[syzbot] [lsm?] general protection fault in hook_inode_free_security

[Mickaël Salaün]

On Wed, Jul 10, 2024 at 02:23:23PM +0200, Mickaël Salaün wrote:
> On Thu, Jun 27, 2024 at 02:28:03PM -0400, Paul Moore wrote:
> > On Thu, Jun 27, 2024 at 9:34 AM Mickaël Salaün <mic at digikod.net> wrote:
> > >
> > > I didn't find specific issues with Landlock's code except the extra
> > > check in hook_inode_free_security().  It looks like inode->i_security is
> > > a dangling pointer, leading to UAF.
> > >
> > > Reading security_inode_free() comments, two things looks weird to me:
> > >
> > > > /**
> > > >  * security_inode_free() - Free an inode's LSM blob
> > > >  * @inode: the inode
> > > >  *
> > > >  * Deallocate the inode security structure and set @inode->i_security to NULL.
> > >
> > > I don't see where i_security is set to NULL.
> > 
> > The function header comments are known to be a bit suspect, a side
> > effect of being detached from the functions for many years, this may
> > be one of those cases.  I tried to fix up the really awful ones when I
> > moved the comments back, back I didn't have time to go through each
> > one in detail.  Patches to correct the function header comments are
> > welcome and encouraged! :)
> > 
> > > >  */
> > > > void security_inode_free(struct inode *inode)
> > > > {
> > >
> > > Shouldn't we add this check here?
> > > if (!inode->i_security)
> > >         return;
> > 
> > Unless I'm remembering something wrong, I believe we *should* always
> > have a valid i_security pointer each time we are called, if not
> > something has gone wrong, e.g. the security_inode_free() hook is no
> > longer being called from the right place.  If we add a NULL check, we
> > should probably have a WARN_ON(), pr_err(), or something similar to
> > put some spew on the console/logs.
> > 
> > All that said, it would be good to hear some confirmation from the VFS
> > folks that the security_inode_free() hook is located in a spot such
> > that once it exits it's current RCU critical section it is safe to
> > release the associated LSM state.
> > 
> > It's also worth mentioning that while we always allocate i_security in
> > security_inode_alloc() right now, I can see a world where we allocate
> > the i_security field based on need using the lsm_blob_size info (maybe
> > that works today?  not sure how kmem_cache handled 0 length blobs?).
> > The result is that there might be a legitimate case where i_security
> > is NULL, yet we still want to call into the LSM using the
> > inode_free_security() implementation hook.
> 
> Looking at existing LSM implementations, even if some helpers (e.g.
> selinux_inode) return NULL if inode->i_security is NULL, this may not be
> handled by the callers.  For instance, SELinux always dereferences the
> blob pointer in the security_inode_permission() hook.  EVM seems to be
> the only one properly handling this case.
> 
> Shouldn't we remove all inode->i_security checks and assume it is always
> set?  This is currently the case anyway, but it would be clearer this
> way and avoid false sense of security (with useless checks).

A patch was sent to do this kind of check:
https://lore.kernel.org/r/20140109101932.0508dec7@gandalf.local.home
but the applied commit 3dc91d4338d6 ("SELinux: Fix possible NULL pointer
dereference in selinux_inode_permission()") didn't include the
i_security check.

Since this commit, the security_inode_free()'s header comment is no
longer correct because i_security is no longer set to NULL.