Messages in this thread | | | Subject | Re: Security issues with local filesystem caching | From | Stephen Smalley <> | Date | Thu, 26 Oct 2006 13:45:06 -0400 |
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On Thu, 2006-10-26 at 18:09 +0100, David Howells wrote: > > and prior to creating a file within the cache, it would set the current > > fscreate attribute (via a security hook call) to that per-cache value, in > > much the same way it is presently setting the fsuid/fsgid. > > That sounds reasonable. There has to be some way to revert it, though.
Yes, that can be done. See below.
> > The fscreate attribute is normally set via the security_setprocattr hook > > (when a task writes to /proc/self/attr/fscreate); > > That makes it sound like fscreate is a per-process attribute, not a per-thread > attribute. The former would definitely be a problem.
It is actually per-task (thread), sorry for the confusing reference to /proc/self.
> > You may also want to convert the context value to a secid once when it is > > first configured, and then later just pass the secid to the hook call for > > setting the fscreate value for efficiency. > > I'm not sure what you mean by that. Can you give a pseudo-code example?
When the daemon writes the context value (a string) to the cachefiles module interface for a given cache, the cachefiles module would do something like the following: /* Map the context to an integer. */ rc = security_secctx_to_secid(value, size, &secid); if (rc) goto err; /* Check permission of current to set this context. */ rc = security_cache_set_context(secid); if (rc) goto err; cache->secid = secid; SELinux would then provide selinux_secctx_to_secid() and selinux_cache_set_context() implementations; the former would just be call to selinux_string_to_sid(), while the latter would require some new permission check to be defined unless we can treat this as equivalent to some existing operation. You'll find that there is already a security_secid_to_secctx() hook defined for LSM, so the first hook just adds the other direction.
Later, when going to create a file in that cache, the cachefiles module would do something like: /* Save and switch the fs secid for creation. */ fssecid = security_getfssecid(); security_setfssecid(cache->secid); <create file> /* Restore the original fs secid. */ security_setfssecid(fssecid); SELinux would then provide selinux_getfsecid() and selinux_setfssecid() implementations that are just: u32 selinux_getfssecid(void) { struct task_security_struct *tsec = current->security; return tsec->create_sid; } void selinux_setfssecid(u32 secid) { struct task_security_struct *tsec = current->security; tsec->create_sid = secid; }
-- Stephen Smalley National Security Agency
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