LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Jason Gunthorpe <jgg@ziepe.ca>
To: Liam Howlett <liam.howlett@oracle.com>
Cc: Luigi Rizzo <lrizzo@google.com>, Jann Horn <jannh@google.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [PATCH] Add mmap_assert_locked() annotations to find_vma*()
Date: Tue, 3 Aug 2021 20:35:42 -0300	[thread overview]
Message-ID: <20210803233542.GH543798@ziepe.ca> (raw)
In-Reply-To: <20210803230725.ao3i2emejyyor36n@revolver>

On Tue, Aug 03, 2021 at 11:07:35PM +0000, Liam Howlett wrote:
> * Luigi Rizzo <lrizzo@google.com> [210803 17:49]:
> > On Tue, Aug 3, 2021 at 6:08 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Sat, Jul 31, 2021 at 10:53:41AM -0700, Luigi Rizzo wrote:
> > > > find_vma() and variants need protection when used.
> > > > This patch adds mmap_assert_lock() calls in the functions.
> > > >
> > > > To make sure the invariant is satisfied, we also need to add a
> > > > mmap_read_loc() around the get_user_pages_remote() call in
> > > > get_arg_page(). The lock is not strictly necessary because the mm
> > > > has been newly created, but the extra cost is limited because
> > > > the same mutex was also acquired shortly before in __bprm_mm_init(),
> > > > so it is hot and uncontended.
> > > >
> > > > Signed-off-by: Luigi Rizzo <lrizzo@google.com>
> > > >  fs/exec.c | 2 ++
> > > >  mm/mmap.c | 2 ++
> > > >  2 files changed, 4 insertions(+)
> > > >
> > > > diff --git a/fs/exec.c b/fs/exec.c
> > > > index 38f63451b928..ac7603e985b4 100644
> > > > +++ b/fs/exec.c
> > > > @@ -217,8 +217,10 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
> > > >        * We are doing an exec().  'current' is the process
> > > >        * doing the exec and bprm->mm is the new process's mm.
> > > >        */
> > > > +     mmap_read_lock(bprm->mm);
> > > >       ret = get_user_pages_remote(bprm->mm, pos, 1, gup_flags,
> > > >                       &page, NULL, NULL);
> > > > +     mmap_read_unlock(bprm->mm);
> > > >       if (ret <= 0)
> > > >               return NULL;
> > >
> > > Wasn't Jann Horn working on something like this too?
> > >
> > > https://lore.kernel.org/linux-mm/20201016225713.1971256-1-jannh@google.com/
> > >
> > > IIRC it was very tricky here, are you sure it is OK to obtain this lock
> > > here?
> > 
> > I cannot comment on Jann's patch series but no other thread knows
> > about this mm at this point in the code so the lock is definitely
> > safe to acquire (shortly before there was also a write lock acquired
> > on the same mm, in the same conditions).
> 
> If there is no other code that knows about this mm, then does one need
> the lock at all?  Is this just to satisfy the new check you added?
> 
> If you want to make this change, I would suggest writing it in a way to
> ensure the call to expand_downwards() in the same function also holds
> the lock.  I believe this is technically required as well?  What do you
> think?

This is essentially what Jann was doing. Since the mm is newly created
we can create it write locked and then we can add proper locking tests
to many of the functions called along this path.

Adding useless locks around each troublesome callsite just seems
really confusing to me.

Jason

  reply	other threads:[~2021-08-03 23:35 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-31 17:53 Luigi Rizzo
2021-08-01 19:33 ` Andrew Morton
2021-08-02  0:16   ` Luigi Rizzo
2021-08-02 21:11     ` Andrew Morton
2021-08-03 16:08 ` Jason Gunthorpe
2021-08-03 21:48   ` Luigi Rizzo
2021-08-03 23:07     ` Liam Howlett
2021-08-03 23:35       ` Jason Gunthorpe [this message]
2021-08-03 23:57         ` Luigi Rizzo
2021-08-04  5:12           ` Liam Howlett
2021-08-04 14:42       ` Jann Horn
2021-08-04 15:21         ` Jason Gunthorpe
2021-08-04 21:22           ` Jann Horn
2021-08-04 17:32         ` Liam Howlett

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210803233542.GH543798@ziepe.ca \
    --to=jgg@ziepe.ca \
    --cc=akpm@linux-foundation.org \
    --cc=jannh@google.com \
    --cc=liam.howlett@oracle.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=lrizzo@google.com \
    --cc=rientjes@google.com \
    --subject='Re: [PATCH] Add mmap_assert_locked() annotations to find_vma*()' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
on how to clone and mirror all data and code used for this inbox