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Re: [Virtio-fs] [PATCH v2] virtiofsd: prevent opening of special files (

From: Liam Merwick
Subject: Re: [Virtio-fs] [PATCH v2] virtiofsd: prevent opening of special files (CVE-2020-35517)
Date: Tue, 26 Jan 2021 10:47:13 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1

On 26/01/2021 10:35, Stefan Hajnoczi wrote:
A well-behaved FUSE client does not attempt to open special files with
FUSE_OPEN because they are handled on the client side (e.g. device nodes
are handled by client-side device drivers).

The check to prevent virtiofsd from opening special files is missing in
a few cases, most notably FUSE_OPEN. A malicious client can cause
virtiofsd to open a device node, potentially allowing the guest to
escape. This can be exploited by a modified guest device driver. It is
not exploitable from guest userspace since the guest kernel will handle
special files inside the guest instead of sending FUSE requests.

This patch adds the missing checks to virtiofsd. This is a short-term
solution because it does not prevent a compromised virtiofsd process
from opening device nodes on the host.

Reported-by: Alex Xu <alex@alxu.ca>
Fixes: CVE-2020-35517

Should this also explicitly

Cc: qemu-stable@nongnu.org

Or would only a long-term fix target that?

Reviewed-by: Liam Merwick <liam.merwick@oracle.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
  * Add doc comment clarifying that symlinks are traversed client-side

This issue was diagnosed on public IRC and is therefore already known
and not embargoed.

A stronger fix, and the long-term solution, is for users to mount the
shared directory and any sub-mounts with nodev, as well as nosuid and
noexec. Unfortunately virtiofsd cannot do this automatically because
bind mounts added by the user after virtiofsd has launched would not be
detected. I suggest the following:

1. Modify libvirt and Kata Containers to explicitly set these mount
2. Then modify virtiofsd to check that the shared directory has the
    necessary options at startup. Refuse to start if the options are
    missing so that the user is aware of the security requirements.

As a bonus this also increases the likelihood that other host processes
besides virtiofsd will be protected by nosuid/noexec/nodev so that a
malicious guest cannot drop these files in place and then arrange for a
host process to come across them.

Additionally, user namespaces have been discussed. They seem like a
worthwhile addition as an unprivileged or privilege-separated mode
although there are limitations with respect to security xattrs and the
actual uid/gid stored on the host file system not corresponding to the
guest uid/gid.
  tools/virtiofsd/passthrough_ll.c | 85 +++++++++++++++++++++-----------
  1 file changed, 57 insertions(+), 28 deletions(-)

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