linux-kernel - Re: [PATCH -next] mm: delete oversized WARN

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Date:   Thu, 2 Dec 2021 21:24:08 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bixuan Cui <cuibixuan@...ux.alibaba.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        w@....eu
Subject: Re: [PATCH -next] mm: delete oversized WARN_ON() in kvmalloc() calls

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.

Thanks

> 
> -- 
> Kees Cook