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

[Kasireddy, Vivek]

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@xxxxxxxxx>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219106
> Closes:
> https://lore.kernel.org/r/CAG48ez0w8HrFEZtJkfmkVKFDhE5aP7nz=obrimeTg
> pD+StkV9w@xxxxxxxxxxxxxx
> Fixes: fbb0de795078 ("Add udmabuf misc device")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
>  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