linux-kernel - Re: [PATCH] firmware: dmi_scan: Prevent dmi

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Date:	Thu, 26 Mar 2015 16:21:53 +0100
From:	Jean Delvare <jdelvare@...e.de>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
Subject: Re: [PATCH] firmware: dmi_scan: Prevent dmi_num integer overflow

Le Thursday 26 March 2015 à 14:47 +0000, Matt Fleming a écrit :
> On Thu, 26 Mar, at 02:15:05PM, Jean Delvare wrote:
> > 
> > I don't actually have a tree, so feel free to pick it.
> 
> OK will do, but this is a regression fix for a potential bug introduced
> in commit 6d9ff4733172 ("firmware: dmi_scan: Fix dmi_len type"), right?

Not really. I'd rather say it complements commit 6d9ff4733172. The
actual bugs were introduced in commit fc43026278b2. But you can't really
call it a regression as SMBIOS 3.0 entry points were not supported at
all before that commit.

Still a reference to commit fc43026278b2 would certainly be appropriate,
as anyone backporting that commit will also want to pick the two
aforementioned fix-ups (plus ce204e9a4bd8 for that matter.)

Thanks,
Jean

> If so, it would be useful for this patch to make reference to that
> commit.
> 
> How about this?
> 
> ---
> 
> From 8aabaf128c85bab02fdbe594481826e137c25da4 Mon Sep 17 00:00:00 2001
> From: Jean Delvare <jdelvare@...e.de>
> Date: Fri, 20 Mar 2015 09:59:47 +0100
> Subject: [PATCH] firmware: dmi_scan: Prevent dmi_num integer overflow
> 
> dmi_num is a u16, dmi_len is a u32, so this construct:
> 
> 	dmi_num = dmi_len / 4;
> 
> would result in an integer overflow for a DMI table larger than
> 256 kB. I've never see such a large table so far, but SMBIOS 3.0
> makes it possible so maybe we'll see such tables in the future.
> 
> So instead of faking a structure count when the entry point does
> not provide it, adjust the loop condition in dmi_table() to properly
> deal with the case where dmi_num is not set.
> 
> This potential overflow was introduced in commit 6d9ff4733172
> ("firmware: dmi_scan: Fix dmi_len type").
> 
> Signed-off-by: Jean Delvare <jdelvare@...e.de>
> Cc: Matt Fleming <matt.fleming@...el.com>
> Cc: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
> Cc: <stable@...r.kernel.org>
> Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Signed-off-by: Matt Fleming <matt.fleming@...el.com>
> ---
>  drivers/firmware/dmi_scan.c | 22 +++++++---------------
>  1 file changed, 7 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
> index 69fac068669f..2eebd28b4c40 100644
> --- a/drivers/firmware/dmi_scan.c
> +++ b/drivers/firmware/dmi_scan.c
> @@ -86,10 +86,13 @@ static void dmi_table(u8 *buf, u32 len, int num,
>  	int i = 0;
>  
>  	/*
> -	 *	Stop when we see all the items the table claimed to have
> -	 *	OR we run off the end of the table (also happens)
> +	 * Stop when we have seen all the items the table claimed to have
> +	 * (SMBIOS < 3.0 only) OR we reach an end-of-table marker OR we run
> +	 * off the end of the table (should never happen but sometimes does
> +	 * on bogus implementations.)
>  	 */
> -	while ((i < num) && (data - buf + sizeof(struct dmi_header)) <= len) {
> +	while ((!num || i < num) &&
> +	       (data - buf + sizeof(struct dmi_header)) <= len) {
>  		const struct dmi_header *dm = (const struct dmi_header *)data;
>  
>  		/*
> @@ -529,21 +532,10 @@ static int __init dmi_smbios3_present(const u8 *buf)
>  	if (memcmp(buf, "_SM3_", 5) == 0 &&
>  	    buf[6] < 32 && dmi_checksum(buf, buf[6])) {
>  		dmi_ver = get_unaligned_be16(buf + 7);
> +		dmi_num = 0;			/* No longer specified */
>  		dmi_len = get_unaligned_le32(buf + 12);
>  		dmi_base = get_unaligned_le64(buf + 16);
>  
> -		/*
> -		 * The 64-bit SMBIOS 3.0 entry point no longer has a field
> -		 * containing the number of structures present in the table.
> -		 * Instead, it defines the table size as a maximum size, and
> -		 * relies on the end-of-table structure type (#127) to be used
> -		 * to signal the end of the table.
> -		 * So let's define dmi_num as an upper bound as well: each
> -		 * structure has a 4 byte header, so dmi_len / 4 is an upper
> -		 * bound for the number of structures in the table.
> -		 */
> -		dmi_num = dmi_len / 4;
> -
>  		if (dmi_walk_early(dmi_decode) == 0) {
>  			pr_info("SMBIOS %d.%d present.\n",
>  				dmi_ver >> 8, dmi_ver & 0xFF);
> -- 
> 2.1.0


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