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SubjectRe: [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@xmission.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@kylehuey.com>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Cc: stable@vger.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@xmission.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

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