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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 27 Jan 2021 23:03:33 +0900 Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:

> On 2021/01/27 21:17, Michal Hocko wrote:
> > 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?
> > 
> 
> There is no point with allowing userspace to allocate 2GB of physically non-contiguous
> memory using kvmalloc(). Size is controlled by userspace, and memdup_user_nul() is used
> for allocating temporary memory which will be released before returning to userspace.
> 
> Sane userspace processes should allocate only one or a few pages using memdup_user_nul().
> Just making insane user processes (like fuzzer) fail memory allocation requests is a
> reasonable decision.

(cc Casey)

I'd say that the immediate problem is in smk_write_syslog().  Obviously
it was implemented expecting small writes, but the fuzzer is passing it a
huge write and things fall apart.