Date: Mon, 12 Dec 2016 16:12:16 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
To: Michal Hocko <mhocko@kernel.org>
cc: Andrey Konovalov <andreyknvl@google.com>,
        Felipe Balbi <balbi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Deepa Dinamani <deepa.kernel@gmail.com>,
        Mathieu Laurendeau <mat.lau@laposte.net>, Bin Liu <b-liu@ti.com>,
        USB list <linux-usb@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>,
        Dmitry Vyukov <dvyukov@google.com>, Kostya Serebryany <kcc@google.com>,
        Cristopher Lameter <cl@linux.com>
Subject: Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask
In-Reply-To: <20161212210504.GB3107@dhcp22.suse.cz>
Message-ID: <Pine.LNX.4.44L0.1612121608270.1502-100000@iolanthe.rowland.org>
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On Mon, 12 Dec 2016, Michal Hocko wrote:

> On Mon 12-12-16 21:32:35, Andrey Konovalov wrote:
> > On Mon, Dec 12, 2016 at 9:31 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> > > Hi!
> > >
> > > While running the syzkaller fuzzer I've got the following error report.
> > >
> > > The issue is that the len argument is not checked for being too big.
> 
> Well, the value is checked in kmalloc_slab. There is a discrepancy
> though. While the page allocator enforces order < MAX_ORDER, slab
> allocators enforce size <= KMALLOC_MAX_SIZE and
> 
> KMALLOC_MAX_SIZE is 1UL << (MAX_ORDER + PAGE_SHIFT) for SLUB which is
> what I expect you have or 1UL << (MAX_ORDER + PAGE_SHIFT - 1) for SLAB
> on most archs.  This means that KMALLOC_MAX_SIZE might be MAX_ORDER for
> SLUB which would explain your warning.
> 
> Let's CC Christoph, this is nothing really new, but I suspect that SLUB
> should change the maximum size to something like SLAB does.
> 
> That being said, what ep_write_iter does sounds quite stupit. It just
> allocates a large continuous buffer which seems to be under user
> control...  Aka no good! It should do that per pages or something like
> that. Something worth fixing

It's not important enough to make the driver do all this work.  If
users want to send large amounts of data, they can send it a page at a
time (or something like that).

If you really want to prevent the driver from attempting to allocate a
large buffer, all that's needed is an upper limit on the total size.  
For example, 64 KB.

Alan Stern