Re: [PATCH v3] mm: memdup_user*() should use same gfp flags

[Michal Hocko <mhocko@suse.com>]

On Wed 27-01-21 12:59:28, Michal Hocko wrote:
> On Wed 27-01-21 19:55:38, Tetsuo Handa wrote:
> > syzbot is reporting that memdup_user_nul() which receives user-controlled
> > size (which can be up to (INT_MAX & PAGE_MASK)) via vfs_write() will hit
> > order >= MAX_ORDER path [1].
> > 
> > Making costly allocations (order > PAGE_ALLOC_COSTLY_ORDER) naturally fail
> > should be better than trying to enforce PAGE_SIZE upper limit, for some of
> > callers accept space-delimited list arguments.
> > 
> > Therefore, let's add __GFP_NOWARN to memdup_user_nul() as with
> > commit 6c8fcc096be9d02f ("mm: don't let userspace spam allocations
> > warnings"). Also use GFP_USER as with other userspace-controllable
> > allocations like memdup_user().
> 
> I absolutely detest hiding this behind __GFP_NOWARN. There should be no
> reason to even try hard for memdup_user_nul. Can you explain why this

this should have been "try hard to get a physicaly contiguous memory for memdup_user_nul"

> cannot use kvmalloc instead?

-- 
Michal Hocko
SUSE Labs