Re: [PATCH] arm64: Avoid premature usercopy failure

[Will Deacon]

On Mon, Jul 12, 2021 at 03:27:46PM +0100, Robin Murphy wrote:
> Al reminds us that the usercopy API must only return complete failure
> if absolutely nothing could be copied. Currently, if userspace does
> something silly like giving us an unaligned pointer to Device memory,
> or a size which overruns MTE tag bounds, we may fail to honour that
> requirement when faulting on a multi-byte access even though a smaller
> access could have succeeded.
> 
> Add a mitigation to the fixup routines to fall back to a single-byte
> copy if we faulted on a larger access before anything has been written
> to the destination, to guarantee making *some* forward progress. We
> needn't be too concerned about the overall performance since this should
> only occur when callers are doing something a bit dodgy in the first
> place. Particularly broken userspace might still be able to trick
> generic_perform_write() into an infinite loop by targeting write() at
> an mmap() of some read-only device register where the fault-in load
> succeeds but any store synchronously aborts such that copy_to_user() is
> genuinely unable to make progress, but, well, don't do that...
> 
> CC: stable@xxxxxxxxxxxxxxx
> Reported-by: Chen Huang <chenhuang5@xxxxxxxxxx>
> Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> ---
> 
> I've started trying the "replay" approach for figuring out more precise
> remainders in general, but that quickly got more complicated with
> rebasing the fault address passing stuff, so I'm resending this now as
> a point fix and will continue to explore that as an improvement on top.

Is it possible to add/extend a selftest for this, please? I think Catalin
mentioned that before, but not sure if he got anywhere with it.

Will