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 <kees@kernel.org>
Cc: Jinjie Ruan <ruanjinjie@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: stable@vger.kernel.org
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")
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
Tested-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Will Deacon <will@kernel.org>

0 files changed, 0 insertions, 0 deletions