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
