[PATCH v3] arm64: ptrace: Keep 'orig_x0' in-sync with x0 on syscall entry

[Will Deacon <will@kernel.org>]

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