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

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

[Will Deacon]

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 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>
---
 arch/arm64/kernel/ptrace.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 4d08598e2891..57e8c6714d44 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 +2432,21 @@ 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 are visible to seccomp
+		 * ptrace exits (SECCOMP_RET_TRACE).
+		 */
+		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 seccomp updates to x0 are visible to audit. */
+	update_syscall_orig_x0_after_ptrace(regs);
+
 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
 		trace_sys_enter(regs, regs->syscallno);
 
-- 
2.55.0.795.g602f6c329a-goog