linux-kernel - Re: [PATCH RESEND] tpm: eventlog: Replace zero-length array with flexible-array member

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Date:   Thu, 7 May 2020 11:02:18 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Peter Huewe <peterhuewe@....de>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH RESEND] tpm: eventlog: Replace zero-length array with
 flexible-array member

On Wed, May 06, 2020 at 11:09:12PM -0500, Gustavo A. R. Silva wrote:
> As mentioned above: "Flexible array members have incomplete type, and
> so the sizeof operator may not be applied. As a quirk of the original
> implementation of zero-length arrays, sizeof evaluates to zero."[1] So,
> the sizeof(flexible-array) can be safely removed to fix the error above.

As in "sizeof(event_header->event) always evaluated to 0, so removing it
has no effect".

> [...]
> diff --git a/drivers/char/tpm/eventlog/tpm2.c b/drivers/char/tpm/eventlog/tpm2.c
> index e741b1157525..351a2989b3c6 100644
> --- a/drivers/char/tpm/eventlog/tpm2.c
> +++ b/drivers/char/tpm/eventlog/tpm2.c
> @@ -51,8 +51,7 @@ static void *tpm2_bios_measurements_start(struct seq_file *m, loff_t *pos)
>  	int i;
>  
>  	event_header = addr;
> -	size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
> -		+ event_header->event_size;
> +	size = sizeof(*event_header) + event_header->event_size;

That said, I think it would be better to stick to the struct_size()
idiom for dealing with flexible arrays here:

	size = struct_size(event_header, event, event_size);

-- 
Kees Cook