From: Jann Horn <jannh@google.com>
To: Yang Shi <shy828301@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
"Zach O'Keefe" <zokeefe@google.com>,
linux-kernel@vger.kernel.org,
David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm/khugepaged: Fix ->anon_vma race
Date: Fri, 13 Jan 2023 20:36:37 +0100 [thread overview]
Message-ID: <CAG48ez3JUGWyDQ1hioVVy==5QCeDw3B=3TbGT0hUa1MygqOasg@mail.gmail.com> (raw)
In-Reply-To: <CAHbLzkqqKWcMOxm9V4xeyDnnqBVzUjgMaPHtxV=+X3yq4NP1zw@mail.gmail.com>
On Thu, Jan 12, 2023 at 2:06 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Wed, Jan 11, 2023 at 5:33 AM Jann Horn <jannh@google.com> wrote:
> >
> > If an ->anon_vma is attached to the VMA, collapse_and_free_pmd() requires
> > it to be locked. retract_page_tables() bails out if an ->anon_vma is
> > attached, but does this check before holding the mmap lock (as the comment
> > above the check explains).
> >
> > If we racily merge an existing ->anon_vma (shared with a child process)
> > from a neighboring VMA, subsequent rmap traversals on pages belonging to
> > the child will be able to see the page tables that we are concurrently
> > removing while assuming that nothing else can access them.
> >
> > Repeat the ->anon_vma check once we hold the mmap lock to ensure that there
> > really is no concurrent page table access.
> >
> > Reported-by: Zach O'Keefe <zokeefe@google.com>
> > Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> > zokeefe@ pointed out to me that the current code (after my last round of patches)
> > can hit a lockdep assert by racing, and after staring at it a bit I've
>
> I'm supposed the lockdep is the one in collapse_and_free_pmd(). It is
> better to have the splat included in the commit log.
I pasted the splat in
https://lore.kernel.org/linux-mm/CAG48ez3434wZBKFFbdx4M9j6eUwSUVPd4dxhzW_k_POneSDF+A@mail.gmail.com/
- which part do you think should go into the commit log? Just the
"WARNING: CPU: 14 PID: 116 at
mm/khugepaged.c:1406 collapse_and_free_pmd+0x364/0x420"? Or the whole
ASAN splat below the lockdep complaint with all three backtraces?
> > convinced myself that this is a real, preexisting bug.
> > (I haven't written a reproducer for it though. One way to hit it might be
> > something along the lines of:
> >
> > - set up a process A with a private-file-mapping VMA V1
> > - let A fork() to create process B, thereby copying V1 in A to V1' in B
> > - let B extend the end of V1'
> > - let B put some anon pages into the extended part of V1'
>
> I don't quite get why we need this step. A cow fault on A's V1 isn't
> enough to have anon_vma for V1? This should not prevent V1 and V2 from
> sharing anon_vma. Did I miss something?
You're right, these steps don't work.
next prev parent reply other threads:[~2023-01-13 19:37 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-11 13:33 [PATCH] mm/khugepaged: Fix ->anon_vma race Jann Horn
2023-01-12 1:06 ` Yang Shi
2023-01-13 19:36 ` Jann Horn [this message]
2023-01-12 8:56 ` Kirill A. Shutemov
2023-01-12 18:12 ` Yang Shi
2023-01-13 0:10 ` Kirill A. Shutemov
2023-01-13 3:22 ` Yang Shi
2023-01-13 19:28 ` Jann Horn
2023-01-15 19:06 ` Kirill A. Shutemov
2023-01-16 12:06 ` Jann Horn
2023-01-16 12:34 ` Kirill A. Shutemov
2023-01-16 12:54 ` Jann Horn
2023-01-16 13:07 ` David Hildenbrand
2023-01-16 13:47 ` Kirill A. Shutemov
2023-01-23 11:07 ` David Hildenbrand
2023-01-24 0:51 ` Kirill A. Shutemov
2023-01-24 10:19 ` David Hildenbrand
2023-01-17 18:57 ` Yang Shi
2023-01-17 19:12 ` Jann Horn
2023-01-17 22:55 ` Andrew Morton
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