Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP

[Robin Murphy <robin.murphy@arm.com>]

On 2022-03-23 19:16, Linus Torvalds wrote:
> On Wed, Mar 23, 2022 at 12:06 PM Robin Murphy <robin.murphy@arm.com> wrote:
>>
>> On 2022-03-23 17:27, Linus Torvalds wrote:
>>>
>>> I'm assuming that the ath9k issue is that it gives DMA mapping a big
>>> enough area to handle any possible packet size, and just expects -
>>> quite reasonably - smaller packets to only fill the part they need.
>>>
>>> Which that "info leak" patch obviously breaks entirely.
>>
>> Except that's the exact case which the new patch is addressing
> 
> Not "addressing". Breaking.
> 
> Which is why it will almost certainly get reverted.
> 
> Not doing DMA to the whole area seems to be quite the sane thing to do
> for things like network packets, and overwriting the part that didn't
> get DMA'd with zeroes seems to be exactly the wrong thing here.
> 
> So the SG_IO - and other random untrusted block command sources - data
> leak will almost certainly have to be addressed differently. Possibly
> by simply allocating the area with GFP_ZERO to begin with.

Er, the point of the block layer case is that whole area *is* zeroed to 
begin with, and a latent memory corruption problem in SWIOTLB itself 
replaces those zeros with random other kernel data unexpectedly. Let me 
try illustrating some sequences for clarity...

Expected behaviour/without SWIOTLB:
                              Memory
---------------------------------------------------
start                        12345678
dma_map(DMA_FROM_DEVICE)      no-op
device writes partial data   12ABC678 <- ABC
dma_unmap(DMA_FROM_DEVICE)   12ABC678


SWIOTLB previously:
                              Memory      Bounce buffer
---------------------------------------------------
start                        12345678    xxxxxxxx
dma_map(DMA_FROM_DEVICE)             no-op
device writes partial data   12345678    xxABCxxx <- ABC
dma_unmap(DMA_FROM_DEVICE)   xxABCxxx <- xxABCxxx


SWIOTLB Now:
                              Memory      Bounce buffer
---------------------------------------------------
start                        12345678    xxxxxxxx
dma_map(DMA_FROM_DEVICE)     12345678 -> 12345678
device writes partial data   12345678    12ABC678 <- ABC
dma_unmap(DMA_FROM_DEVICE)   12ABC678 <- 12ABC678


Now, sure we can prevent any actual information leakage by initialising 
the bounce buffer slot with zeros, but then we're just corrupting the 
not-written-to parts of the mapping with zeros instead of anyone else's 
old data. That's still fundamentally not OK. The only thing SWIOTLB can 
do to be correct is treat DMA_FROM_DEVICE as a read-modify-write of the 
entire mapping, because it has no way to know how much of it is actually 
going to be modified.

I'll admit I still never quite grasped the reason for also adding the 
override to swiotlb_sync_single_for_device() in aa6f8dcbab47, but I 
think by that point we were increasingly tired and confused and starting 
to second-guess ourselves (well, I was, at least). I don't think it's 
wrong per se, but as I said I do think it can bite anyone who's been 
doing dma_sync_*() wrong but getting away with it until now. If 
ddbd89deb7d3 alone turns out to work OK then I'd be inclined to try a 
partial revert of just that one hunk.

Thanks,
Robin.