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 <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
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")
Tested-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 2a72c61a8af9c8ac1580eb46ab48c656e19c64e1..4955166723b88a7cb5210d158ceb11e93cc19628 100644 (file)
--- 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;
 }