[PATCH] tools/kvm_stat: fix attack vector with user controlled FUSE mounts

[Matthias Gerstner <matthias.gerstner@suse.de>]

The first field in /proc/mounts can be influenced by unprivileged users
through the widespread `fusermount` setuid-root program. Example:

```
user$ mkdir ~/mydebugfs
user$ export _FUSE_COMMFD=0
user$ fusermount ~/mydebugfs -ononempty,fsname=debugfs
user$ grep debugfs /proc/mounts
debugfs /home/user/mydebugfs fuse rw,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
```

If there is no debugfs already mounted in the system then this can be
used by unprivileged users to trick kvm_stat into using a user
controlled file system location for obtaining KVM statistics.

To exploit this also a race condition has to be won, since the code
checks for the existence of the 'kvm' subdirectory of the resulting
path. This doesn't work on a FUSE mount, because the root user is not
allowed to access non-root FUSE mounts for security reasons. If an
attacker manages to unmount the FUSE mount in time again then kvm_stat
would be using the resulting path, though.

The impact if winning the race condition is mostly a denial-of-service
or damaged information integrity. The files in debugfs are only opened
for reading. So the attacker can cause very large data to be read in by
kvm_stat or fake data to be processed by kvm_stat. I don't see any
viable way to turn this into a privilege escalation.

The fix is simply to use the file system type field instead. Whitespace
in the mount path is escaped in /proc/mounts thus no further safety
measures in the parsing should be necessary to make this correct.
---
 tools/kvm/kvm_stat/kvm_stat | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/kvm/kvm_stat/kvm_stat b/tools/kvm/kvm_stat/kvm_stat
index 9c366b3a676d..88a73999aa58 100755
--- a/tools/kvm/kvm_stat/kvm_stat
+++ b/tools/kvm/kvm_stat/kvm_stat
@@ -1756,7 +1756,7 @@ def assign_globals():
 
     debugfs = ''
     for line in open('/proc/mounts'):
-        if line.split(' ')[0] == 'debugfs':
+        if line.split(' ')[2] == 'debugfs':
             debugfs = line.split(' ')[1]
             break
     if debugfs == '':
-- 
2.37.3