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Date:   Tue, 14 Apr 2020 13:04:29 +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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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