Rather than add hooks to the core seccomp code, instead move the
synchronisation code into the ptrace GPR and syscall setting code so
that 'orig_x0' is kept up to date with x0 whenever we're stopped on the
syscall entry path.

Cc: Kees Cook <kees@kernel.org>
Cc: Jinjie Ruan <ruanjinjie@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://sashiko.dev/#/patchset/20260716120640.6590-1-will@kernel.org [1]
Reported-by: Yiqi Sun <sunyiqixm@gmail.com>
Link: https://lore.kernel.org/all/20260529065444.1336608-1-sunyiqixm@gmail.com/
Fixes: e057b9477232 ("arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates")
Fixes: a5cd110cb836 ("arm64/ptrace: run seccomp after ptrace")
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/arm64/kernel/ptrace.c | 50 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

v2: https://lore.kernel.org/r/20260716120640.6590-1-will@kernel.org

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 4d08598e2891..2b716fa11c02 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -560,6 +560,42 @@ static int gpr_get(struct task_struct *target,
 	return membuf_write(&to, uregs, sizeof(*uregs));
 }
 
+static void update_syscall_orig_x0_after_ptrace(struct task_struct *target)
+{
+	struct pt_regs *regs = task_pt_regs(target);
+	struct kernel_siginfo *info = target->last_siginfo;
+
+	/* We should only be called when target is in a ptrace stop */
+	if (WARN_ON_ONCE(!info))
+		return;
+
+	/*
+	 * Skip the update for NO_SYSCALL (set either by the user or the
+	 * tracer), as regs[0] holds the return value (see the comment in
+	 * el0_svc_common()) and can be unwound using syscall_rollback().
+	 */
+	if (regs->syscallno == NO_SYSCALL)
+		return;
+
+	/*
+	 * For compat tasks, orig_r0 is provided directly through GPR index
+	 * 17.
+	 */
+	if (is_compat_thread(task_thread_info(target)))
+		return;
+
+	/*
+	 * Don't update orig_x0 for a syscall-exit-stop, as x0 now contains the
+	 * return value of the system call.
+	 */
+	if ((info->si_code & ~0x80) == SIGTRAP &&
+	    target->ptrace_message == PTRACE_EVENTMSG_SYSCALL_EXIT) {
+		return;
+	}
+
+	regs->orig_x0 = regs->regs[0];
+}
+
 static int gpr_set(struct task_struct *target, const struct user_regset *regset,
 		   unsigned int pos, unsigned int count,
 		   const void *kbuf, const void __user *ubuf)
@@ -575,6 +611,14 @@ static int gpr_set(struct task_struct *target, const struct user_regset *regset,
 		return -EINVAL;
 
 	task_pt_regs(target)->user_regs = newregs;
+
+	/*
+	 * Keep orig_x0 authoritative so that seccomp (via
+	 * syscall_get_arguments()), audit and the restart path all see the same
+	 * first argument the syscall is dispatched with, even if it has been
+	 * updated by a tracer.
+	 */
+	update_syscall_orig_x0_after_ptrace(target);
 	return 0;
 }
 
@@ -753,6 +797,12 @@ static int system_call_set(struct task_struct *target,
 		return ret;
 
 	task_pt_regs(target)->syscallno = syscallno;
+
+	/*
+	 * Re-sync orig_x0 in case the syscall number has been changed
+	 * from NO_SYSCALL.
+	 */
+	update_syscall_orig_x0_after_ptrace(target);
 	return ret;
 }
 
