Date:   Thu, 2 Dec 2021 13:23:13 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Leon Romanovsky <leon@kernel.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Bixuan Cui <cuibixuan@linux.alibaba.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        w@1wt.eu
Subject: Re: [PATCH -next] mm: delete oversized WARN_ON() in kvmalloc() calls
Message-ID: <202112021320.87AB40A@keescook>
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On Thu, Dec 02, 2021 at 09:24:08PM +0200, Leon Romanovsky wrote:
> On Thu, Dec 02, 2021 at 11:08:34AM -0800, Kees Cook wrote:
> > On Thu, Dec 02, 2021 at 06:08:40PM +0200, Leon Romanovsky wrote:
> > > On Thu, Dec 02, 2021 at 03:29:47PM +0000, Matthew Wilcox wrote:
> > > > On Thu, Dec 02, 2021 at 05:23:42PM +0200, Leon Romanovsky wrote:
> > > > > The problem is that this WARN_ON() is triggered by the users.
> > > > 
> > > > ... or the problem is that you don't do a sanity check between the user
> > > > and the MM system.  I mean, that's what this conversation is about --
> > > > is it a bug to be asking for this much memory in the first place?
> > > 
> > > We do a lot of checks, and in this case, user provided valid input.
> > > He asked size that doesn't cross his address space.
> > > https://elixir.bootlin.com/linux/v5.16-rc3/source/drivers/infiniband/core/umem_odp.c#L67
> > > 
> > > 		start = ALIGN_DOWN(umem_odp->umem.address, page_size);
> > > 		if (check_add_overflow(umem_odp->umem.address,
> > > 				       (unsigned long)umem_odp->umem.length,
> > > 				       &end))
> > > 			return -EOVERFLOW;
> > > 
> > > There is a feature called ODP (on-demand-paging) which is supported
> > > in some RDMA NICs. It allows to the user "export" their whole address
> > > space to the other RDMA node without pinning the pages. And once the
> > > other node sends data to not-pinned page, the RDMA NIC will prefetch
> > > it.
> > 
> > I think we have two cases:
> > 
> > - limiting kvmalloc allocations to INT_MAX
> > - issuing a WARN when that limit is exceeded
> > 
> > The argument for the having the WARN is "that amount should never be
> > allocated so we want to find the pathological callers".
> > 
> > But if the actual issue is that >INT_MAX is _acceptable_, then we have
> > to do away with the entire check, not just the WARN.
> 
> First we need to get rid from WARN_ON(), which is completely safe thing to do.
> 
> Removal of the check can be done in second step as it will require audit
> of whole kvmalloc* path.

If those are legit sizes, I'm fine with dropping the WARN. (But I still
think if they're legit sizes, we must also drop the INT_MAX limit.)

-- 
Kees Cook