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 | 221 ++++++++++++++++++++-----------
 1 file changed, 145 insertions(+), 76 deletions(-)

-- 
2.29.2

Patchew URL: https://patchew.org/QEMU/20210203113719.83633-1-stefanha@redhat.com/



Hi,

This series seems to have some coding style problems. See output below for
more information:

Type: series
Message-id: 20210203113719.83633-1-stefanha@redhat.com
Subject: [PATCH v4 0/3] virtiofsd: prevent opening of special files (CVE-2020-35517)

=== TEST SCRIPT BEGIN ===
#!/bin/bash
git rev-parse base > /dev/null || exit 0
git config --local diff.renamelimit 0
git config --local diff.renames True
git config --local diff.algorithm histogram
./scripts/checkpatch.pl --mailback base..
=== TEST SCRIPT END ===

Updating 3c8cf5a9c21ff8782164d1def7f44bd888713384
Switched to a new branch 'test'
9e44f0e virtiofsd: prevent opening of special files (CVE-2020-35517)
e3ddfae virtiofsd: optionally return inode pointer from lo_do_lookup()
a6c73fd virtiofsd: extract lo_do_open() from lo_open()

=== OUTPUT BEGIN ===
1/3 Checking commit a6c73fd0a630 (virtiofsd: extract lo_do_open() from lo_open())
ERROR: return of an errno should typically be -ve (return -ENOMEM)
#70: FILE: tools/virtiofsd/passthrough_ll.c:1674:
+        return ENOMEM;

total: 1 errors, 0 warnings, 114 lines checked

Patch 1/3 has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.

2/3 Checking commit e3ddfaebb90a (virtiofsd: optionally return inode pointer from lo_do_lookup())
3/3 Checking commit 9e44f0e0be3a (virtiofsd: prevent opening of special files (CVE-2020-35517))
=== OUTPUT END ===

Test command exited with code: 1


The full log is available at
http://patchew.org/logs/20210203113719.83633-1-stefanha@redhat.com/testing.checkpatch/?type=message.
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