linux-kernel - Re: [PATCH v7 2/2] firmware_loader: fix use-after-free in firmware_fallback

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Date:   Mon, 26 Jul 2021 11:22:11 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Anirudh Rayabharam <mail@...rudhrb.com>, mcgrof@...nel.org,
        gregkh@...uxfoundation.org, rafael@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+de271708674e2093097b@...kaller.appspotmail.com,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH v7 2/2] firmware_loader: fix use-after-free in
 firmware_fallback_sysfs

On 7/24/21 6:11 AM, Anirudh Rayabharam wrote:
> This use-after-free happens when a fw_priv object has been freed but
> hasn't been removed from the pending list (pending_fw_head). The next
> time fw_load_sysfs_fallback tries to insert into the list, it ends up
> accessing the pending_list member of the previoiusly freed fw_priv.
> 
> The root cause here is that all code paths that abort the fw load
> don't delete it from the pending list. For example:
> 
> 	_request_firmware()
> 	  -> fw_abort_batch_reqs()
> 	      -> fw_state_aborted()
> 
> To fix this, delete the fw_priv from the list in __fw_set_state() if
> the new state is DONE or ABORTED. This way, all aborts will remove
> the fw_priv from the list. Accordingly, remove calls to list_del_init
> that were being made before calling fw_state_(aborted|done).
> 
> Also, in fw_load_sysfs_fallback, don't add the fw_priv to the pending
> list if it is already aborted. Instead, just jump out and return early.
> 
> Fixes: bcfbd3523f3c ("firmware: fix a double abort case with fw_load_sysfs_fallback")
> Reported-by: syzbot+de271708674e2093097b@...kaller.appspotmail.com
> Tested-by: syzbot+de271708674e2093097b@...kaller.appspotmail.com
> Signed-off-by: Anirudh Rayabharam <mail@...rudhrb.com>
> ---
>   drivers/base/firmware_loader/fallback.c | 10 +++++++---
>   drivers/base/firmware_loader/firmware.h |  6 +++++-
>   drivers/base/firmware_loader/main.c     |  2 ++
>   3 files changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/base/firmware_loader/fallback.c b/drivers/base/firmware_loader/fallback.c
> index 1a48be0a030e..5d9f372fc9e7 100644
> --- a/drivers/base/firmware_loader/fallback.c
> +++ b/drivers/base/firmware_loader/fallback.c
> @@ -91,10 +91,9 @@ static void __fw_load_abort(struct fw_priv *fw_priv)
>   	 * There is a small window in which user can write to 'loading'
>   	 * between loading done and disappearance of 'loading'
>   	 */

Update the comment to "between loading done/abort"

Rest looks good to me.

> -	if (fw_sysfs_done(fw_priv))
> +	if (fw_state_is_aborted(fw_priv) || fw_sysfs_done(fw_priv))
>   		return;
>   
> -	list_del_init(&fw_priv->pending_list);
>   	fw_state_aborted(fw_priv);
>   }
>   
> @@ -280,7 +279,6 @@ static ssize_t firmware_loading_store(struct device *dev,
>   			 * Same logic as fw_load_abort, only the DONE bit
>   			 * is ignored and we set ABORT only on failure.
>   			 */
> -			list_del_init(&fw_priv->pending_list);
>   			if (rc) {
>   				fw_state_aborted(fw_priv);
>   				written = rc;
> @@ -513,6 +511,11 @@ static int fw_load_sysfs_fallback(struct fw_sysfs *fw_sysfs, long timeout)
>   	}
>   
>   	mutex_lock(&fw_lock);
> +	if (fw_state_is_aborted(fw_priv)) {
> +		mutex_unlock(&fw_lock);
> +		retval = -EINTR;
> +		goto out;
> +	}
>   	list_add(&fw_priv->pending_list, &pending_fw_head);
>   	mutex_unlock(&fw_lock);
>   
> @@ -538,6 +541,7 @@ static int fw_load_sysfs_fallback(struct fw_sysfs *fw_sysfs, long timeout)
>   	} else if (fw_priv->is_paged_buf && !fw_priv->data)
>   		retval = -ENOMEM;
>   
> +out:
>   	device_del(f_dev);
>   err_put_dev:
>   	put_device(f_dev);
> diff --git a/drivers/base/firmware_loader/firmware.h b/drivers/base/firmware_loader/firmware.h
> index 63bd29fdcb9c..36bdb413c998 100644
> --- a/drivers/base/firmware_loader/firmware.h
> +++ b/drivers/base/firmware_loader/firmware.h
> @@ -117,8 +117,12 @@ static inline void __fw_state_set(struct fw_priv *fw_priv,
>   
>   	WRITE_ONCE(fw_st->status, status);
>   
> -	if (status == FW_STATUS_DONE || status == FW_STATUS_ABORTED)
> +	if (status == FW_STATUS_DONE || status == FW_STATUS_ABORTED) {
> +#ifdef CONFIG_FW_LOADER_USER_HELPER

Might be helpful to add a comment that this covers all abort/done
paths

> +		list_del_init(&fw_priv->pending_list);
> +#endif
>   		complete_all(&fw_st->completion);
> +	}
>   }
>   
>   static inline void fw_state_aborted(struct fw_priv *fw_priv)
> diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
> index 4fdb8219cd08..68c549d71230 100644
> --- a/drivers/base/firmware_loader/main.c
> +++ b/drivers/base/firmware_loader/main.c
> @@ -783,8 +783,10 @@ static void fw_abort_batch_reqs(struct firmware *fw)
>   		return;
>   
>   	fw_priv = fw->priv;
> +	mutex_lock(&fw_lock);
>   	if (!fw_state_is_aborted(fw_priv))
>   		fw_state_aborted(fw_priv);
> +	mutex_unlock(&fw_lock);
>   }
>   
>   /* called from request_firmware() and request_firmware_work_func() */
> 

thanks,
-- Shuah