linux-kernel - Re: [PATCH 2/2] usb: tegra: Fix zero length memory allocation

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Date:   Wed, 15 Jul 2020 09:43:42 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Thierry Reding <thierry.reding@...il.com>
CC:     Mathias Nyman <mathias.nyman@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH 2/2] usb: tegra: Fix zero length memory allocation


On 14/07/2020 10:32, Thierry Reding wrote:
> On Sun, Jul 12, 2020 at 11:28:37AM +0100, Jon Hunter wrote:
>> After commit cad064f1bd52 ("devres: handle zero size in devm_kmalloc()")
>> was added system suspend started failing on Tegra186. The kernel log
>> showed that the Tegra XHCI driver was crashing on entry to suspend when
>> attemptin the save the USB context. The problem is caused because we
>> are trying to allocate a zero length array for the IPFS context on
>> Tegra186 and following commit cad064f1bd52 ("devres: handle zero size
>> in devm_kmalloc()") this now causes a NULL pointer deference crash
>> when we try to access the memory. Fix this by only allocating memory
>> for both the IPFS and FPCI contexts when required.
>>
>> Cc: stable@...r.kernel.org
>>
>> Fixes: 5c4e8d3781bc ("usb: host: xhci-tegra: Add support for XUSB context save/restore")
>>
>> Signed-off-by: Jon Hunter <jonathanh@...dia.com>
>> ---
>>  drivers/usb/host/xhci-tegra.c | 22 ++++++++++++++--------
>>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> Actually it would seem to me that this is no longer a bug after your fix
> in patch 1. We only ever access tegra->context.ipfs if
> tegra->soc->ipfs.num_offsets > 0, so the special ZERO_SIZE_PTR case will
> not actually cause an issue anymore.
> 
> The reason why this was crashing was because tegra->context.fpci was
> allocated with a zero size (because of the bug that you fixed in patch
> 1) and then that zero-size pointer was dereferenced because the code was
> correctly checking for tegra->soc->fpci.num_offsets > 0 in the context
> save and restore.
> 
> So I don't think there's a bug here. It's not wrong to allocate a zero-
> size buffer. It's only a bug to then go and dereference it. Are you
> still seeing the issue if you leave out this patch and only apply patch
> 1?

Ah yes you are right. OK, we can drop this. I will update the commit
message to patch 1/1.

Jon

-- 
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