[PATCH] signal: Always attempt to allocate siginfo for SIGSTOP

[ebiederm@xmission.com (Eric W. Biederman)]

Since 2.5.34 the code has had the potential to not allocate siginfo
for SIGSTOP signals.  Except for ptrace this is perfectly fine as only
ptrace can use PTRACE_PEEK_SIGINFO and see what the contents of
the delivered siginfo are.

Users of PTRACE_PEEK_SIGINFO that care about the contents siginfo
for SIGSTOP are rare, but they do exist.  A seccomp self test
has cared and lldb cares.

Jack Andersen <jackoalan@gmail.com> writes:

> The patch titled
> `signal: Never allocate siginfo for SIGKILL or SIGSTOP`
> created a regression for users of PTRACE_GETSIGINFO needing to
> discern signals that were raised via the tgkill syscall.
>
> A notable user of this tgkill+ptrace combination is lldb while
> debugging a multithreaded program. Without the ability to detect a
> SIGSTOP originating from tgkill, lldb does not have a way to
> synchronize on a per-thread basis and falls back to SIGSTOP-ing the
> entire process.

Everyone affected by this please note.  The kernel can still fail to
allocate a siginfo structure.  The allocation is with GFP_KERNEL and
is best effort only.  If memory is tight when the signal allocation
comes in this will fail to allocate a siginfo.

So I strongly recommend looking at more robust solutions for
synchronizing with a single thread such as PTRACE_INTERRUPT.  Or if
that does not work persuading your friendly local kernel developer to
build the interface you need.

Reported-by: Tycho Andersen <tycho@tycho.ws>
Reported-by: Kees Cook <keescook@chromium.org>
Reported-by: Jack Andersen <jackoalan@gmail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Christian Brauner <christian@brauner.io>
Fixes: f149b3155744 ("signal: Never allocate siginfo for SIGKILL or SIGSTOP")
Fixes: 6dfc88977e42 ("[PATCH] shared thread signals")
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---

Unless someone objects I will drop this in my tree for linux-next and
send Linus a pull request in a couple of days.

 kernel/signal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index e1d7ad8e6ab1..45298b3a8ffc 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1057,10 +1057,10 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
 
 	result = TRACE_SIGNAL_DELIVERED;
 	/*
-	 * Skip useless siginfo allocation for SIGKILL SIGSTOP,
+	 * Skip useless siginfo allocation for SIGKILL,
 	 * and kernel threads.
 	 */
-	if (sig_kernel_only(sig) || (t->flags & PF_KTHREAD))
+	if ((sig == SIGKILL) || (t->flags & PF_KTHREAD))
 		goto out_set;
 
 	/*
-- 
2.17.1