compat: Fix RT signal mask corruption via sigprocmask

compat_sys_sigprocmask reads a smaller signal mask from userspace than
sigprogmask accepts for setting.  So the high word of blocked.sig[0]
will be cleared, releasing any potentially blocked RT signal.

This was discovered via userspace code that relies on get/setcontext.
glibc's i386 versions of those functions use sigprogmask instead of
rt_sigprogmask to save/restore signal mask and caused RT signal
unblocking this way.

As suggested by Linus, this replaces the sys_sigprocmask based compat
version with one that open-codes the required logic, including the merge
of the existing blocked set with the new one provided on SIG_SETMASK.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/compat.c

@@ -372,25 +372,54 @@ asmlinkage long compat_sys_sigpending(compat_old_sigset_t __user *set)
 
 #ifdef __ARCH_WANT_SYS_SIGPROCMASK
 
-asmlinkage long compat_sys_sigprocmask(int how, compat_old_sigset_t __user *set,
+/*
+ * sys_sigprocmask SIG_SETMASK sets the first (compat) word of the
+ * blocked set of signals to the supplied signal set
+ */
+static inline void compat_sig_setmask(sigset_t *blocked, compat_sigset_word set)
+{
+	memcpy(blocked->sig, &set, sizeof(set));
+}
+
+asmlinkage long compat_sys_sigprocmask(int how,
+				       compat_old_sigset_t __user *nset,
 				       compat_old_sigset_t __user *oset)
 {
-	old_sigset_t s;
-	long ret;
-	mm_segment_t old_fs;
+	old_sigset_t old_set, new_set;
+	sigset_t new_blocked;
+
+	old_set = current->blocked.sig[0];
 
-	if (set && get_user(s, set))
+	if (nset) {
+		if (get_user(new_set, nset))
 			return -EFAULT;
-	old_fs = get_fs();
-	set_fs(KERNEL_DS);
-	ret = sys_sigprocmask(how,
-			      set ? (old_sigset_t __user *) &s : NULL,
-			      oset ? (old_sigset_t __user *) &s : NULL);
-	set_fs(old_fs);
-	if (ret == 0)
-		if (oset)
-			ret = put_user(s, oset);
-	return ret;
+		new_set &= ~(sigmask(SIGKILL) | sigmask(SIGSTOP));
+
+		new_blocked = current->blocked;
+
+		switch (how) {
+		case SIG_BLOCK:
+			sigaddsetmask(&new_blocked, new_set);
+			break;
+		case SIG_UNBLOCK:
+			sigdelsetmask(&new_blocked, new_set);
+			break;
+		case SIG_SETMASK:
+			compat_sig_setmask(&new_blocked, new_set);
+			break;
+		default:
+			return -EINVAL;
+		}
+
+		set_current_blocked(&new_blocked);
+	}
+
+	if (oset) {
+		if (put_user(old_set, oset))
+			return -EFAULT;
+	}
+
+	return 0;
 }
 
 #endif