4.20-stable review patch. If anyone has any objections, please let me know.
------------------
From: Eric W. Biederman ebiederm@xmission.com
commit a692933a87691681e880feb708081681ff32400a upstream.
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 Cc: stable@vger.kernel.org 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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/signal.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)
--- a/kernel/signal.c +++ b/kernel/signal.c @@ -1057,10 +1057,9 @@ static int __send_signal(int sig, struct
result = TRACE_SIGNAL_DELIVERED; /* - * Skip useless siginfo allocation for SIGKILL SIGSTOP, - * and kernel threads. + * 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;
/*