linux-kernel - Re: [PATCH 15/20] signal/sparc32: Exit with a fatal signal when try_to_clear_window

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Date:   Thu, 21 Oct 2021 09:34:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Miller <davem@...emloft.net>, sparclinux@...r.kernel.org
Subject: Re: [PATCH 15/20] signal/sparc32: Exit with a fatal signal when
 try_to_clear_window_buffer fails

On Wed, Oct 20, 2021 at 12:44:01PM -0500, Eric W. Biederman wrote:
> The function try_to_clear_window_buffer is only called from
> rtrap_32.c.  After it is called the signal pending state is retested,

nit: rtrap_32.S

> and signals are handled if TIF_SIGPENDING is set.  This allows
> try_to_clear_window_buffer to call force_fatal_signal and then rely on
> the signal being delivered to kill the process, without any danger of
> returning to userspace, or otherwise using possible corrupt state on
> failure.

The TIF_SIGPENDING test happens in do_notify_resume(), though I see
other code before that:

...
        call    try_to_clear_window_buffer
        add    %sp, STACKFRAME_SZ, %o0

        b       signal_p
...
signal_p:
        andcc   %g2, _TIF_DO_NOTIFY_RESUME_MASK, %g0
        bz,a    ret_trap_continue
        ld     [%sp + STACKFRAME_SZ + PT_PSR], %t_psr

        mov     %g2, %o2
        mov     %l6, %o1
        call    do_notify_resume

Will the ret_trap_continue always be skipped?

Also I see the "tp->w_saved = 0" never happens due to the "return" in
try_to_clear_window_buffer. Is that okay? Only synchronize_user_stack()
uses it, and that could be called in do_sigreturn(). Should the "return"
be removed?

-- 
Kees Cook