-- 
2.55.0.229.g6434b31f56-goog

On 7/18/2026 2:27 AM, Will Deacon wrote:
> Commit e057b9477232 ("arm64: syscall: Ensure saved x0 is kept in-sync
> with tracer updates") attempted to resolve a long-standing issue with
> syscall entry tracing, where a tracer is able to manipulate the first
> syscall argument without being subjected to seccomp or audit checking.
> 
> Unfortunately, that fix was incomplete [1], as it failed to update
> 'orig_x0' between a tracer updating x0 during a seccomp ptrace exit
> (SECCOMP_RET_TRACE) and the seccomp filter being re-evaluated.
> 
> Rather than add hooks to the core seccomp code, instead move the
> synchronisation code into the ptrace GPR and syscall setting code so
> that 'orig_x0' is kept up to date with x0 whenever we're stopped on the
> syscall entry path.
> 
> Cc: Kees Cook <kees@kernel.org>
> Cc: Jinjie Ruan <ruanjinjie@huawei.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Link: https://sashiko.dev/#/patchset/20260716120640.6590-1-will@kernel.org [1]
> Reported-by: Yiqi Sun <sunyiqixm@gmail.com>
> Link: https://lore.kernel.org/all/20260529065444.1336608-1-sunyiqixm@gmail.com/
> Fixes: e057b9477232 ("arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates")
> Fixes: a5cd110cb836 ("arm64/ptrace: run seccomp after ptrace")
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kernel/ptrace.c | 50 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
> 
> v2: https://lore.kernel.org/r/20260716120640.6590-1-will@kernel.org
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 4d08598e2891..2b716fa11c02 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -560,6 +560,42 @@ static int gpr_get(struct task_struct *target,
>  	return membuf_write(&to, uregs, sizeof(*uregs));
>  }
>  
> +static void update_syscall_orig_x0_after_ptrace(struct task_struct *target)
> +{
> +	struct pt_regs *regs = task_pt_regs(target);
> +	struct kernel_siginfo *info = target->last_siginfo;
> +
> +	/* We should only be called when target is in a ptrace stop */
> +	if (WARN_ON_ONCE(!info))
> +		return;
> +
> +	/*
> +	 * Skip the update for NO_SYSCALL (set either by the user or the
> +	 * tracer), as regs[0] holds the return value (see the comment in
> +	 * el0_svc_common()) and can be unwound using syscall_rollback().
> +	 */
> +	if (regs->syscallno == NO_SYSCALL)
> +		return;
> +
> +	/*
> +	 * For compat tasks, orig_r0 is provided directly through GPR index
> +	 * 17.
> +	 */
> +	if (is_compat_thread(task_thread_info(target)))
> +		return;
> +
> +	/*
> +	 * Don't update orig_x0 for a syscall-exit-stop, as x0 now contains the
> +	 * return value of the system call.
> +	 */
> +	if ((info->si_code & ~0x80) == SIGTRAP &&
> +	    target->ptrace_message == PTRACE_EVENTMSG_SYSCALL_EXIT) {
> +		return;
> +	}
> +
> +	regs->orig_x0 = regs->regs[0];
> +}
> +
>  static int gpr_set(struct task_struct *target, const struct user_regset *regset,
>  		   unsigned int pos, unsigned int count,
>  		   const void *kbuf, const void __user *ubuf)
> @@ -575,6 +611,14 @@ static int gpr_set(struct task_struct *target, const struct user_regset *regset,
>  		return -EINVAL;
>  
>  	task_pt_regs(target)->user_regs = newregs;
> +
> +	/*
> +	 * Keep orig_x0 authoritative so that seccomp (via
> +	 * syscall_get_arguments()), audit and the restart path all see the same
> +	 * first argument the syscall is dispatched with, even if it has been
> +	 * updated by a tracer.
> +	 */
> +	update_syscall_orig_x0_after_ptrace(target);

Tested-by: Jinjie Ruan <ruanjinjie@huawei.com>

>  	return 0;
>  }
>  
> @@ -753,6 +797,12 @@ static int system_call_set(struct task_struct *target,
>  		return ret;
>  
>  	task_pt_regs(target)->syscallno = syscallno;
> +
> +	/*
> +	 * Re-sync orig_x0 in case the syscall number has been changed
> +	 * from NO_SYSCALL.
> +	 */
> +	update_syscall_orig_x0_after_ptrace(target);
>  	return ret;
>  }
>