Re: [AppArmor 32/44] Enable LSM hooks to distinguish operations on file descriptors from operations on pathnames

[Stephen Smalley <sds@tycho.nsa.gov>]

On Thu, 2007-06-28 at 20:15 +0200, Andreas Gruenbacher wrote:
> On Thursday 28 June 2007 18:12, James Morris wrote:
> > Are you trying to cater for the case where you're holding an open fd for a 
> > file which has been deleted, and thus has no pathname?
> 
> Yes, see the AA_CHECK_FD flag in security/apparmor/main.c:aa_perm_dentry(). We 
> want to distinguish between the following two cases:
> 
>  - process performs an operation on an open file descriptor,
> 
>  - process performs an operation on a pathname, and between the dentry
>    lookup and the LSM permission check, the file gets deleted.
> 
> In the former case, we obviously want to continue giving the process access to 
> his fd (the classical pattern: open temporary file; delete it so that it will 
> self-recycle, continue using the open file descriptor).
> 
> In the latter case, The file still existed at the time of the lookup but not 
> anymore at the time of the permission check. The file obviously doesn't have 
> a filename anymore, so we cannot check permissions. If we granted access in 
> that case, processes could bypass their profile permissions in that race 
> window. We close the race by returning -ENOENT in that case, the same result 
> as if the file had already been deleted before the lookup.

So you don't actually need/use the struct file pointer; you just need a
flag indicating whether or not access was by open file descriptor or by
pathname?

And what does this mean for a process that has "changed hats"?  Which
might not be authorized to access the file anymore, even via an already
opened descriptor.

-- 
Stephen Smalley
National Security Agency