Messages in this thread | | | Subject | Re: [RFC] android: ion: How to properly clean caches for uncached allocations | From | Laura Abbott <> | Date | Thu, 8 Mar 2018 18:01:10 -0800 |
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On 03/08/2018 04:45 PM, Liam Mark wrote: > On Wed, 7 Mar 2018, Laura Abbott wrote: > >> On 02/28/2018 09:18 PM, Liam Mark wrote: >>> The issue: >>> >>> Currently in ION if you allocate uncached memory it is possible that there >>> are still dirty lines in the cache. And often these dirty lines in the >>> cache are the zeros which were meant to clear out any sensitive kernel >>> data. >>> >>> What this means is that if you allocate uncached memory from ION, and then >>> subsequently write to that buffer (using the uncached mapping you are >>> provided by ION) then the data you have written could be corrupted at some >>> point in the future if a dirty line is evicted from the cache. >>> >>> Also this means there is a potential security issue. If an un-privileged >>> userspace user allocated uncached memory (for example from the system heap) >>> and then if they were to read from that buffer (through the un-cached >>> mapping they are provided by ION), and if some of the zeros which were >>> written to that memory are still in the cache then this un-privileged >>> userspace user could read potentially sensitive kernel data. >> >> For the use case you are describing we don't actually need the >> memory to be non-cached until it comes time to do the dma mapping. >> Here's a proposal to shoot holes in: >> >> - Before any dma_buf attach happens, all mmap mappings are cached >> - At the time attach happens, we shoot down any existing userspace >> mappings, do the dma_map with appropriate flags to clean the pages >> and then allow remapping to userspace as uncached. Really this >> looks like a variation on the old Ion faulting code which I removed >> except it's for uncached buffers instead of cached buffers. >> > > Thanks Laura, I will take a look to see if I can think of any concerns. > > Initial thoughts. > - What about any kernel mappings (kmap/vmap) the client has made? >
We could either synchronize with dma_buf_{begin,end}_cpu_access or just disallow the mapping to happen if there's an outstanding kmap or vmap. Is this an actual problem or only theoretical?
> - I guess it would be tempting to only do this behavior for memory that > came from buddy (as opposed to the pool since it should be clean), but we > would need to be careful that no pages sneak into the pool without being > cleaned (example: client allocs then frees without ever call > dma_buf_attach). >
You're welcome to try that optimization but I think we should focus on the basics first. Honestly it might make sense just to have a single pool at this point since the cost of syncing is not happening on the allocation path.
>> Potential problems: >> - I'm not 100% about the behavior here if the attaching device >> is already dma_coherent. I also consider uncached mappings >> enough of a device specific optimization that you shouldn't >> do them unless you know it's needed. > > I don't believe we want to allow uncached memory to be dma mapped by an > io-coherent device and this is something I would like to eventually block. > > Since there is always a kernel cached mapping for ION uncached memory then > speculative access could still be putting lines in the cache, so when an > io-coherent device tries to read this uncached memory its snoop into the > cache could find one of these 'stale' clean cache lines and end up using > stale data. > Agree? >
Sounds right.
>> - The locking/sequencing with userspace could be tricky >> since userspace may not like us ripping mappings out from >> underneath if it's trying to access. > > Perhaps delay this work to the dma_map_attachment call since when the data > is dma mapped the CPU shouldn't be accessing it? > > Or worst case perhaps fail all map attempts to uncached memory until the > memory has been dma mapped (and cleaned) at least once? >
My concern was mostly concurrent userspace access on a buffer that's being dma_mapped but that sounds racy to begin with.
I suggested disallowing mmap until dma_mapping before and I thought that was not possible?
Thanks, Laura
> Thanks, > Liam > > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, > a Linux Foundation Collaborative Project >
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