linux-kernel - Re: [PATCH] /dev/mem: Add missing memory barriers for devmem

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Date:   Thu, 16 Jul 2020 08:53:30 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Russell King <linux@....linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] /dev/mem: Add missing memory barriers for devmem_inode

On Wed, Jul 15, 2020 at 11:07 PM Eric Biggers <ebiggers@...nel.org> wrote:
>
> From: Eric Biggers <ebiggers@...gle.com>
>
> WRITE_ONCE() isn't the correct way to publish a pointer to a data
> structure, since it doesn't include a write memory barrier.  Therefore
> other tasks may see that the pointer has been set but not see that the
> pointed-to memory has finished being initialized yet.  Instead a
> primitive with "release" semantics is needed.
>
> Use smp_store_release() for this.
>
> The use of READ_ONCE() on the read side is still potentially correct if
> there's no control dependency, i.e. if all memory being "published" is
> transitively reachable via the pointer itself.  But this pairing is
> somewhat confusing and error-prone.  So just upgrade the read side to
> smp_load_acquire() so that it clearly pairs with smp_store_release().
>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Russell King <linux@....linux.org.uk>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Fixes: 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims the region")
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Makes sense:

Acked-by: Dan Williams <dan.j.williams@...el.com>