Re: [PATCH 2/2] ptrace: Return the signal to continue with from ptrace_stop

Kees Cook
2022-03-18T17:28:56Z

On Fri, Mar 18, 2022 at 09:52:46AM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
>
> > On Tue, Mar 15, 2022 at 06:22:26PM -0500, Eric W. Biederman wrote:
> >>
> >> The signal a task should continue with after a ptrace stop is
> >> inconsistently read, cleared, and sent. Solve this by reading and
> >> clearing the signal to be sent in ptrace_stop.
> >>
> >> In an ideal world everything except ptrace_signal would share a common
> >> implementation of continuing with the signal, so ptracers could count
> >> on the signal they ask to continue with actually being delivered. For
> >> now retain bug compatibility and just return with the signal number
> >> the ptracer requested the code continue with.
> >>
> >> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> >> ---
> >> include/linux/ptrace.h | 12 ++++++------
> >> kernel/signal.c | 31 ++++++++++++++++++-------------
> >> 2 files changed, 24 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
> >> index 3e6b46e2b7be..15b3d176b6b4 100644
> >> --- a/include/linux/ptrace.h
> >> +++ b/include/linux/ptrace.h
> >> @@ -60,7 +60,7 @@ extern int ptrace_writedata(struct task_struct *tsk, char __user *src, unsigned
> >> extern void ptrace_disable(struct task_struct *);
> >> extern int ptrace_request(struct task_struct *child, long request,
> >> unsigned long addr, unsigned long data);
> >> -extern void ptrace_notify(int exit_code, unsigned long message);
> >> +extern int ptrace_notify(int exit_code, unsigned long message);
> >> [...]
> >> -static void ptrace_stop(int exit_code, int why, int clear_code,
> >> +static int ptrace_stop(int exit_code, int why, int clear_code,
> >> unsigned long message, kernel_siginfo_t *info)
> >> [...]
> >> -static void ptrace_do_notify(int signr, int exit_code, int why, unsigned long message)
> >> +static int ptrace_do_notify(int signr, int exit_code, int why, unsigned long message)
> >> [...]
> >> -void ptrace_notify(int exit_code, unsigned long message)
> >> +int ptrace_notify(int exit_code, unsigned long message)
> >
> > Just for robustness, how about marking the functions that have switched
> > from void to int return as __must_check (or at least just ptrace_notify)?
>
> We can't. There are historical cases that simply don't check if a
> signal should be sent after the function, and they exist for every
> function that is modified.

This seems at least worth documenting with a comment, otherwise we're
just trading one kind of "weirdness" (setting/clearing
current->exit_code) with another (ignoring the signal returned by
ptrace_notify()).

I see only two cases that would need comments:

static inline void ptrace_event(int event, unsigned long message)
{
if (unlikely(ptrace_event_enabled(current, event))) {
ptrace_notify((event << 8) | SIGTRAP, message);
} else if (event == PTRACE_EVENT_EXEC) {
/* legacy EXEC report via SIGTRAP */
if ((current->ptrace & (PT_PTRACED|PT_SEIZED)) == PT_PTRACED)
send_sig(SIGTRAP, current, 0);
}
}

static void signal_delivered(struct ksignal *ksig, int stepping)
{
...
if (stepping)
ptrace_notify(SIGTRAP, 0);
}

--
Kees Cook