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

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.

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.
- 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.


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