[Virtio-fs] [PATCH 1/2] virtiofsd: cap-ng helpers

[Dr. David Alan Gilbert (git)]

From: "Dr. David Alan Gilbert" <dgilbert at redhat.com>

libcap-ng reads /proc during capng_get_caps_process, and virtiofsd's
sandboxing doesn't have /proc mounted; thus we have to do the
caps read before we sandbox it and save/restore the state.

Signed-off-by: Dr. David Alan Gilbert <dgilbert at redhat.com>
---
 Makefile                           |  2 +-
 contrib/virtiofsd/passthrough_ll.c | 72 ++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 237cbdfee1..1484c23a9b 100644
--- a/Makefile
+++ b/Makefile
@@ -323,7 +323,7 @@ HELPERS-$(call land,$(CONFIG_SOFTMMU),$(CONFIG_LINUX)) = qemu-bridge-helper$(EXE
 
 ifdef CONFIG_LINUX
 ifdef CONFIG_SECCOMP
-ifdef CONFIG_LIBCAP
+ifdef CONFIG_LIBCAP_NG
 HELPERS-y += virtiofsd$(EXESUF)
 vhost-user-json-y += contrib/virtiofsd/50-qemu-virtiofsd.json
 endif
diff --git a/contrib/virtiofsd/passthrough_ll.c b/contrib/virtiofsd/passthrough_ll.c
index 1688071c10..139261efc1 100644
--- a/contrib/virtiofsd/passthrough_ll.c
+++ b/contrib/virtiofsd/passthrough_ll.c
@@ -47,6 +47,7 @@
 #include <string.h>
 #include <syslog.h>
 #include <limits.h>
+#include <cap-ng.h>
 #include <dirent.h>
 #include <assert.h>
 #include <errno.h>
@@ -216,6 +217,13 @@ static const struct fuse_opt lo_opts[] = {
 };
 static bool use_syslog = false;
 static int current_log_level;
+static struct {
+	pthread_mutex_t mutex;
+	void *saved;
+} cap;
+/* That we loaded cap-ng in the current thread from the saved */
+static __thread bool cap_loaded = 0;
+
 static void unref_inode_lolocked(struct lo_data *lo, struct lo_inode *inode, uint64_t n);
 static void put_shared(struct lo_data *lo, struct lo_inode *inode);
 
@@ -242,6 +250,37 @@ static struct lo_data *lo_data(fuse_req_t req)
 	return (struct lo_data *) fuse_req_userdata(req);
 }
 
+/*
+ * Load capng's state from our saved state if the current thread
+ * hadn't previously been loaded.
+ * returns 0 on success
+ */
+static int load_capng(void)
+{
+	if (!cap_loaded) {
+		pthread_mutex_lock(&cap.mutex);
+		capng_restore_state(&cap.saved);
+		/*
+		 * restore_state free's the saved copy
+		 * so make another.
+		 */
+		cap.saved = capng_save_state();
+		if (!cap.saved) {
+			fuse_log(FUSE_LOG_ERR, "capng_save_state (thread)\n");
+			return -EINVAL;
+		}
+		pthread_mutex_unlock(&cap.mutex);
+
+		/*
+		 * We want to use the loaded state for our pid,
+		 * not the original
+		 */
+		capng_setpid(gettid());
+		cap_loaded = true;
+	}
+	return 0;
+}
+
 /* Helpers for dropping and regaining effective capabilities. Returns 0
  * on success, error otherwise  */
 static int drop_effective_cap(const char *cap_name, bool *cap_dropped)
@@ -2754,6 +2793,35 @@ static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
 	}
 }
 
+/*
+ * Capture the capability state, we'll need to restore this for individual
+ * threads later; see load_capng.
+ */
+static void setup_capng(void)
+{
+	/* Note this accesses /proc so has to happen before the sandbox */
+	if (capng_get_caps_process()) {
+		fuse_log(FUSE_LOG_ERR, "capng_get_caps_process\n");
+		exit(1);
+	}
+	pthread_mutex_init(&cap.mutex, NULL);
+	pthread_mutex_lock(&cap.mutex);
+	cap.saved = capng_save_state();
+	if (!cap.saved) {
+		fuse_log(FUSE_LOG_ERR, "capng_save_state\n");
+		exit(1);
+	}
+	pthread_mutex_unlock(&cap.mutex);
+}
+
+static void cleanup_capng(void)
+{
+	free(cap.saved);
+	cap.saved = NULL;
+	pthread_mutex_destroy(&cap.mutex);
+}
+
+
 /*
  * Make the source directory our root so symlinks cannot escape and no other
  * files are accessible.  Assumes unshare(CLONE_NEWNS) was already called.
@@ -3173,6 +3241,9 @@ int main(int argc, char *argv[])
 
 	setup_nofile_rlimit();
 
+	/* Must be before sandbox since it wants /proc */
+	setup_capng();
+
 	setup_sandbox(&lo, se, opts.syslog);
 
 	setup_root(&lo, &lo.root);
@@ -3195,6 +3266,7 @@ int main(int argc, char *argv[])
 
 err_out4:
 	fuse_session_unmount(se);
+	cleanup_capng();
 err_out3:
 	fuse_remove_signal_handlers(se);
 err_out2:
-- 
2.23.0