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

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Message-ID: <IA0PR11MB7185EFC7E59546204A617DE9F8372@IA0PR11MB7185.namprd11.prod.outlook.com>
Date: Wed, 4 Dec 2024 09:08:59 +0000
From: "Kasireddy, Vivek" <vivek.kasireddy@...el.com>
To: Jann Horn <jannh@...gle.com>, 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>
CC: "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

Hi Jann,

> 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.
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()?

Thanks,
Vivek

>  		ret = check_memfd_seals(memfd);
> -		if (ret < 0) {
> -			fput(memfd);
> -			goto err;
> -		}
> +		if (ret)
> +			goto out_unlock;
> 
>  		ret = udmabuf_pin_folios(ubuf, memfd, list[i].offset,
>  					 list[i].size, folios);
> +out_unlock:
> +		inode_unlock_shared(memfd->f_inode);
>  		fput(memfd);
>  		if (ret)
>  			goto err;
> 
> --
> 2.47.0.338.g60cca15819-goog