Revert "arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates"

This reverts commit e057b94772328221405b067c3a85fe479b915dc8.

Sashiko points out that updating 'orig_x0' after secure_computing()
has returned is too late to handle the case where a seccomp filter is
re-evaluated after initially returning SECCOMP_RET_TRACE. This means
that a tracer can manipulate the first argument of the syscall behind
seccomp's back.

For now, revert the initial fix and we'll have another crack at it soon.
Since the incorrect fix was cc'd to stable, do the same here with an
appropriate fixes tag.

Cc: stable@vger.kernel.org
Fixes: e057b9477232 ("arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates")
Link: https://sashiko.dev/#/patchset/20260716120640.6590-1-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 390c9b2bd96627605d3d1ec7eaf9e153d7e666f5..4d08598e2891d3b1a8d56b3a7d3ff963f3acc9b6 100644 (file)
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -2408,21 +2408,6 @@ 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();
@@ -2432,26 +2417,12 @@ 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);