linux-hardening - Re: [PATCH 1/2] signal: Don't always set SA

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Date:   Thu, 18 Nov 2021 17:13:52 -0800
From:   Kyle Huey <me@...ehuey.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Righi <andrea.righi@...onical.com>,
        Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        linux-hardening@...r.kernel.org,
        "Robert O'Callahan" <rocallahan@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Oliver Sang <oliver.sang@...el.com>, lkp@...ts.01.org,
        kbuild test robot <lkp@...el.com>
Subject: Re: [PATCH 1/2] signal: Don't always set SA_IMMUTABLE for forced signals

On Thu, Nov 18, 2021 at 2:05 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
>
> Recently to prevent issues with SECCOMP_RET_KILL and similar signals
> being changed before they are delivered SA_IMMUTABLE was added.
>
> Unfortunately this broke debuggers[1][2] which reasonably expect to be
> able to trap synchronous SIGTRAP and SIGSEGV even when the target
> process is not configured to handle those signals.
>
> Update force_sig_to_task to support both the case when we can
> allow the debugger to intercept and possibly ignore the
> signal and the case when it is not safe to let userspace
> known about the signal until the process has exited.

s/known/know/

>
> Reported-by: Kyle Huey <me@...ehuey.com>
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Cc: stable@...r.kernel.org
> [1] https://lkml.kernel.org/r/CAP045AoMY4xf8aC_4QU_-j7obuEPYgTcnQQP3Yxk=2X90jtpjw@mail.gmail.com
> [2] https://lkml.kernel.org/r/20211117150258.GB5403@xsang-OptiPlex-902

This link doesn't work.

> Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  kernel/signal.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 7c4b7ae714d4..02058c983bd6 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1298,6 +1298,12 @@ int do_send_sig_info(int sig, struct kernel_siginfo *info, struct task_struct *p
>         return ret;
>  }
>
> +enum sig_handler {
> +       HANDLER_CURRENT, /* If reachable use the current handler */
> +       HANDLER_SIG_DFL, /* Always use SIG_DFL handler semantics */
> +       HANDLER_EXIT,    /* Only visible as the proces exit code */
> +};
> +
>  /*
>   * Force a signal that the process can't ignore: if necessary
>   * we unblock the signal and change any SIG_IGN to SIG_DFL.
> @@ -1310,7 +1316,8 @@ int do_send_sig_info(int sig, struct kernel_siginfo *info, struct task_struct *p
>   * that is why we also clear SIGNAL_UNKILLABLE.
>   */
>  static int
> -force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool sigdfl)
> +force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
> +       enum sig_handler handler)
>  {
>         unsigned long int flags;
>         int ret, blocked, ignored;
> @@ -1321,9 +1328,10 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
>         action = &t->sighand->action[sig-1];
>         ignored = action->sa.sa_handler == SIG_IGN;
>         blocked = sigismember(&t->blocked, sig);
> -       if (blocked || ignored || sigdfl) {
> +       if (blocked || ignored || (handler != HANDLER_CURRENT)) {
>                 action->sa.sa_handler = SIG_DFL;
> -               action->sa.sa_flags |= SA_IMMUTABLE;
> +               if (handler == HANDLER_EXIT)
> +                       action->sa.sa_flags |= SA_IMMUTABLE;
>                 if (blocked) {
>                         sigdelset(&t->blocked, sig);
>                         recalc_sigpending_and_wake(t);
> @@ -1343,7 +1351,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
>
>  int force_sig_info(struct kernel_siginfo *info)
>  {
> -       return force_sig_info_to_task(info, current, false);
> +       return force_sig_info_to_task(info, current, HANDLER_CURRENT);
>  }
>
>  /*
> @@ -1660,7 +1668,7 @@ void force_fatal_sig(int sig)
>         info.si_code = SI_KERNEL;
>         info.si_pid = 0;
>         info.si_uid = 0;
> -       force_sig_info_to_task(&info, current, true);
> +       force_sig_info_to_task(&info, current, HANDLER_SIG_DFL);
>  }
>
>  /*
> @@ -1693,7 +1701,7 @@ int force_sig_fault_to_task(int sig, int code, void __user *addr
>         info.si_flags = flags;
>         info.si_isr = isr;
>  #endif
> -       return force_sig_info_to_task(&info, t, false);
> +       return force_sig_info_to_task(&info, t, HANDLER_CURRENT);
>  }
>
>  int force_sig_fault(int sig, int code, void __user *addr
> @@ -1813,7 +1821,8 @@ int force_sig_seccomp(int syscall, int reason, bool force_coredump)
>         info.si_errno = reason;
>         info.si_arch = syscall_get_arch(current);
>         info.si_syscall = syscall;
> -       return force_sig_info_to_task(&info, current, force_coredump);
> +       return force_sig_info_to_task(&info, current,
> +               force_coredump ? HANDLER_EXIT : HANDLER_CURRENT);
>  }
>
>  /* For the crazy architectures that include trap information in
> --
> 2.20.1

- Kyle