Subject: Re: [PATCH 1/2] can: peak_canfd: decorate pciefd_board.can with __counted_by()

From: Kees Cook <[email-masked]>
Date: Mon, 10 Jun 2024 11:34:40 -0700

On Sun, Jun 09, 2024 at 01:54:18PM +0900, Vincent Mailhol wrote:
> A new __counted_by() attribute was introduced in [1]. It makes the
> compiler's sanitizer aware of the actual size of a flexible array
> member, allowing for additional runtime checks.
> 
> Move the end of line comments to the previous line to make room and
> apply the __counted_by() attribute to the can flexible array member of
> struct pciefd_board.
> 
> [1] commit dd06e72e68bc ("Compiler Attributes: Add __counted_by macro")
> Link: https://git.kernel.org/torvalds/c/dd06e72e68bc
> 
> CC: Kees Cook <[email-masked]>
> Signed-off-by: Vincent Mailhol <[email-masked]>
> ---
>  drivers/net/can/peak_canfd/peak_pciefd_main.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/can/peak_canfd/peak_pciefd_main.c b/drivers/net/can/peak_canfd/peak_pciefd_main.c
> index 1df3c4b54f03..636102103a88 100644
> --- a/drivers/net/can/peak_canfd/peak_pciefd_main.c
> +++ b/drivers/net/can/peak_canfd/peak_pciefd_main.c
> @@ -190,8 +190,10 @@ struct pciefd_board {
>  	void __iomem *reg_base;
>  	struct pci_dev *pci_dev;
>  	int can_count;
> -	spinlock_t cmd_lock;		/* 64-bits cmds must be atomic */
> -	struct pciefd_can *can[];	/* array of network devices */
> +	/* 64-bits cmds must be atomic */
> +	spinlock_t cmd_lock;
> +	/* array of network devices */
> +	struct pciefd_can *can[] __counted_by(can_count);
>  };
>  
>  /* supported device ids. */

You'll need to adjust the code logic that manipulates "can_count", as
accesses to "can" will trap when they're seen as out of bounds. For
example:

        pciefd = devm_kzalloc(&pdev->dev, struct_size(pciefd, can, can_count),
                              GFP_KERNEL);
	...
	/* pciefd->can_count is "0" now */

        while (pciefd->can_count < can_count) {
		...
                pciefd_can_probe(pciefd);
		/* which does: */
        		pciefd->can[pciefd->can_count] = priv;	/// HERE
		...
                pciefd->can_count++;
	}

The access at "HERE" above will trap: "can" is believed to have
"can_count" elements (0 on the first time through the loop).
This needs to be adjusted to increment "can_count" first.

Perhaps:

diff --git a/drivers/net/can/peak_canfd/peak_pciefd_main.c b/drivers/net/can/peak_canfd/peak_pciefd_main.c
index 1df3c4b54f03..df8304b2d291 100644
--- a/drivers/net/can/peak_canfd/peak_pciefd_main.c
+++ b/drivers/net/can/peak_canfd/peak_pciefd_main.c
@@ -676,7 +676,8 @@ static int pciefd_can_probe(struct pciefd_board *pciefd)
 	spin_lock_init(&priv->tx_lock);
 
 	/* save the object address in the board structure */
-	pciefd->can[pciefd->can_count] = priv;
+	pciefd->can_count++;
+	pciefd->can[pciefd->can_count - 1] = priv;
 
 	dev_info(&pciefd->pci_dev->dev, "%s at reg_base=0x%p irq=%d\n",
 		 ndev->name, priv->reg_base, ndev->irq);
@@ -800,8 +801,6 @@ static int peak_pciefd_probe(struct pci_dev *pdev,
 		err = pciefd_can_probe(pciefd);
 		if (err)
 			goto err_free_canfd;
-
-		pciefd->can_count++;
 	}
 
 	/* set system timestamps counter in RST mode */

-- 
Kees Cook