Re: [PATCH v2] arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates

[Jinjie Ruan <ruanjinjie@huawei.com>]

On 7/16/2026 8:06 PM, Will Deacon wrote:
> When seccomp support was originally added to arm64 in a1ae65b21941
> ("arm64: add seccomp support"), seccomp was erroneously called _before_
> the ptrace syscall-enter-stop and therefore the tracer could trivially
> manipulate the syscall register state after the seccomp check had
> passed. This was subsequently fixed in a5cd110cb836 ("arm64/ptrace: run
> seccomp after ptrace") by moving the seccomp check after the tracer has
> run. Unfortunately, a decade later, that fix has been reported to be
> incomplete.
> 
> On arm64, both the first argument to a syscall and its eventual return
> value are allocated to register x0. In order to facilitate syscall
> restarting and querying of syscall arguments on the syscall exit path,
> the original value of x0 is stashed in 'struct pt_regs::orig_x0' early
> during the syscall entry path and is returned for the first argument by
> syscall_get_arguments(). Unlike 32-bit Arm, this stashed value is not
> directly exposed via ptrace() and so changes to register x0 made by the
> tracer on a syscall-enter-stop are not reflected in 'orig_x0'. This
> means that seccomp, syscall tracepoints and audit can observe a stale
> value for the register compared to the argument that will be observed by
> the actual syscall.
> 
> Re-sync 'orig_x0' from x0 on the syscall entry path following a
> potential ptrace stop (i.e. PTRACE_EVENTMSG_SYSCALL_ENTRY or
> SECCOMP_RET_TRACE). This behaviour is limited to native tasks (because
> compat tasks expose 'orig_r0' to ptrace) where the syscall is not being
> skipped (because x0 is updated to hold the return value of -ENOSYS in
> that case).
> 
> Cc: Kees Cook <kees@kernel.org>
> Cc: Jinjie Ruan <ruanjinjie@huawei.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Reported-by: Yiqi Sun <sunyiqixm@gmail.com>
> Link: https://lore.kernel.org/all/20260529065444.1336608-1-sunyiqixm@gmail.com/
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Fixes: a5cd110cb836 ("arm64/ptrace: run seccomp after ptrace")
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> 
> Changes since v1 (https://lore.kernel.org/r/20260714143600.23853-1-will@kernel.org):
>   * Reworded and expanded comments and commit message per Jinjie's
>     feedback
> 
>  arch/arm64/kernel/ptrace.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 4d08598e2891..390c9b2bd966 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -2408,6 +2408,21 @@ static void report_syscall_exit(struct pt_regs *regs)
>  	}
>  }
>  
> +static void update_syscall_orig_x0_after_ptrace(struct pt_regs *regs)
> +{
> +	/*
> +	 * 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. Skip this 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().
> +	 * For compat tasks, orig_r0 is provided directly through GPR index 17.
> +	 */
> +	if (!is_compat_task() && regs->syscallno != NO_SYSCALL)
> +		regs->orig_x0 = regs->regs[0];
> +}
> +
>  int syscall_trace_enter(struct pt_regs *regs)
>  {
>  	unsigned long flags = read_thread_flags();
> @@ -2417,12 +2432,26 @@ int syscall_trace_enter(struct pt_regs *regs)
>  		ret = report_syscall_entry(regs);
>  		if (ret || (flags & _TIF_SYSCALL_EMU))
>  			return NO_SYSCALL;
> +
> +		/*
> +		 * Ensure ptrace changes to x0 during a regular
> +		 * syscall-enter-stop (PTRACE_SYSCALL) are visible to
> +		 * subsequent seccomp checks, tracepoints and audit.
> +		 */
> +		update_syscall_orig_x0_after_ptrace(regs);
>  	}
>  
>  	/* Do the secure computing after ptrace; failures should be fast. */
>  	if (secure_computing() == -1)
>  		return NO_SYSCALL;
>  
> +	/*
> +	 * Ensure tracer changes to x0 during seccomp ptrace exit
> +	 * processing (SECCOMP_RET_TRACE) are visible to tracepoints and
> +	 * audit.
> +	 */
> +	update_syscall_orig_x0_after_ptrace(regs);

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

> +
>  	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
>  		trace_sys_enter(regs, regs->syscallno);
>