virtiofsd: Drop CAP_FSETID if client asked for it

If client requested killing setuid/setgid bits on file being written, drop
CAP_FSETID capability so that setuid/setgid bits are cleared upon write
automatically.

pjdfstest chown/12.t needs this.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
  dgilbert: reworked for libcap-ng
Reviewed-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/passthrough_ll.c

@@ -201,6 +201,91 @@ static int load_capng(void)
     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)
+{
+    int cap, ret;
+
+    cap = capng_name_to_capability(cap_name);
+    if (cap < 0) {
+        ret = errno;
+        fuse_log(FUSE_LOG_ERR, "capng_name_to_capability(%s) failed:%s\n",
+                 cap_name, strerror(errno));
+        goto out;
+    }
+
+    if (load_capng()) {
+        ret = errno;
+        fuse_log(FUSE_LOG_ERR, "load_capng() failed\n");
+        goto out;
+    }
+
+    /* We dont have this capability in effective set already. */
+    if (!capng_have_capability(CAPNG_EFFECTIVE, cap)) {
+        ret = 0;
+        goto out;
+    }
+
+    if (capng_update(CAPNG_DROP, CAPNG_EFFECTIVE, cap)) {
+        ret = errno;
+        fuse_log(FUSE_LOG_ERR, "capng_update(DROP,) failed\n");
+        goto out;
+    }
+
+    if (capng_apply(CAPNG_SELECT_CAPS)) {
+        ret = errno;
+        fuse_log(FUSE_LOG_ERR, "drop:capng_apply() failed\n");
+        goto out;
+    }
+
+    ret = 0;
+    if (cap_dropped) {
+        *cap_dropped = true;
+    }
+
+out:
+    return ret;
+}
+
+static int gain_effective_cap(const char *cap_name)
+{
+    int cap;
+    int ret = 0;
+
+    cap = capng_name_to_capability(cap_name);
+    if (cap < 0) {
+        ret = errno;
+        fuse_log(FUSE_LOG_ERR, "capng_name_to_capability(%s) failed:%s\n",
+                 cap_name, strerror(errno));
+        goto out;
+    }
+
+    if (load_capng()) {
+        ret = errno;
+        fuse_log(FUSE_LOG_ERR, "load_capng() failed\n");
+        goto out;
+    }
+
+    if (capng_update(CAPNG_ADD, CAPNG_EFFECTIVE, cap)) {
+        ret = errno;
+        fuse_log(FUSE_LOG_ERR, "capng_update(ADD,) failed\n");
+        goto out;
+    }
+
+    if (capng_apply(CAPNG_SELECT_CAPS)) {
+        ret = errno;
+        fuse_log(FUSE_LOG_ERR, "gain:capng_apply() failed\n");
+        goto out;
+    }
+    ret = 0;
+
+out:
+    return ret;
+}
+
 static void lo_map_init(struct lo_map *map)
 {
     map->elems = NULL;
@@ -1577,6 +1662,7 @@ static void lo_write_buf(fuse_req_t req, fuse_ino_t ino,
     (void)ino;
     ssize_t res;
     struct fuse_bufvec out_buf = FUSE_BUFVEC_INIT(fuse_buf_size(in_buf));
+    bool cap_fsetid_dropped = false;
 
     out_buf.buf[0].flags = FUSE_BUF_IS_FD | FUSE_BUF_FD_SEEK;
     out_buf.buf[0].fd = lo_fi_fd(req, fi);
@@ -1588,12 +1674,31 @@ static void lo_write_buf(fuse_req_t req, fuse_ino_t ino,
                  out_buf.buf[0].size, (unsigned long)off);
     }
 
+    /*
+     * If kill_priv is set, drop CAP_FSETID which should lead to kernel
+     * clearing setuid/setgid on file.
+     */
+    if (fi->kill_priv) {
+        res = drop_effective_cap("FSETID", &cap_fsetid_dropped);
+        if (res != 0) {
+            fuse_reply_err(req, res);
+            return;
+        }
+    }
+
     res = fuse_buf_copy(&out_buf, in_buf);
     if (res < 0) {
         fuse_reply_err(req, -res);
     } else {
         fuse_reply_write(req, (size_t)res);
     }
+
+    if (cap_fsetid_dropped) {
+        res = gain_effective_cap("FSETID");
+        if (res) {
+            fuse_log(FUSE_LOG_ERR, "Failed to gain CAP_FSETID\n");
+        }
+    }
 }
 
 static void lo_statfs(fuse_req_t req, fuse_ino_t ino)