Re: [PATCH] usercopy: Skip multi-page bounds checking on SLOB

[Michal Hocko]

On Thu 18-08-16 10:21:58, Rik van Riel wrote:
> On Wed, 2016-08-17 at 15:29 -0700, Kees Cook wrote:
> > When an allocator does not mark all allocations as PageSlab, or does
> > not
> > mark multipage allocations with __GFP_COMP, hardened usercopy cannot
> > correctly validate the allocation. SLOB lacks this, so short-circuit
> > the checking for the allocators that aren't marked with
> > CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR. This also updates the config
> > help and corrects a typo in the usercopy comments.
> > 
> > Reported-by: xiaolong.ye@xxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> There may still be some subsystems that do not
> go through kmalloc for multi-page allocations,
> and also do not use __GFP_COMP
> 
> I do not know whether there are, but if they exist
> those would still trip up the same way SLOB got
> tripped up before your patch.
> 
> One big question I have for Linus is, do we want
> to allow code that does a higher order allocation,
> and then frees part of it in smaller orders, or
> individual pages, and keeps using the remainder?

We even have an API for that alloc_pages_exact. I do not think anybody
uses that for copying from/to userspace but this pattern is not all that
rare.
-- 
Michal Hocko
SUSE Labs