Re: [PATCH 1/2] bpf: do not use KMALLOC_SHIFT_MAX

[Michal Hocko <mhocko@kernel.org>]

On Fri 16-12-16 16:28:21, Alexei Starovoitov wrote:
> On Sat, Dec 17, 2016 at 12:39:17AM +0100, Michal Hocko wrote:
> > On Fri 16-12-16 15:23:42, Alexei Starovoitov wrote:
> > > On Fri, Dec 16, 2016 at 11:02:35PM +0100, Michal Hocko wrote:
> > > > On Fri 16-12-16 10:02:10, Alexei Starovoitov wrote:
> > > > > On Thu, Dec 15, 2016 at 05:47:21PM +0100, Michal Hocko wrote:
> > > > > > From: Michal Hocko <mhocko@suse.com>
> > > > > > 
> > > > > > 01b3f52157ff ("bpf: fix allocation warnings in bpf maps and integer
> > > > > > overflow") has added checks for the maximum allocateable size. It
> > > > > > (ab)used KMALLOC_SHIFT_MAX for that purpose. While this is not incorrect
> > > > > > it is not very clean because we already have KMALLOC_MAX_SIZE for this
> > > > > > very reason so let's change both checks to use KMALLOC_MAX_SIZE instead.
> > > > > > 
> > > > > > Cc: Alexei Starovoitov <ast@kernel.org>
> > > > > > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > > > > 
> > > > > Nack until the patches 1 and 2 are reversed.
> > > > 
> > > > I do not insist on ordering. The thing is that it shouldn't matter all
> > > > that much. Or are you worried about bisectability?
> > > 
> > > This patch 1 strongly depends on patch 2 !
> > > Therefore order matters.
> > > The patch 1 by itself is broken.
> > > The commit log is saying
> > > '(ab)used KMALLOC_SHIFT_MAX for that purpose .. use KMALLOC_MAX_SIZE instead'
> > > that is also incorrect. We cannot do that until KMALLOC_MAX_SIZE is fixed.
> > > So please change the order
> > 
> > Yes, I agree that using KMALLOC_MAX_SIZE could lead to a warning with
> > the current ordering. Why that matters all that much is less clear to
> > me. The allocation would simply fail and you would return ENOMEM rather
> > than E2BIG. Does this really matter?
> > 
> > Anyway, as I've said, I do not really insist on the current ordering and
> > the will ask Andrew to reorder them. I am just really wondering about
> > such a strong pushback about something that barely matters. Or maybe I
> > am just missing your point and checking KMALLOC_MAX_SIZE without an
> > update would lead to a wrong behavior, user space breakage, crash or
> > anything similar.
> 
> if admin set ulimit for locked memory high enough for the particular user,
> that non-root user will be able to trigger warn_on_once in __alloc_pages_slowpath
> which is not acceptable.

But why is the warning such a big deal?

Also note that such a setup would be inherently dangerous. Even the
default ulimit for the locked memory allows to allocat 64k which means
that an untrusted user will be able to request PAGE_ALLOC_COSTLY_ORDER
and potentially deplete those larger blocks to the extend it hits the
OOM killer with the current gfp flags.

I think what you really want is a GFP_NORETRY for size > PAGE_SIZE and
fallback to the vmalloc for failure. But that is a separate topic.

> Also see the comment in hashtab.c
>   if (htab->map.value_size >= (1 << (KMALLOC_SHIFT_MAX - 1)) -
>       MAX_BPF_STACK - sizeof(struct htab_elem))
>           /* if value_size is bigger, the user space won't be able to
>            * access the elements via bpf syscall. This check also makes
>            * sure that the elem_size doesn't overflow and it's
>            * kmalloc-able later in htab_map_update_elem()
>            */
>           goto free_htab;

I have seen this comment before, but honestly, I do not understand it
(well apart from the overflow part).
htab_map_update_elem has to be able to handle the allocation failure in
any case. Note that any allocation larger than 64kB is likely to fail
anyway.

> 
> > > and fix the commit log to say that KMALLOC_MAX_SIZE
> > > is actually valid limit now.
> > 
> > KMALLOC_MAX_SIZE has always been the right limit. It's value has been
> > incorrect but that is to be fixed now. Using KMALLOC_SHIFT_MAX is simply
> > abusing an internal constant. So I am not sure what should be fixed in
> > the changelog.
> 
> that's exactly my problem with this patch and the commit log.
> You think it's abusing KMALLOC_SHIFT_MAX whereas it's doing so
> for reasons stated above.
> That piece of code cannot use KMALLOC_MAX_SIZE until it's fixed.
> So commit log should say something like:
> "now since KMALLOC_MAX_SIZE is fixed and size < KMALLOC_MAX_SIZE condition
> guarantees warn free allocation in kmalloc(value_size, GFP_USER | __GFP_NOWARN);
> we can safely use KMALLOC_MAX_SIZE instead of KMALLOC_SHIFT_MAX"

OK, fair enough, I will update the changelog

-- 
Michal Hocko
SUSE Labs

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