Re: [PATCH] mm: Don't warn if memdup_user fails

David Rientjes
2012-01-12T22:29:17Z

On Thu, 12 Jan 2012, Andrew Morton wrote:

> > I think it's good to fix ecryptfs like Tyler is doing and, at the same
> > time, ensure that the len passed to memdup_user() makes sense prior to
> > kmallocing memory with GFP_KERNEL. Perhaps something like
> >
> > if (WARN_ON(len > PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
> > return ERR_PTR(-ENOMEM);
> >
> > in which case __GFP_NOWARN is irrelevant.
>
> If someone is passing huge size_t's into kmalloc() and getting failures
> then that's probably a bug. So perhaps we should add a warning to
> kmalloc itself if the size_t is out of bounds, and !__GFP_NOWARN.
>

That's already done. For slub, for example, the largest object size
handled by the allocator itself is an order-1 page; everything else gets
passed through to the page allocator and its limitation is MAX_ORDER,
which is the warning that we're seeing in Sasha's changelog when
!__GFP_NOWARN.

> But none of this will be very effective. If someone is passing an
> unchecked size_t into kmalloc then normal testing will not reveal the
> problem because the testers won't pass stupid numbers into their
> syscalls.
>

They'll get the same warning that Sasha got, which is because the page
allocator can't handle larger than MAX_ORDER orders. The intention in my
WARN_ON() above specifically for memdup_user() is to avoid the infinite
loop in the page allocator for GFP_KERNEL allocations where the order
is less than PAGE_ALLOC_COSTLY_ORDER and avoid the oom killer. It returns
immediately rather than passing __GFP_NORETRY since we don't want to incur
the side-effects of direct reclaim or compaction as well.

The real fix would be to convert all callers to pass gfp flags into
memdup_user() to determine the behavior they want, though.