On Tue, Jul 13, 2021 at 05:59:58PM +0100, Will Deacon wrote:
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@vger.kernel.org Reported-by: Chen Huang chenhuang5@huawei.com Suggested-by: Al Viro viro@zeniv.linux.org.uk Reviewed-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Robin Murphy robin.murphy@arm.com
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.
It's on my to-do list but going on holiday soon. If Robin is keen on this, I don't really mind ;).