[PATCH v5 0/3] virtiofsd: prevent opening of special files (CVE-2020-35517)

[Stefan Hajnoczi]

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 <alex@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(-)

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2.29.2