Date: Sat, 12 Mar 2011 20:03:45 +0300 From: Vasiliy Kulikov <segoon@...nwall.com> To: oss-security@...ts.openwall.com Subject: Untrusted fs and invalid filenames This is a resumption of the subject "Physical access vulnerabilities and auto-mounting" brought by Dan Rosenberg. The previous discussion was about possible attacks the kernel, now I'd like to talk about attacks userland programs. While POSIX restricts the character set used in filenames, some Linux filesystems (at least ext2) permit reserved filenames ".", ".." and filenames with "/" inside. I have a crafted flash drive with ext2 that has such files: root@...atros:/media# ls cdrom/ -la итого 28 drwxr-xr-x 4 root root 4096 2011-03-12 18:55 . drwxr-xr-x 3 root root 4096 2011-03-12 18:48 .. drwxr-xr-x 3 root root 4096 2011-03-12 18:48 .. drwx------ 2 root root 16384 2011-03-12 18:54 lost+found root@...atros:/media# ls a2f202b6-a3ef-45b5-bce4-01c4d35af4a0/ -la итого 28 drwxr-xr-x 4 root root 4096 2011-03-12 18:55 . drwxr-xr-x 4 root root 4096 2011-03-12 19:08 .. drwx------ 2 root root 16384 2011-03-12 18:54 lost+found -rw-r--r-- 1 root root 3146 2011-03-12 19:07 lost+found/../../../etc/passwd Guess what does "rm" with such filenames :-) Another example of crafted fs is ext2 partition with EXT2_ERRORS_PANIC option set in superblock and corrupted root directory. When run "ls" on the fs, the kernel would panic. While it was said that such attacks have low impact, some systems already try to protect itself from untrusted external filesystems. E.g. automounting of flash drives in Ubuntu is processed with "-o nodev,nosuid". I read this as external flash drives are not fully trusted and may contain some dangerous files. If some automatic file processing of files on drives with specially crafted filenames is started then it might have a security impact. I don't know such popular apps, though. What I suggest is something like "-o untrusted" option to mount. This would mean that the system considers the input from such fs as a malicious input. Such mounted fs would try to consider the data on disk as untrusted and to be as robust as possible, e.g. check against "/"-filenames, against corrupted fs structures, etc. I'd be happy to hear opinions about the usefulness of this feature. Thanks, -- Vasiliy Kulikov http://www.openwall.com - bringing security into open computing environments
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