Messages in this thread | | | From | Kyle Moffett <> | Subject | Re: [RESEND][RFC][PATCH 2/7] implementation of LSM hooks | Date | Thu, 20 Apr 2006 02:51:07 -0400 |
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On Apr 19, 2006, at 02:56:28, Valdis.Kletnieks@vt.edu wrote: > On Wed, 19 Apr 2006 02:40:25 EDT, Kyle Moffett said: >> Perhaps the SELinux model should be extended to handle (dir-inode, >> path-entry) pairs. For example, if I want to protect the /etc/ >> shadow file regardless of what tool is used to safely modify it, I >> would set > > Some of us think that the tools can protect /etc/shadow just fine > on their own, and are concerned with rogue software that abuses / > etc/shadow without bothering to safely modify it..
Here I'm talking about using SELinux to protect /etc/shadow both before _and_ after it's edited with vipw. A tool like vipw does creat () write() rename() to overwrite the /etc/shadow file, so any SELinux system relying _only_ on inode is guaranteed to break. In any case, the point here is to provide 2 clean semantics for protecting files: inodes and directory entries; They have entirely different use- cases. For an inode all you need is (<inode>), for a directory entry you need (<dir-inode>, <path-component>). The latter would only be used when referencing the file by name, if you already have a filehandle it would not be used.
>> o Protect the "/" and "/etc" directory inodes as usual under SELinux >> (with attributes on directory inodes). >> o Create pairs with (etc_inode,"shadow") and (etc_inode,"gshadow") >> and apply security attributes to those potentially nonexistent pairs. > > *bzzt* wrong. Why should "gshadow" matter? (Think carefully about > what happens when a setUID program gets exploited and used to > scribble on /etc/shadow - black hats rarely bother to do locking > and other such niceties....)
/etc/gshadow matters because it's the group shadow file, no? If you're going to bother protecting the user shadow file, you should protect the group shadow file too.
>> I'm not terribly familiar with the exact internal semantics of >> SELinux, but that should provide a 90% solution (it fixes bind >> mounts and namespaces). > > 90% doesn't give the security guys warm-and-fuzzies....
True, but that's why I address the following 2 items below:
>> The remaining 2 issues are hardlinks and fd-passing. For >> hardlinks you don't care about other links to that data, you're >> concerned with protecting a particular filesystem location, not >> particular contents, so you just need to prevent _new_ hardlinks >> to a protected (dir_inode, path_elem) pair, which doesn't seem >> very hard. > > It's not. include/linux/security.h: > > * @inode_link: > * Check permission before creating a new hard link to a file. > * @old_dentry contains the dentry structure for an existing > link to the file. > * @dir contains the inode structure of the parent directory > of the new link. > * @new_dentry contains the dentry structure for the new link. > * Return 0 if permission is granted.
I _think_ SELinux provides a way to hook this, but I'm not entirely sure. If not, it should be added, but I suspect it does.
>> For fd-passing, I don't know what to do. Perhaps nothing. > > include/linux/security.h: > > * @file_receive: > * This hook allows security modules to control the ability of > a process > * to receive an open file descriptor via socket IPC. > * @file contains the file structure being received. > * Return 0 if permission is granted. > > Already a solved problem.
Likewise, this should be solved with inode-based security if it's a problem. The use-case I'm addressing (legitimate use of /etc/shadow with vipw) doesn't care about that. For example, if I want to only allow vipw access to /etc/shadow, then associating that policy with the /etc/shadow inode is useless, because the first rogue program would just remove and rewrite /etc/shadow, getting a different inode. What I need to do is protect the "shadow" entry in the dentry for ("/etc") in the particular namespace I care about, right?
Cheers, Kyle Moffett
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