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

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Message-ID: <20200714093256.GG141356@ulmo>
Date:   Tue, 14 Jul 2020 11:32:56 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Jon Hunter <jonathanh@...dia.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 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?

Thierry

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