Re: [PATCH] firmware: dmi_scan: Prevent dmi_num integer overflow

[Ard Biesheuvel]

On 20 March 2015 at 09:59, Jean Delvare <jdelvare@xxxxxxx> wrote:
> 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.
>
> Signed-off-by: Jean Delvare <jdelvare@xxxxxxx>
> Cc: Matt Fleming <matt.fleming@xxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Ivan Khoronzhuk <ivan.khoronzhuk@xxxxxxxxxx>


Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>


> ---
>  drivers/firmware/dmi_scan.c |   22 +++++++---------------
>  1 file changed, 7 insertions(+), 15 deletions(-)
>
> --- linux-4.0-rc4.orig/drivers/firmware/dmi_scan.c      2015-03-19 14:19:56.384426825 +0100
> +++ linux-4.0-rc4/drivers/firmware/dmi_scan.c   2015-03-20 09:23:37.689542485 +0100
> @@ -86,10 +86,13 @@ static void dmi_table(u8 *buf, u32 len,
>         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(co
>         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);
>
>
> --
> Jean Delvare
> SUSE L3 Support
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