Fix bug #7067 - Linux asynchronous IO (aio) can cause smbd to fail to respond to a read or write.

Only works on Linux kernels 2.6.26 and above. Grants CAP_KILL capability
to allow Linux threads under different euids to send signals to each other.

Same as mater commit 899bd0005f56dcc1e95c3988d41ab3f628bb15db.

Jeremy.
(cherry picked from commit cbf09baa90f5c4cfa8a0019ccc79211d72d13629)

3 files changed, 71 insertions, 5 deletions

diff --git a/source/include/smb.h b/source/include/smb.h
index 327f2129067..3825c6397ef 100644
--- a/source/include/smb.h
+++ b/source/include/smb.h
@@ -1684,7 +1684,8 @@ minimum length == 18.
 enum smbd_capability {
     KERNEL_OPLOCK_CAPABILITY,
     DMAPI_ACCESS_CAPABILITY,
-    LEASE_CAPABILITY
+    LEASE_CAPABILITY,
+    KILL_CAPABILITY
 };
 
 /* if a kernel does support oplocks then a structure of the following
diff --git a/source/lib/system.c b/source/lib/system.c
index 36745b112f0..fd18928598c 100644
--- a/source/lib/system.c
+++ b/source/lib/system.c
@@ -705,6 +705,11 @@ int sys_chroot(const char *dname)
 
 #if defined(HAVE_POSIX_CAPABILITIES)
 
+/* This define hasn't made it into the glibc capabilities header yet. */
+#ifndef SECURE_NO_SETUID_FIXUP
+#define SECURE_NO_SETUID_FIXUP          2
+#endif
+
 /**************************************************************************
  Try and abstract process capabilities (for systems that have them).
 ****************************************************************************/
@@ -735,6 +740,32 @@ static bool set_process_capability(enum smbd_capability capability,
 	}
 #endif
 
+#if defined(HAVE_PRCTL) && defined(PR_SET_SECUREBITS) && defined(SECURE_NO_SETUID_FIXUP)
+        /* New way of setting capabilities as "sticky". */
+
+	/*
+	 * Use PR_SET_SECUREBITS to prevent setresuid()
+	 * atomically dropping effective capabilities on
+	 * uid change. Only available in Linux kernels
+	 * 2.6.26 and above.
+	 *
+	 * See here:
+	 * http://www.kernel.org/doc/man-pages/online/pages/man7/capabilities.7.html
+	 * for details.
+	 *
+	 * Specifically the CAP_KILL capability we need
+	 * to allow Linux threads under different euids
+	 * to send signals to each other.
+	 */
+
+	if (prctl(PR_SET_SECUREBITS, 1 << SECURE_NO_SETUID_FIXUP)) {
+		DEBUG(0,("set_process_capability: "
+			"prctl PR_SET_SECUREBITS failed with error %s\n",
+			strerror(errno) ));
+		return false;
+	}
+#endif
+
 	cap = cap_get_proc();
 	if (cap == NULL) {
 		DEBUG(0,("set_process_capability: cap_get_proc failed: %s\n",
@@ -763,6 +794,11 @@ static bool set_process_capability(enum smbd_capability capability,
 			cap_vals[num_cap_vals++] = CAP_LEASE;
 #endif
 			break;
+		case KILL_CAPABILITY:
+#ifdef CAP_KILL
+			cap_vals[num_cap_vals++] = CAP_KILL;
+#endif
+			break;
 	}
 
 	SMB_ASSERT(num_cap_vals <= ARRAY_SIZE(cap_vals));
@@ -772,16 +808,37 @@ static bool set_process_capability(enum smbd_capability capability,
 		return True;
 	}
 
-	cap_set_flag(cap, CAP_EFFECTIVE, num_cap_vals, cap_vals,
-		enable ? CAP_SET : CAP_CLEAR);
+	/*
+	 * Ensure the capability is effective. We assume that as a root
+	 * process it's always permitted.
+	 */
+
+	if (cap_set_flag(cap, CAP_EFFECTIVE, num_cap_vals, cap_vals,
+			enable ? CAP_SET : CAP_CLEAR) == -1) {
+		DEBUG(0, ("set_process_capability: cap_set_flag effective "
+			"failed (%d): %s\n",
+			(int)capability,
+			strerror(errno)));
+		cap_free(cap);
+		return false;
+	}
 
 	/* We never want to pass capabilities down to our children, so make
 	 * sure they are not inherited.
 	 */
-	cap_set_flag(cap, CAP_INHERITABLE, num_cap_vals, cap_vals, CAP_CLEAR);
+	if (cap_set_flag(cap, CAP_INHERITABLE, num_cap_vals,
+			cap_vals, CAP_CLEAR) == -1) {
+		DEBUG(0, ("set_process_capability: cap_set_flag inheritable "
+			"failed (%d): %s\n",
+			(int)capability,
+			strerror(errno)));
+		cap_free(cap);
+		return false;
+	}
 
 	if (cap_set_proc(cap) == -1) {
-		DEBUG(0, ("set_process_capability: cap_set_proc failed: %s\n",
+		DEBUG(0, ("set_process_capability: cap_set_flag (%d) failed: %s\n",
+			(int)capability,
 			strerror(errno)));
 		cap_free(cap);
 		return False;
diff --git a/source/smbd/server.c b/source/smbd/server.c
index 51294847307..23c8c12a625 100644
--- a/source/smbd/server.c
+++ b/source/smbd/server.c
@@ -1240,6 +1240,14 @@ extern void build_options(bool screen);
 	gain_root_privilege();
 	gain_root_group_privilege();
 
+	/*
+	 * Ensure we have CAP_KILL capability set on Linux,
+	 * where we need this to communicate with threads.
+	 * This is inherited by new threads, but not by new
+	 * processes across exec().
+	 */
+	set_effective_capability(KILL_CAPABILITY);
+
 	fault_setup((void (*)(void *))exit_server_fault);
 	dump_core_setup("smbd");