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
Date: Tue, 14 Apr 2020 13:04:29 +0200 [thread overview]
Message-ID: <20200414110429.GF4629@dhcp22.suse.cz> (raw)
In-Reply-To: <CAHk-=whwRqkwdaJQf4g0-Evd6RmXR3dkkKyfnPjbnkeia=b1ug@mail.gmail.com>
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
next prev parent reply other threads:[~2020-04-14 11:04 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-08 1:40 [PATCH 0/2] mm: Two small fixes for recent syzbot reports Peter Xu
2020-04-08 1:40 ` [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal Peter Xu
2020-04-08 10:21 ` Michal Hocko
2020-04-08 14:20 ` Peter Xu
2020-04-08 14:30 ` Michal Hocko
2020-04-08 15:24 ` Peter Xu
2020-04-08 15:26 ` Michal Hocko
2020-04-09 7:02 ` Michal Hocko
2020-04-09 12:52 ` Peter Xu
2020-04-09 13:00 ` Peter Xu
2020-04-09 13:53 ` Michal Hocko
2020-04-09 16:42 ` Linus Torvalds
2020-04-14 11:04 ` Michal Hocko [this message]
2020-04-14 13:49 ` Peter Xu
2020-04-14 14:18 ` Michal Hocko
2020-04-20 12:47 ` Michal Hocko
2020-04-20 17:31 ` Linus Torvalds
2020-04-21 7:09 ` Michal Hocko
2020-04-08 1:40 ` [PATCH 2/2] mm/gup: Mark lock taken only after a successful retake Peter Xu
2020-04-09 0:47 ` [PATCH 0/2] mm: Two small fixes for recent syzbot reports Andrew Morton
2020-04-09 11:49 ` Matthew Wilcox
2020-04-09 13:00 ` Dmitry Vyukov
2020-04-09 18:16 ` Andrew Morton
2020-04-09 18:53 ` Linus Torvalds
2020-04-09 19:12 ` Andrew Morton
2020-04-09 19:46 ` Linus Torvalds
2020-04-09 19:56 ` Matthew Wilcox
2020-04-09 19:58 ` Linus Torvalds
2020-04-09 20:27 ` Eric Biggers
2020-04-09 20:34 ` Linus Torvalds
2020-04-09 23:34 ` Stephen Rothwell
2020-04-10 1:11 ` Theodore Y. Ts'o
2020-04-09 12:55 ` Dmitry Vyukov
2020-04-09 16:32 ` Linus Torvalds
2020-04-09 16:58 ` Qian Cai
2020-04-09 17:05 ` Linus Torvalds
2020-04-09 17:58 ` Qian Cai
2020-04-09 18:06 ` Linus Torvalds
2020-04-09 21:14 ` Qian Cai
2020-04-10 13:12 ` Tetsuo Handa
2020-04-10 14:26 ` Qian Cai
2020-04-10 17:26 ` Andrew Morton
2020-04-10 19:46 ` Qian Cai
2020-04-09 23:29 ` Stephen Rothwell
2020-04-13 22:06 ` Qian Cai
2020-04-13 23:05 ` Jens Axboe
2020-04-14 11:12 ` Dmitry Vyukov
2020-04-14 11:59 ` Qian Cai
2020-04-14 12:05 ` Dmitry Vyukov
2020-04-14 19:28 ` Dan Rue
2020-04-15 11:09 ` Dmitry Vyukov
2020-04-15 16:23 ` Dan Rue
2020-04-16 0:34 ` Stephen Rothwell
2020-05-11 15:29 ` Dmitry Vyukov
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