[newlib-cygwin] Cygwin: fix potential SEGV in sigwaitinfo/signalfd scenario

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=92115a83a4579635e253be2887d3706d28b477fd

commit 92115a83a4579635e253be2887d3706d28b477fd
Author: Corinna Vinschen <corinna@vinschen.de>
Date:   Mon Aug 12 22:40:50 2019 +0200

    Cygwin: fix potential SEGV in sigwaitinfo/signalfd scenario
    
    On sigwaitinfo or reading from a signalfd, signal processing sets up
    signal handling via sigdelayed even if the handler address is NULL.
    This doesn't have any impact on sigwaitinfo scenarios (or at least, I
    wasn't able to come up with a reproducer) but it breaks signalfd
    scenarios, where eventually a call to call_signal_handler from
    sigdelayed will try to call the NULL function.
    
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/exceptions.cc | 17 ++++++++++++++---
 winsup/cygwin/release/3.1.0 |  3 +++
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/winsup/cygwin/exceptions.cc b/winsup/cygwin/exceptions.cc
index 1765f43..9bdf9f0 100644
--- a/winsup/cygwin/exceptions.cc
+++ b/winsup/cygwin/exceptions.cc
@@ -1628,7 +1628,7 @@ _cygtls::call_signal_handler ()
       if (retaddr () == (__tlsstack_t) sigdelayed)
 	pop ();
 
-      debug_only_printf ("dealing with signal %d", sig);
+      debug_only_printf ("dealing with signal %d, handler %p", sig, func);
       this_sa_flags = sa_flags;
 
       sigset_t this_oldmask = set_process_mask_delta ();
@@ -1647,8 +1647,12 @@ _cygtls::call_signal_handler ()
 
       ucontext_t *thiscontext = NULL, context_copy;
 
-      /* Only make a context for SA_SIGINFO handlers */
-      if (this_sa_flags & SA_SIGINFO)
+      /* Only make a context for SA_SIGINFO handlers, only if a handler
+         exists.  If handler is NULL, drop SA_SIGINFO flag to avoid
+	 accidental context access later in the function. */
+      if (!thisfunc)
+	this_sa_flags &= ~SA_SIGINFO;
+      else if (this_sa_flags & SA_SIGINFO)
 	{
 	  context.uc_link = 0;
 	  context.uc_flags = 0;
@@ -1710,6 +1714,11 @@ _cygtls::call_signal_handler ()
       sig = 0;		/* Flag that we can accept another signal */
       unlock ();	/* unlock signal stack */
 
+      /* Handler may be NUll in sigwaitinfo/signalfd scenario.  Avoid
+	 crashing by calling a NULL function. */
+      if (!thisfunc)
+	goto skip_calling_handler;
+
       /* Alternate signal stack requested for this signal and alternate signal
 	 stack set up for this thread? */
       if (this_sa_flags & SA_ONSTACK
@@ -1817,6 +1826,8 @@ _cygtls::call_signal_handler ()
 	   signal handler. */
 	thisfunc (thissig, &thissi, thiscontext);
 
+skip_calling_handler:
+
       incyg = true;
 
       set_signal_mask (_my_tls.sigmask, (this_sa_flags & SA_SIGINFO)
diff --git a/winsup/cygwin/release/3.1.0 b/winsup/cygwin/release/3.1.0
index ccb63c3..c9cb7c0 100644
--- a/winsup/cygwin/release/3.1.0
+++ b/winsup/cygwin/release/3.1.0
@@ -71,3 +71,6 @@ Bug Fixes
 - 64 bit only: Avoid collisions between memory maps created with shmat
   and Windows datastructures during fork.
   Addresses: https://cygwin.com/ml/cygwin/2019-08/msg00107.html
+
+- Avoid a SEGV after using signalfd.
+  Addresses: https://cygwin.com/ml/cygwin/2019-08/msg00148.html