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SubjectRe: No check of the size passed to unmap_single in swiotlb
Hi Eric,

On 23/11/17 09:08, Eric Yang wrote:
>> -----Original Message----- From: Robin Murphy
>> [] Sent: Tuesday, November 21, 2017
>> 12:50 AM To: Konrad Rzeszutek Wilk <>; Eric
>> Yang <>; Cc:
>> Daniel Borkmann <>; Kees Cook
>> <>; Geert Uytterhoeven
>> <>; Greg Kroah-Hartman
>> <>; linux-; David
>> Miller <>; Al Viro <>;
>> Andrey Ryabinin <>; Andrew Morton
>> <>; Ingo Molnar <>
>> Subject: Re: No check of the size passed to unmap_single in
>> swiotlb
>> On 20/11/17 16:26, Konrad Rzeszutek Wilk wrote:
>>> On Mon, Nov 20, 2017 at 08:17:14AM +0000, Eric Yang wrote:
>>>> Hi all,
>>> Hi!
>>>> During debug a device only support 32bits DMA(Qualcomm Atheros
>>>> AP) in
>> our LS1043A 64bits ARM SOC, we found that the invoke of
>> dma_unmap_single - -> swiotlb_tbl_unmap_single will unmap the
>> passed "size" anyway even when the "size" is incorrect.
>>>> If the size is larger than it should, the extra entries in
>>>> io_tlb_orig_addr array
>> will be refilled by INVALID_PHYS_ADDR, and it will cause the bounce
>> buffer copy not happen when the one who really used the mis-freed
>> entries doing DMA data transfers, and this will cause further
>> unknow behaviors.
>>>> Here we just fix it temporarily by adding a judge of the "size"
>>>> in the
>> swiotlb_tbl_unmap_single, if it is larger than it deserves, just
>> unmap the right size only. Like the code:
>>> Did the DMA debug API (CONFIG_DMA_API_DEBUG) help in figuring
>>> this issue
>> as well?
>>>> [yangyu@titan dash-lts]$ git diff ./lib/swiotlb.c diff --git
>>>> a/lib/swiotlb.c b/lib/swiotlb.c index
>>>> ad1d2962d129..58c97ede9d78 100644 --- a/lib/swiotlb.c +++
>>>> b/lib/swiotlb.c @@ -591,7 +591,10 @@ void
>>>> swiotlb_tbl_unmap_single(struct device
>> *hwdev, phys_addr_t tlb_addr,
>>>> */ for (i = index + nslots - 1; i >= index; i--) {
>>>> io_tlb_list[i] = ++count; -
>>>> io_tlb_orig_addr[i] = INVALID_PHYS_ADDR; +
>>>> if(io_tlb_orig_addr[i] != orig_addr) +
>>>> printk("======size wrong, ally down ally down!===\n"); +
>>>> else + io_tlb_orig_addr[i] =
>>>> INVALID_PHYS_ADDR; } /* * Step 2: merge the returned slots with
>>>> the preceding slots,
>>>> Although pass a right size of DMA buffer is the responsibility
>>>> of the drivers,
>> but Is it useful to add some size check code to prevent real
>> damage happen?
>> There doesn't seem to be much good reason for SWIOTLB to be more
>> special than other DMA API backends, and not all of them have
>> enough internal state to be able to make such a check. It's also
>> not necessarily possible to "prevent damage" anyway - if a driver
>> does pass a bogus size for dma_unmap_single(..., DMA_FROM_DEVICE),
>> SWIOTLB might be able to keep itself internally consistent, but it
>> still can't prevent the arch code in the middle from invalidating
>> the wrong cache lines and potentially corrupting adjacent memory.
>> In short, trying to work around broken drivers is a much worse idea
>> than just fixing those drivers, and that's what we already have
>> dma-debug for.
>> Robin.
> Hi Robin,
> I agree that hacking kernel to fix broken drivers is not acceptable,
> actually we spent days to fight driver supplier with this, they do
> not want to change their code and want fix it directly in kernel.

So their code misuses an API in a manner which has never been correct,
and is *impossible* for many common implementations of that API to
validate, and they think it's upstream's job to work around it? Wow...
you have my sympathy :)

> I tried Dma-debug yesterday, it works very well, but I think only
> the size mismatch check may not be enough for the map entry corrupt
> situation, some run-time warning may be better when the real
> corruption happen.
> For most of the dma-api backend, the size mismatch may do no harm at
> all, and even in SWIOTLB itself when the bounce buffer is not used,
> the size mismatch do no harm either. In our case, the same buggy
> driver works well when board has 2G DDR, but panic frequently in 4G
> DDR because of the use of bounce buffer and these corrupted map
> entries. it is hard to catch this kind of bugs, for when the
> corruption happen, the kernel has all kind of reasons to panic, but
> not even one may directly point to the real source.

As I said, just because things appear to work for your test cases on
your system in the non-bounced case doesn't mean it's universally fine.
If this device can be integrated into non-cache-coherent systems, then
over-unmapping of device-writable buffers will eventually cause random
corruption and data loss to somebody, somewhere, by invalidating dirty
cache lines in the wrong place. If this device can be integrated behind
an IOMMU (and if it's available with a PCI/PCIe interface, assume that
it will be), then any over-unmapping will remove other devices'
translations and cause random DMA problems which can be even less
obvious to debug, and cannot be 'worked around' at all (certainly on the
arm64 and x86 implementations).

> Add the warning messages is a big convenience for figure this kind of
> issues, at least to me and the AP driver supplier, such warnings may
> save weeks of hard debug time.

I don't get it - if driver developers are writing buggy drivers and not
testing with basic well-established features like dma-debug, that's on
the driver developers. Optimising for the case where BSP developers
happen to get lucky with a particular configuration in which they might
see driver bugs tickle warnings elsewhere doesn't seem sensible. Yes, it
wouldn't be utterly unacceptable for SWIOTLB to print a warning when it
detects some (address,size) combination that looks like it may have gone
out-of-bounds, but at that point swiotlb_bounce() has presumably already
done the damage of overwriting something it shouldn't have with who
knows what, and it's still only one specific case - for instance, you
wouldn't detect if the size is too small and you haven't bounced
*enough* data, but that would still make your I/O misbehave.

In the end, it comes down to the difference between a) I/O going wrong
and the system crashing, and b) the user *possibly* getting a warning
they can't do anything about before I/O going wrong and the system
crashing. Ultimately the driver developer still has to fix their bug, so
why add code to occasionally antagonise the user when a developer
feature tailor-made for catching such bugs immediately has existed for
nearly 9 years?


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