From 449e8171f96a6a944d1f3b7d3627ae059eae21ca Mon Sep 17 00:00:00 2001

From: Vivek Goyal <vgoyal@redhat.com>

Date: Tue, 25 Jan 2022 13:51:14 -0500

Subject: [PATCH] virtiofsd: Drop membership of all supplementary groups

 (CVE-2022-0358)

At the start, drop membership of all supplementary groups. This is

not required.

If we have membership of "root" supplementary group and when we switch

uid/gid using setresuid/setsgid, we still retain membership of existing

supplemntary groups. And that can allow some operations which are not

normally allowed.

For example, if root in guest creates a dir as follows.

$ mkdir -m 03777 test_dir

This sets SGID on dir as well as allows unprivileged users to write into

this dir.

And now as unprivileged user open file as follows.

$ su test

$ fd = open("test_dir/priviledge_id", O_RDWR|O_CREAT|O_EXCL, 02755);

This will create SGID set executable in test_dir/.

And that's a problem because now an unpriviliged user can execute it,

get egid=0 and get access to resources owned by "root" group. This is

privilege escalation.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2044863

Fixes: CVE-2022-0358

Reported-by: JIETAO XIAO <shawtao1125@gmail.com>

Suggested-by: Miklos Szeredi <mszeredi@redhat.com>

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

Message-Id: <YfBGoriS38eBQrAb@redhat.com>

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

  dgilbert: Fixed missing {}'s style nit

---

 tools/virtiofsd/passthrough_ll.c | 27 +++++++++++++++++++++++++++

 1 file changed, 27 insertions(+)

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

index 64b5b4fbb1..b3d0674f6d 100644

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

+++ b/tools/virtiofsd/passthrough_ll.c

@@ -54,6 +54,7 @@

 #include <sys/wait.h>

 #include <sys/xattr.h>

 #include <syslog.h>

+#include <grp.h>

 #include "qemu/cutils.h"

 #include "passthrough_helpers.h"

@@ -1161,6 +1162,30 @@ static void lo_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)

 #define OURSYS_setresuid SYS_setresuid

 #endif

+static void drop_supplementary_groups(void)

+{

+    int ret;

+

+    ret = getgroups(0, NULL);

+    if (ret == -1) {

+        fuse_log(FUSE_LOG_ERR, "getgroups() failed with error=%d:%s\n",

+                 errno, strerror(errno));

+        exit(1);

+    }

+

+    if (!ret) {

+        return;

+    }

+

+    /* Drop all supplementary groups. We should not need it */

+    ret = setgroups(0, NULL);

+    if (ret == -1) {

+        fuse_log(FUSE_LOG_ERR, "setgroups() failed with error=%d:%s\n",

+                 errno, strerror(errno));

+        exit(1);

+    }

+}

+

 /*

  * Change to uid/gid of caller so that file is created with

  * ownership of caller.

@@ -3926,6 +3951,8 @@ int main(int argc, char *argv[])

     qemu_init_exec_dir(argv[0]);

+    drop_supplementary_groups();

+

     pthread_mutex_init(&lo.mutex, NULL);

     lo.inodes = g_hash_table_new(lo_key_hash, lo_key_equal);

     lo.root.fd = -1;

-- 

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