Re: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and user_write_access_begin/end

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Apr 02, 2020 at 11:35:57AM -0700, Kees Cook wrote:

> Yup, I think it's a weakness of the ARM implementation and I'd like to
> not extend it further. AFAIK we should never nest, but I would not be
> surprised at all if we did.
> 
> If we were looking at a design goal for all architectures, I'd like
> to be doing what the public PaX patchset did for their memory access
> switching, which is to alarm if calling into "enable" found the access
> already enabled, etc. Such a condition would show an unexpected nesting
> (like we've seen with similar constructs with set_fs() not getting reset
> during an exception handler, etc etc).

FWIW, maybe I'm misreading the ARM uaccess logics, but... it smells like
KERNEL_DS is somewhat more dangerous there than on e.g. x86.

Look: with CONFIG_CPU_DOMAINS, set_fs(KERNEL_DS) tells MMU to ignore
per-page permission bits in DOMAIN_KERNEL (i.e. for kernel address
ranges), allowing them even if they would normally be denied.  We need
that for actual uaccess loads/stores, since those use insns that pretend
to be done in user mode and we want them to access the kernel pages.
But that affects the normal loads/stores as well; unless I'm misreading
that code, it will ignore (supervisor) r/o on a page.  And that's not
just for the code inside the uaccess blocks; *everything* done under
KERNEL_DS is subject to that.

Why do we do that (modify_domain(), that is) inside set_fs() and not
in uaccess_enable() et.al.?