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>

1 files changed, 46 insertions, 17 deletions

diff --git a/kernel/compat.c b/kernel/compat.c
index 74ff8498809a..d2c67aa49ae6 100644
--- 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