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>

diff --git a/kernel/compat.c b/kernel/compat.c
index 74ff8498809a0af4b60c9df461825f80c5007260..d2c67aa49ae667f85d41e2616ece39654ba9d0b3 100644 (file)
--- a/kernel/compat.c
+++ b/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,
-               compat_old_sigset_t __user *oset)
+/*
+ * 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)
 {
-       old_sigset_t s;
-       long ret;
-       mm_segment_t old_fs;
+       memcpy(blocked->sig, &set, sizeof(set));
+}
 
-       if (set && get_user(s, set))
-               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;
+asmlinkage long compat_sys_sigprocmask(int how,
+                                      compat_old_sigset_t __user *nset,
+                                      compat_old_sigset_t __user *oset)
+{
+       old_sigset_t old_set, new_set;
+       sigset_t new_blocked;
+
+       old_set = current->blocked.sig[0];
+
+       if (nset) {
+               if (get_user(new_set, nset))
+                       return -EFAULT;
+               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