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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Hangyu Hua <hbh25y@gmail.com>, fbarrat@linux.ibm.com,
        ajd@linux.ibm.com, arnd@arndb.de, gregkh@linuxfoundation.org,
        alastair@d-silva.org
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Hangyu Hua <hbh25y@gmail.com>
Subject: Re: [PATCH] misc: ocxl: fix possible double free in
 ocxl_file_register_afu
In-Reply-To: <20220418085758.38145-1-hbh25y@gmail.com>
References: <20220418085758.38145-1-hbh25y@gmail.com>
Date:   Thu, 21 Apr 2022 08:54:04 +1000
Message-ID: <87czhbfjsj.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Hangyu Hua <hbh25y@gmail.com> writes:
> info_release() will be called in device_unregister() when info->dev's
> reference count is 0. So there is no need to call ocxl_afu_put() and
> kfree() again.

Double frees are often exploitable. But it looks to me like this error
path is not easily reachable by an attacker.

ocxl_file_register_afu() is only called from ocxl_probe(), and we only
go to err_unregister if the sysfs or cdev initialisation fails, which
should only happen if we hit ENOMEM, or we have a duplicate device which
would be a device-tree/hardware error. But maybe Fred can check more
closely, I don't know the driver that well.

cheers


> Fix this by adding free_minor() and return to err_unregister error path.
>
> Fixes: 75ca758adbaf ("ocxl: Create a clear delineation between ocxl backend & frontend")
> Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
> ---
>  drivers/misc/ocxl/file.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/misc/ocxl/file.c b/drivers/misc/ocxl/file.c
> index d881f5e40ad9..6777c419a8da 100644
> --- a/drivers/misc/ocxl/file.c
> +++ b/drivers/misc/ocxl/file.c
> @@ -556,7 +556,9 @@ int ocxl_file_register_afu(struct ocxl_afu *afu)
>  
>  err_unregister:
>  	ocxl_sysfs_unregister_afu(info); // safe to call even if register failed
> +	free_minor(info);
>  	device_unregister(&info->dev);
> +	return rc;
>  err_put:
>  	ocxl_afu_put(afu);
>  	free_minor(info);
> -- 
> 2.25.1