[Qemu-devel] [PATCH 2/2] linux-user: Don't allow guest to block SIGSEGV

[Peter Maydell <peter.maydell@linaro.org>]

Don't allow the linux-user guest to block SIGSEGV -- QEMU needs this
signal to detect accesses to pages which it has marked read-only
because it has cached translated code from them.

We implement this by making the do_sigprocmask() wrapper suppress
SIGSEGV when doing the host process signal mask manipulation; instead
we store the current state of SIGSEGV in the TaskState struct.

If we get a SIGSEGV for the guest when the guest has blocked the
signal, we treat it as if the default SEGV handler was in place,
as the kernel does for forced SIGSEGV delivery.

This patch is based on an idea by Alex Barcelo, but rather than
simply lying to the guest about the SIGSEGV state we track it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Alex Barcelo <abarcelo@ac.upc.edu>
---
 linux-user/qemu.h   |  1 +
 linux-user/signal.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 4d24e74..36d4a73 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -126,6 +126,7 @@ typedef struct TaskState {
 #endif
     uint32_t stack_base;
     int used; /* non zero if used */
+    bool sigsegv_blocked; /* SIGSEGV blocked by guest */
     struct image_info *info;
     struct linux_binprm *bprm;
 
diff --git a/linux-user/signal.c b/linux-user/signal.c
index c8df584..f18f3d3 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -204,7 +204,46 @@ void target_to_host_old_sigset(sigset_t *sigset,
  */
 int do_sigprocmask(int how, const sigset_t *set, sigset_t *oldset)
 {
-    return sigprocmask(how, set, oldset);
+    int ret;
+    sigset_t val;
+    sigset_t *temp = NULL;
+    CPUState *cpu = thread_cpu;
+    TaskState *ts = (TaskState *)cpu->opaque;
+    bool segv_was_blocked = ts->sigsegv_blocked;
+
+    if (set) {
+        bool has_sigsegv = sigismember(set, SIGSEGV);
+        val = *set;
+        temp = &val;
+
+        sigdelset(temp, SIGSEGV);
+
+        switch (how) {
+        case SIG_BLOCK:
+            if (has_sigsegv) {
+                ts->sigsegv_blocked = true;
+            }
+            break;
+        case SIG_UNBLOCK:
+            if (has_sigsegv) {
+                ts->sigsegv_blocked = false;
+            }
+            break;
+        case SIG_SETMASK:
+            ts->sigsegv_blocked = has_sigsegv;
+            break;
+        default:
+            g_assert_not_reached();
+        }
+    }
+
+    ret = sigprocmask(how, temp, oldset);
+
+    if (oldset && segv_was_blocked) {
+        sigaddset(oldset, SIGSEGV);
+    }
+
+    return ret;
 }
 
 /* siginfo conversion */
@@ -468,6 +507,19 @@ int queue_signal(CPUArchState *env, int sig, target_siginfo_t *info)
     k = &ts->sigtab[sig - 1];
     queue = gdb_queuesig ();
     handler = sigact_table[sig - 1]._sa_handler;
+
+    if (ts->sigsegv_blocked && sig == TARGET_SIGSEGV) {
+        /* Guest has blocked SIGSEGV but we got one anyway. Assume this
+         * is a forced SIGSEGV (ie one the kernel handles via force_sig_info
+         * because it got a real MMU fault). A blocked SIGSEGV in that
+         * situation is treated as if using the default handler. This is
+         * not correct if some other process has randomly sent us a SIGSEGV
+         * via kill(), but that is not easy to distinguish at this point,
+         * so we assume it doesn't happen.
+         */
+        handler = TARGET_SIG_DFL;
+    }
+
     if (!queue && handler == TARGET_SIG_DFL) {
         if (sig == TARGET_SIGTSTP || sig == TARGET_SIGTTIN || sig == TARGET_SIGTTOU) {
             kill(getpid(),SIGSTOP);
@@ -5726,6 +5778,14 @@ void process_pending_signals(CPUArchState *cpu_env)
         handler = sa->_sa_handler;
     }
 
+    if (ts->sigsegv_blocked && sig == TARGET_SIGSEGV) {
+        /* Guest has blocked SIGSEGV but we got one anyway. Assume this
+         * is a forced SIGSEGV (ie one the kernel handles via force_sig_info
+         * because it got a real MMU fault), and treat as if default handler.
+         */
+        handler = TARGET_SIG_DFL;
+    }
+
     if (handler == TARGET_SIG_DFL) {
         /* default handler : ignore some signal. The other are job control or fatal */
         if (sig == TARGET_SIGTSTP || sig == TARGET_SIGTTIN || sig == TARGET_SIGTTOU) {
-- 
1.9.0