From e057b94772328221405b067c3a85fe479b915dc8 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Thu, 16 Jul 2026 13:06:39 +0100 Subject: [PATCH] arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates 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 Cc: Jinjie Ruan Cc: Mark Rutland Cc: stable@vger.kernel.org Reported-by: Yiqi Sun Link: https://lore.kernel.org/all/20260529065444.1336608-1-sunyiqixm@gmail.com/ Suggested-by: Catalin Marinas Fixes: a5cd110cb836 ("arm64/ptrace: run seccomp after ptrace") Reviewed-by: Jinjie Ruan Tested-by: Jinjie Ruan Signed-off-by: Will Deacon --- 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); + if (test_thread_flag(TIF_SYSCALL_TRACEPOINT)) trace_sys_enter(regs, regs->syscallno);