From 88b839ce497ccb1ff92f7ae742c78dd2937ba572 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Thu, 30 Jul 2026 14:26:50 +0100 Subject: [PATCH] arm64: ptrace: Keep 'orig_x0' in-sync with x0 on syscall entry 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 Cc: Jinjie Ruan Cc: Mark Rutland Link: https://sashiko.dev/#/patchset/20260716120640.6590-1-will@kernel.org [1] Reported-by: Yiqi Sun 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") Tested-by: Jinjie Ruan Signed-off-by: Will Deacon --- arch/arm64/kernel/ptrace.c | 50 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+) diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c index 2a72c61a8af9..4955166723b8 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; + + /* + * 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; + + /* We should only be called when target is in a ptrace stop */ + if (WARN_ON_ONCE(!info)) + 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; }