[PATCH] arm64: Avoid premature usercopy failure

[Catalin Marinas]

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 at vger.kernel.org
> > Reported-by: Chen Huang <chenhuang5 at huawei.com>
> > Suggested-by: Al Viro <viro at zeniv.linux.org.uk>
> > Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> > Signed-off-by: Robin Murphy <robin.murphy at 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 ;).

-- 
Catalin