Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict cache maintenance to dma mapped memory

From: Liam Mark <lmark@codeaurora.org>
To: "Sumit Semwal" <sumit.semwal@linaro.org>,
	"Ørjan Eide" <Orjan.Eide@arm.com>
Cc: Brian Starkey <Brian.Starkey@arm.com>,
	"devel@driverdev.osuosl.org" <devel@driverdev.osuosl.org>,
	"tkjos@android.com" <tkjos@android.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"linaro-mm-sig@lists.linaro.org" <linaro-mm-sig@lists.linaro.org>,
	"arve@android.com" <arve@android.com>,
	"joel@joelfernandes.org" <joel@joelfernandes.org>,
	nd <nd@arm.com>, "maco@android.com" <maco@android.com>,
	"christian@brauner.io" <christian@brauner.io>
Subject: Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict cache maintenance to dma mapped memory
Date: Thu, 28 Feb 2019 15:49:26 -0800 (PST)	[thread overview]
Message-ID: <alpine.DEB.2.10.1902281541260.16374@lmark-linux.qualcomm.com> (raw)
In-Reply-To: <20190206154021.GC3768@e106893-lin.trondheim.arm.com>

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+ Sumit

Hi Sumit,

Do you have any thoughts on this patch? 

It fixes a potential crash in on older kernel and I think limiting 
begin/end_cpu_access to only apply cache maintenance when the buffer is 
dma mapped makes sense from a logical perspective and performance 
perspective.

On Wed, 6 Feb 2019, Ørjan Eide wrote:

> 
> I've run some testing, and this patch does indeed fix the crash in
> dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg
> list.
> 
> Tested-by: Ørjan Eide <orjan.eide@arm.com>
> 
> I tested this on an older kernel, v4.14, since the dma-mapping code
> moved, in v4.19, to ignore the dma_address and instead use sg_phys() to
> get a valid address from the page, which is always valid in the ion sg
> lists. While this wouldn't crash on newer kernels, it's still good to
> avoid the unnecessary work when no CMO is needed.
> 

Isn't a fix like this also required from a stability perspective for 
future kernels? I understand from your analysis below that the crash has 
been fixed after 4.19 by using sg_phys to get the address but aren't we 
breaking the DMA API contract by calling dma_sync_* without first dma 
mapping the memory, if so then we have no guarantee that future 
implementations of functions like dma_direct_sync_sg_for_cpu will properly 
handle calls to dma_sync_* if the memory is not dma mapped.

> Is this patch a candidate for the relevant stable kernels, those that 
> have this bug exposed to user space via Ion and DMA_BUF_IOCTL_SYNC?
> 

My belief is that is relevant for older kernels otherwise an unprivileged 
malicious userspace application may be able to crash the system if they 
can call DMA_BUF_IOCTL_SYNC at the right time.

BTW thanks Ørjan testing and anaalsyis you have carried out on this 
change.

Liam

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