a3fdbbc7f2
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 fixes this issue by introducing the lo_inode_open() function to check the file type before opening it. This is a short-term solution because it does not prevent a compromised virtiofsd process from opening device nodes on the host. Restructure lo_create() to try O_CREAT | O_EXCL first. Note that O_CREAT | O_EXCL does not follow symlinks, so O_NOFOLLOW masking is not necessary here. If the file exists and the user did not specify O_EXCL, open it via lo_do_open(). Reported-by: Alex Xu <alex@alxu.ca> Fixes: CVE-2020-35517 Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Vivek Goyal <vgoyal@redhat.com> Reviewed-by: Greg Kurz <groug@kaod.org> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Message-Id: <20210204150208.367837-4-stefanha@redhat.com> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> |
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