[PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

[Al Viro]

On Thu, Mar 02, 2023 at 11:54:03AM -0800, Kees Cook wrote:
> On Thu, Mar 02, 2023 at 07:19:49PM +0000, Al Viro wrote:
> > On Thu, Mar 02, 2023 at 11:10:03AM -0800, Linus Torvalds wrote:
> > > On Thu, Mar 2, 2023 at 11:03 AM Linus Torvalds
> > > <torvalds at linux-foundation.org> wrote:
> > > >
> > > > It might be best if we actually exposed it as a SLAB_SKIP_ZERO thing,
> > > > just to make it possible to say - exactly in situations like this -
> > > > that this particular slab cache has no advantage from pre-zeroing.
> > > 
> > > Actually, maybe it's just as well to keep it per-allocation, and just
> > > special-case getname_flags() itself.
> > > 
> > > We could replace the __getname() there with just a
> > > 
> > >         kmem_cache_alloc(names_cachep, GFP_KERNEL | __GFP_SKIP_ZERO);
> > > 
> > > we're going to overwrite the beginning of the buffer with the path we
> > > copy from user space, and then we'd have to make people comfortable
> > > with the fact that even with zero initialization hardening on, the
> > > space after the filename wouldn't be initialized...
> > 
> > ACK; same in getname_kernel() and sys_getcwd(), at the very least.
> 
> FWIW, much earlier analysis suggested opting out these kmem caches:
> 
> 	buffer_head
> 	names_cache
> 	mm_struct
> 	anon_vma
> 	skbuff_head_cache
> 	skbuff_fclone_cache

I would probably add dentry_cache to it; the only subtle part is
->d_iname and I'm convinced that explicit "make sure there's a NUL
at the very end" is enough.