Re: [PATCH] netfs: Fix UAF in netfs_unbuffered_write() on failed preparation

[ChenXiaoSong]

Hi hongao and David,

After applying this patch, I can still reproduce the use-after-free issue.

C reproducer: https://syzkaller.appspot.com/text?tag=ReproC&x=171b697e580000

Note that QEMU should be started with the following virtfs option:

  -virtfs ...,mount_tag=syz,...

On 5/30/26 09:14, hongao wrote:
> If write subrequest preparation fails, netfs_unbuffered_write() calls
> netfs_write_subrequest_terminated() and then reads subreq->error to set
> wreq->error.
>
> However, netfs_write_subrequest_terminated() consumes a reference to the
> subrequest through netfs_put_subrequest(), so the subrequest may be freed
> before netfs_unbuffered_write() reads subreq->error again.  This can
> trigger a slab-use-after-free.
>
> Save the error locally before terminating the subrequest, and use the
> saved value afterwards.
>
> Fixes: a0b4c7a49137 ("netfs: Fix unbuffered/DIO writes to dispatch subrequests in strict sequence")
> Reported-by: syzbot+3c74b1f0c372e98efc32@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=3c74b1f0c372e98efc32
>
> Signed-off-by: hongao <hongao@xxxxxxxxxxxxx>
> ---
>   fs/netfs/direct_write.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
> index 25f8ceb15fad..2d5361702076 100644
> --- a/fs/netfs/direct_write.c
> +++ b/fs/netfs/direct_write.c
> @@ -115,8 +115,9 @@ static int netfs_unbuffered_write(struct netfs_io_request *wreq)
>   
>   		/* Check if (re-)preparation failed. */
>   		if (unlikely(test_bit(NETFS_SREQ_FAILED, &subreq->flags))) {
> -			netfs_write_subrequest_terminated(subreq, subreq->error);
> -			wreq->error = subreq->error;
> +			ret = subreq->error;
> +			wreq->error = ret;
> +			netfs_write_subrequest_terminated(subreq, ret);
>   			break;
>   		}
>   

--
ChenXiaoSong <chenxiaosong@xxxxxxxxxxxxxxxx>
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