v4: * Patch 1: Return positive errno if openat(2) fails in lo_do_open() [Greg] * Patch 3: Return -fd instead or -errno after lo_inode_open() in lo_do_open() [Greg] * Patch 3: Use De Morgan's Law to simplify the boolean expression in lo_create() [Vivek] * Patch 3: Add missing errno = -truncfd after lo_inode_open() call in lo_setattr v3: * Restructure lo_create() to handle externally-created files (we need to allocate an inode for them) [Greg] * Patch 1 & 2 refactor the code so that Patch 3 can implement the CVE fix v3: * Protect lo_create() [Greg] v2: * Add doc comment clarifying that symlinks are traversed client-side [Daniel]
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 series 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. This issue was diagnosed on public IRC and is therefore already known and not embargoed. Reported-by: Alex Xu <a...@alxu.ca> Fixes: CVE-2020-35517 Stefan Hajnoczi (3): virtiofsd: extract lo_do_open() from lo_open() virtiofsd: optionally return inode pointer from lo_do_lookup() virtiofsd: prevent opening of special files (CVE-2020-35517) tools/virtiofsd/passthrough_ll.c | 224 ++++++++++++++++++++----------- 1 file changed, 148 insertions(+), 76 deletions(-) -- 2.29.2
