[PATCH v2 2/4] spi: Fix cache corruption due to DMA/PIO overlap

[Robin Murphy]

On 2022-09-30 12:20, Marek Szyprowski wrote:
> Hi,
> 
> CCed: Christoph and Robin, as the issue is partially dma-mapping related.
> 
> On 27.09.2022 13:21, Vincent Whitchurch wrote:
>> The SPI core DMA mapping support performs cache management once for the
>> entire message and not between transfers, and this leads to cache
>> corruption if a message has two or more RX transfers with both
>> transfers targeting the same cache line, and the controller driver
>> decides to handle one using DMA and the other using PIO (for example,
>> because one is much larger than the other).
>>
>> Fix it by syncing before/after the actual transfers.  This also means
>> that we can skip the sync during the map/unmap of the message.
>>
>> Fixes: 99adef310f68 ("spi: Provide core support for DMA mapping transfers")
>> Signed-off-by: Vincent Whitchurch <vincent.whitchurch at axis.com>
>> ---
> 
> This patch landed in linux next-20220929 as commit 0c17ba73c08f ("spi:
> Fix cache corruption due to DMA/PIO overlap"). Unfortunately it causes
> kernel oops on one of my test systems:
> 
> 8<--- cut here ---
> Unable to handle kernel NULL pointer dereference at virtual address 0000000c
> [0000000c] *pgd=00000000
> Internal error: Oops: 5 [#1] PREEMPT SMP ARM
> Modules linked in: cmac bnep btsdio hci_uart btbcm s5p_mfc btintel
> brcmfmac bluetooth videobuf2_dma_contig videobuf2_memops videobuf2_v4l2
> videobuf2_common videodev cfg80211 mc ecdh_generic ecc brcmutil
> CPU: 0 PID: 12 Comm: kworker/0:1 Not tainted
> 6.0.0-rc7-next-20220929-dirty #12903
> Hardware name: Samsung Exynos (Flattened Device Tree)
> Workqueue: events ax88796c_work
> PC is at dma_direct_sync_sg_for_device+0x24/0xb8
> LR is at spi_transfer_one_message+0x4c4/0xabc
> pc : [<c01cbcf0>]    lr : [<c0739fcc>]    psr: 20000013
> ...
> Process kworker/0:1 (pid: 12, stack limit = 0xca429928)
> Stack: (0xe0071d38 to 0xe0072000)
> ...
>    dma_direct_sync_sg_for_device from spi_transfer_one_message+0x4c4/0xabc
>    spi_transfer_one_message from __spi_pump_transfer_message+0x300/0x770
>    __spi_pump_transfer_message from __spi_sync+0x304/0x3f4
>    __spi_sync from spi_sync+0x28/0x40
>    spi_sync from axspi_read_rxq+0x98/0xc8
>    axspi_read_rxq from ax88796c_work+0x7a8/0xf6c
>    ax88796c_work from process_one_work+0x288/0x774
>    process_one_work from worker_thread+0x44/0x504
>    worker_thread from kthread+0xf0/0x124
>    kthread from ret_from_fork+0x14/0x2c
> Exception stack(0xe0071fb0 to 0xe0071ff8)
> ...
> ---[ end trace 0000000000000000 ]---
> 
> This happens because sg_free_table() doesn't clear table->orig_nents nor
> table->nents. If the given spi xfer object is reused without dma-mapped
> buffer, then a NULL pointer de-reference happens at table->sgl
> spi_dma_sync_for_device()/spi_dma_sync_for_cpu(). A possible fix would
> be to zero table->orig_nents in spi_unmap_buf_attrs(). I will send a
> patch for this soon.
> 
> However, I think that clearing table->orig_nents and table->nents should
> be added to __sg_free_table() in lib/scatterlist.c to avoid this kind of
> issue in the future. This however will be a significant change that
> might break code somewhere, if it relies on the nents/orig_nents value
> after calling sg_free_table(). Christoph, Robin - what is your opinion?

Yes, that makes sense to me: the table->nents etc. fields logically 
describe the list that table->sgl points to, so when it sets that to 
NULL it seems right to also update the corresponding fields accordingly. 
I don't see much good reason for code to poking into an sg_table after 
it's been freed, other that to reinitialise it with sg_alloc_table() 
which would overwrite those fields anyway, so I can't imagine it's a 
particularly risky change.

That said, maybe this is something that's better to catch than to paper 
over? Arguably the real bug here is that spi_unmap_buf() and the new 
sync functions should use the same "{tx,rx}_buf != NULL" condition that 
spi_map_buf() used for the DMA mapping decision in the first place.

Thanks,
Robin.

> 
> 
>>    drivers/spi/spi.c | 109 +++++++++++++++++++++++++++++++++++++---------
>>    1 file changed, 88 insertions(+), 21 deletions(-)
>>
>> ...
> 
> Best regards