linux-kernel - Re: [PATCH 1/3] udmabuf: fix racy memfd sealing check

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Message-ID: <CAG48ez0VJr=nuWciFZPLgUFOVca=Sac+4-_8Amqorv9WsBF6vQ@mail.gmail.com>
Date: Wed, 4 Dec 2024 17:08:06 +0100
From: Jann Horn <jannh@...gle.com>
To: "Kasireddy, Vivek" <vivek.kasireddy@...el.com>
Cc: Gerd Hoffmann <kraxel@...hat.com>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Christian König <christian.koenig@....com>, 
	Simona Vetter <simona.vetter@...ll.ch>, John Stultz <john.stultz@...aro.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	"Joel Fernandes (Google)" <joel@...lfernandes.org>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, 
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>, 
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Julian Orth <ju.orth@...il.com>, 
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 1/3] udmabuf: fix racy memfd sealing check

On Wed, Dec 4, 2024 at 10:09 AM Kasireddy, Vivek
<vivek.kasireddy@...el.com> wrote:
> > Subject: [PATCH 1/3] udmabuf: fix racy memfd sealing check
> >
> > The current check_memfd_seals() is racy: Since we first do
> > check_memfd_seals() and then udmabuf_pin_folios() without holding any
> > relevant lock across both, F_SEAL_WRITE can be set in between.
> > This is problematic because we can end up holding pins to pages in a
> > write-sealed memfd.
> >
> > Fix it using the inode lock, that's probably the easiest way.
> > In the future, we might want to consider moving this logic into memfd,
> > especially if anyone else wants to use memfd_pin_folios().
> >
> > Reported-by: Julian Orth <ju.orth@...il.com>
> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219106
> > Closes:
> > https://lore.kernel.org/r/CAG48ez0w8HrFEZtJkfmkVKFDhE5aP7nz=obrimeTg
> > pD+StkV9w@...l.gmail.com
> > Fixes: fbb0de795078 ("Add udmabuf misc device")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Jann Horn <jannh@...gle.com>
> > ---
> >  drivers/dma-buf/udmabuf.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> > index
> > 8ce1f074c2d32a0a9f59ff7184359e37d56548c6..662b9a26e06668bf59ab36d0
> > 7c0648c7b02ee5ae 100644
> > --- a/drivers/dma-buf/udmabuf.c
> > +++ b/drivers/dma-buf/udmabuf.c
> > @@ -436,14 +436,15 @@ static long udmabuf_create(struct miscdevice
> > *device,
> >                       goto err;
> >               }
> >
> > +             inode_lock_shared(memfd->f_inode);
> I think having inode_lock_shared(file_inode(memfd)) looks a bit more cleaner.

Good idea, changed that.

> Also, wouldn't it be more appropriate here to take the writer's lock instead
> of the reader's lock given what we are doing (pinning) in udmabuf_create()?

I don't see why that would require taking the inode lock in write
mode. I am taking the inode lock to provide exclusion against
memfd_add_seals(), which uses inode_lock(); in other words, the
inode_lock is to protect the sealing status of the file from changing
(which is a reader-like requirement). I'll add a comment in v2 to
clarify this.