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Date:   Mon, 20 Apr 2020 14:47:59 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Peter Xu <peterx@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        syzbot+693dc11fcb53120b5559@syzkaller.appspotmail.com
Subject: Re: [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal
 signal
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Any opinion on this Linus? Should I just repost the patch?

On Tue 14-04-20 13:04:32, Michal Hocko wrote:
> On Thu 09-04-20 09:42:20, Linus Torvalds wrote:
> > On Thu, Apr 9, 2020 at 12:03 AM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > This patch however doesn't go all the way to revert it because 0 return
> > > value is impossible.
> > 
> > I'm not convinced it's impossible.
> 
> __get_user_pages is documented as
>  * -- If nr_pages is 0, returns 0.
>  * -- If nr_pages is >0, but no pages were pinned, returns -errno.
>  * -- If nr_pages is >0, and some pages were pinned, returns the number of
>  *    pages pinned. Again, this may be less than nr_pages.
> 
> but let me double check the actual code... There seem to be only one
> exception the above rule AFAICS. faultin_page returning EBUSY will
> be overriden to either 0 for the first page or return the number of
> already pinned pages. So nr_pages > 0 && ret = 0 is indeed possible
> from __get_user_pages :/ That will be the case only for VM_FAULT_RETRY,
> thoug.
> 
> Now __get_user_pages_locked behaves differently. It keeps retrying the
> fault until it succeeds unless FOLL_NOWAIT is specified. Then it would
> return 0. Why we need to return 0 is not really clear to me but it
> seem to be a long term behavior. I believe we need to document it.
> 
> > And if it is, then the current code is harmless.
> 
> Yes from the above it seems that the check is indeed harmless becasue
> this path doesn't use FOLL_NOWAIT and so it will never see 0 return.
> I find a reference to EINTR confusing so I would still love to change
> that.
> 
> > Now, I do agree that we probably should go through and clarify the
> > whole range of different get_user_pages() cases of returning zero (or
> > not doing so), but right now it's so confusing that I'd prefer to keep
> > that (possibly unnecessary) belt-and-suspenders check for zero in
> > there.
> >
> > If/when somebody actually does a real audit and the result is "these
> > functions cannot return zero" and it's documented, then we can remove
> > those checks.
> 
> Would you mind this patch instead?
> 
> commit bc6c0fa7c7fb5eb54963dca65ae4a62ba04c9efa
> Author: Michal Hocko <mhocko@suse.com>
> Date:   Thu Apr 9 08:26:57 2020 +0200
> 
>     mm, mempolicy: fix up gup usage in lookup_node
>     
>     ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal signal") has
>     added a special casing for 0 return value because that was a possible
>     gup return value when interrupted by fatal signal. This has been fixed
>     by ae46d2aa6a7f ("mm/gup: Let __get_user_pages_locked() return -EINTR
>     for fatal signal") in the mean time so ba841078cd05 can be reverted.
>     
>     This patch however doesn't go all the way to revert it because the check
>     for 0 is wrong and confusing here. Firstly it is inherently unsafe to
>     access the page when get_user_pages_locked returns 0 (aka no page
>     returned).
>     Fortunatelly this will not happen because get_user_pages_locked will not
>     return 0 when nr_pages > 0 unless FOLL_NOWAIT is specified which is not
>     the case here. Document this potential error code in gup code while we
>     are at it.
>     
>     Signed-off-by: Michal Hocko <mhocko@suse.com>
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 50681f0286de..a8575b880baf 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -980,6 +980,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>   * -- If nr_pages is >0, but no pages were pinned, returns -errno.
>   * -- If nr_pages is >0, and some pages were pinned, returns the number of
>   *    pages pinned. Again, this may be less than nr_pages.
> + * -- 0 return value is possible when the fault would need to be retried.
>   *
>   * The caller is responsible for releasing returned @pages, via put_page().
>   *
> @@ -1247,6 +1248,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
>  }
>  EXPORT_SYMBOL_GPL(fixup_user_fault);
>  
> +/*
> + * Please note that this function, unlike __get_user_pages will not
> + * return 0 for nr_pages > 0 without FOLL_NOWAIT
> + */
>  static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
>  						struct mm_struct *mm,
>  						unsigned long start,
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 48ba9729062e..1965e2681877 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -927,10 +927,7 @@ static int lookup_node(struct mm_struct *mm, unsigned long addr)
>  
>  	int locked = 1;
>  	err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
> -	if (err == 0) {
> -		/* E.g. GUP interrupted by fatal signal */
> -		err = -EFAULT;
> -	} else if (err > 0) {
> +	if (err > 0) {
>  		err = page_to_nid(p);
>  		put_page(p);
>  	}
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs