linux-kernel - Re: [PATCH] staging: android: ion: Skip sync if not mapped

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Date:   Tue, 14 Apr 2020 18:11:14 +0200
From:   Ørjan Eide <orjan.eide@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     nd@....com, anders.pedersen@....com, john.stultz@...aro.org,
        Laura Abbott <labbott@...hat.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        "Darren Hart (VMware)" <dvhart@...radead.org>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Arnd Bergmann <arnd@...db.de>, devel@...verdev.osuosl.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org
Subject: Re: [PATCH] staging: android: ion: Skip sync if not mapped

On Tue, Apr 14, 2020 at 04:28:10PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Apr 14, 2020 at 04:18:47PM +0200, ï¿½rjan Eide wrote:
> > Only sync the sg-list of an Ion dma-buf attachment when the attachment
> > is actually mapped on the device.
> > 
> > dma-bufs may be synced at any time. It can be reached from user space
> > via DMA_BUF_IOCTL_SYNC, so there are no guarantees from callers on when
> > syncs may be attempted, and dma_buf_end_cpu_access() and
> > dma_buf_begin_cpu_access() may not be paired.
> > 
> > Since the sg_list's dma_address isn't set up until the buffer is used
> > on the device, and dma_map_sg() is called on it, the dma_address will be
> > NULL if sync is attempted on the dma-buf before it's mapped on a device.
> > 
> > Before v5.0 (commit 55897af63091 ("dma-direct: merge swiotlb_dma_ops
> > into the dma_direct code")) this was a problem as the dma-api (at least
> > the swiotlb_dma_ops on arm64) would use the potentially invalid
> > dma_address. How that failed depended on how the device handled physical
> > address 0. If 0 was a valid address to physical ram, that page would get
> > flushed a lot, while the actual pages in the buffer would not get synced
> > correctly. While if 0 is an invalid physical address it may cause a
> > fault and trigger a crash.
> > 
> > In v5.0 this was incidentally fixed by commit 55897af63091 ("dma-direct:
> > merge swiotlb_dma_ops into the dma_direct code"), as this moved the
> > dma-api to use the page pointer in the sg_list, and (for Ion buffers at
> > least) this will always be valid if the sg_list exists at all.
> > 
> > But, this issue is re-introduced in v5.3 with
> > commit 449fa54d6815 ("dma-direct: correct the physical addr in
> > dma_direct_sync_sg_for_cpu/device") moves the dma-api back to the old
> > behaviour and picks the dma_address that may be invalid.
> > 
> > dma-buf core doesn't ensure that the buffer is mapped on the device, and
> > thus have a valid sg_list, before calling the exporter's
> > begin_cpu_access.
> > 
> > Signed-off-by: ï¿½rjan Eide <orjan.eide@....com>
> > ---
> >  drivers/staging/android/ion/ion.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > Resubmit without disclaimer, sorry about that.
> > 
> > This seems to be part of a bigger issue where dma-buf exporters assume
> > that their dma-buf begin_cpu_access and end_cpu_access callbacks have a
> > certain guaranteed behavior, which isn't ensured by dma-buf core.
> > 
> > This patch fixes this in ion only, but it also needs to be fixed for
> > other exporters, either handled like this in each exporter, or in
> > dma-buf core before calling into the exporters.
> > 
> > diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
> > index 38b51eace4f9..7b752ba0cb6d 100644
> > --- a/drivers/staging/android/ion/ion.c
> > +++ b/drivers/staging/android/ion/ion.c
> 
> Now that we have the dma-buff stuff in the tree, do we even need the
> ion code in the kernel anymore?  Can't we delete it now?

It looks like the new dma-heaps have the same issue as ion. The
heap-helpers also do dma_sync_sg_for_device() unconditionally on
end_cpu_access which may happen before dma_map_sg(), leading to use of
the 0 dma_address in the sg list of a, yet unmapped, attachment.

It could be fixed in dma-heaps just like this patch does for ion. Is
patch a valid way to fix this problem? Or, should this rather be handled
in dma-buf core by tracking the mapped state of attachments there?

-- 
ï¿½rjan