From: Vivek Goyal <vgoyal@redhat.com>

Date: Mon, 27 Jan 2020 19:01:33 +0000

Subject: [PATCH] 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>

(cherry picked from commit ee88465224b3aed2596049caa28f86cbe0d5a3d0)

---

 tools/virtiofsd/passthrough_ll.c | 105 +++++++++++++++++++++++++++++++

 1 file changed, 105 insertions(+)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c

index 97e7c75667..d53cb1e005 100644

--- a/tools/virtiofsd/passthrough_ll.c

+++ b/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)