linux-kernel - Re: [PATCH] drm/panfrost: Avoid user size passed to kvmalloc()

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Date:   Fri, 17 Dec 2021 12:28:17 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Steven Price <steven.price@....com>
Cc:     Rob Herring <robh@...nel.org>, Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/panfrost: Avoid user size passed to kvmalloc()

On Fri, Dec 17, 2021 at 09:16:19AM +0000, Steven Price wrote:
> On 17/12/2021 09:10, Dan Carpenter wrote:
> > On Fri, Dec 17, 2021 at 08:55:50AM +0000, Steven Price wrote:
> >> However this one is harder to fix without setting an arbitrary cap on
> >> the number of BOs during a sumbit. I'm not sure how other drivers handle
> >> this - the ones I've looked at so far all have the same issue. There's
> >> obviously the list that Dan already sent, but e.g. msm has the same bug
> >> in msm_gem_submit.c:submit_create() with an amusing bug where the check
> >> for (sz > SIZE_MAX) will never hit, although the call is to kzalloc() so
> >> large allocations are going to fail anyway.
> > 
> > sz is u64 and SIZE_MAX is ULONG_MAX so the (sz > SIZE_MAX) condition
> > does work to prevent an integer overflow on 32bit systems.  But it's not
> > beautiful.
> 
> sz is the result of struct_size() which returns a size_t, and SIZE_MAX
> in case of an overflow.

Correct.

> However the check is *greater than* SIZE_MAX
> which will never occur even on 32 bit systems.

You've missed a part.  We add ((u64)nr_cmds * sizeof(submit->cmd[0]))
to SIZE_MAX.  If nr_cmds is zero then, whatever, the kzmalloc() will
fail.  No big deal.  But if it's non-zero then "sz" is larger than
SIZE_MAX and we allocate a smaller buffer than expected leading to
memory corruption.

Btw, it turns out that I had a hand in writing that check so hooray for
me.  :)  #HoorayForMe

regards,
dan carpenter