Re: [PATCH] libnvdimm/pmem: Bypass CONFIG_HARDENED_USERCOPY overhead

[Dan Williams]

On Fri, May 17, 2019 at 8:57 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Fri, May 17, 2019 at 08:08:27AM -0700, Dan Williams wrote:
> > As far as I can see it's mostly check_heap_object() that is the
> > problem, so I'm open to finding a way to just bypass that sub-routine.
> > However, as far as I can see none of the other block / filesystem user
> > copy implementations submit to the hardened checks, like
> > bio_copy_from_iter(), and iov_iter_copy_from_user_atomic() . So,
> > either those need to grow additional checks, or the hardened copy
> > implementation is targeting single object copy use cases, not
> > necessarily block-I/O. Yes, Kees, please advise.
>
> The intention is mainly for copies that haven't had explicit bounds
> checking already performed on them, yes. Is there something getting
> checked out of the slab, or is it literally just the overhead of doing
> the "is this slab?" check that you're seeing?

It's literally the overhead of "is this slab?" since it needs to go
retrieve the struct page and read that potentially cold cacheline. In
the case where that page is on memory media that is higher latency
than DRAM we get the ~37% performance loss that Jeff measured.

The path is via the filesystem ->write_iter() file operation. In the
DAX case the filesystem traps that path early, before submitting block
I/O, and routes it to the dax_iomap_actor() routine. That routine
validates that the logical file offset is within bounds of the file,
then it does a sector-to-pfn translation which validates that the
physical mapping is within bounds of the block device.

It seems dax_iomap_actor() is not a path where we'd be worried about
needing hardened user copy checks.