Date:   Mon, 14 Jan 2019 17:09:05 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Willy Tarreau <w@....eu>
Cc:     Silvio Cesare <silvio.cesare@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Will Deacon <will.deacon@....com>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH 2/8] libertas: change snprintf to scnprintf for possible overflow

On Sat, Jan 12, 2019 at 7:28 AM Willy Tarreau <w@....eu> wrote:
>
> From: Silvio Cesare <silvio.cesare@...il.com>
>
> Change snprintf to scnprintf. There are generally two cases where using
> snprintf causes problems.
>
> 1) Uses of size += snprintf(buf, SIZE - size, fmt, ...)
> In this case, if snprintf would have written more characters than what the
> buffer size (SIZE) is, then size will end up larger than SIZE. In later
> uses of snprintf, SIZE - size will result in a negative number, leading
> to problems. Note that size might already be too large by using
> size = snprintf before the code reaches a case of size += snprintf.
>
> 2) If size is ultimately used as a length parameter for a copy back to user
> space, then it will potentially allow for a buffer overflow and information
> disclosure when size is greater than SIZE. When the size is used to index
> the buffer directly, we can have memory corruption. This also means when
> size = snprintf... is used, it may also cause problems since size may become
> large.  Copying to userspace is mitigated by the HARDENED_USERCOPY kernel
> configuration.
>
> The solution to these issues is to use scnprintf which returns the number of
> characters actually written to the buffer, so the size variable will never
> exceed SIZE.
>
> Signed-off-by: Silvio Cesare <silvio.cesare@...il.com>
> Cc: Kalle Valo <kvalo@...eaurora.org>
> Cc: Dan Carpenter <dan.carpenter@...cle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Greg KH <greg@...ah.com>
> Signed-off-by: Willy Tarreau <w@....eu>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

>
> ---
>  drivers/net/wireless/marvell/libertas/debugfs.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/net/wireless/marvell/libertas/debugfs.c b/drivers/net/wireless/marvell/libertas/debugfs.c
> index c83f44f9ddf1..ec73bd3a10db 100644
> --- a/drivers/net/wireless/marvell/libertas/debugfs.c
> +++ b/drivers/net/wireless/marvell/libertas/debugfs.c
> @@ -41,9 +41,9 @@ static ssize_t lbs_dev_info(struct file *file, char __user *userbuf,
>         if (!buf)
>                 return -ENOMEM;
>
> -       pos += snprintf(buf+pos, len-pos, "state = %s\n",
> +       pos += scnprintf(buf+pos, len-pos, "state = %s\n",
>                                 szStates[priv->connect_status]);
> -       pos += snprintf(buf+pos, len-pos, "region_code = %02x\n",
> +       pos += scnprintf(buf+pos, len-pos, "region_code = %02x\n",
>                                 (u32) priv->regioncode);
>
>         res = simple_read_from_buffer(userbuf, count, ppos, buf, pos);
> @@ -105,7 +105,7 @@ static ssize_t lbs_sleepparams_read(struct file *file, char __user *userbuf,
>         if (ret)
>                 goto out_unlock;
>
> -       pos += snprintf(buf, len, "%d %d %d %d %d %d\n", sp.sp_error,
> +       pos += scnprintf(buf, len, "%d %d %d %d %d %d\n", sp.sp_error,
>                         sp.sp_offset, sp.sp_stabletime,
>                         sp.sp_calcontrol, sp.sp_extsleepclk,
>                         sp.sp_reserved);
> @@ -170,7 +170,7 @@ static ssize_t lbs_host_sleep_read(struct file *file, char __user *userbuf,
>         if (!buf)
>                 return -ENOMEM;
>
> -       pos += snprintf(buf, len, "%d\n", priv->is_host_sleep_activated);
> +       pos += scnprintf(buf, len, "%d\n", priv->is_host_sleep_activated);
>
>         ret = simple_read_from_buffer(userbuf, count, ppos, buf, pos);
>
> @@ -251,7 +251,7 @@ static ssize_t lbs_threshold_read(uint16_t tlv_type, uint16_t event_mask,
>                 freq  = got->freq;
>                 events = le16_to_cpu(subscribed->events);
>
> -               pos += snprintf(buf, len, "%d %d %d\n", value, freq,
> +               pos += scnprintf(buf, len, "%d %d %d\n", value, freq,
>                                 !!(events & event_mask));
>         }
>
> @@ -446,7 +446,7 @@ static ssize_t lbs_rdmac_read(struct file *file, char __user *userbuf,
>         ret = lbs_get_reg(priv, CMD_MAC_REG_ACCESS, priv->mac_offset, &val);
>         mdelay(10);
>         if (!ret) {
> -               pos = snprintf(buf, len, "MAC[0x%x] = 0x%08x\n",
> +               pos = scnprintf(buf, len, "MAC[0x%x] = 0x%08x\n",
>                                 priv->mac_offset, val);
>                 ret = simple_read_from_buffer(userbuf, count, ppos, buf, pos);
>         }
> @@ -516,7 +516,7 @@ static ssize_t lbs_rdbbp_read(struct file *file, char __user *userbuf,
>         ret = lbs_get_reg(priv, CMD_BBP_REG_ACCESS, priv->bbp_offset, &val);
>         mdelay(10);
>         if (!ret) {
> -               pos = snprintf(buf, len, "BBP[0x%x] = 0x%08x\n",
> +               pos = scnprintf(buf, len, "BBP[0x%x] = 0x%08x\n",
>                                 priv->bbp_offset, val);
>                 ret = simple_read_from_buffer(userbuf, count, ppos, buf, pos);
>         }
> @@ -588,7 +588,7 @@ static ssize_t lbs_rdrf_read(struct file *file, char __user *userbuf,
>         ret = lbs_get_reg(priv, CMD_RF_REG_ACCESS, priv->rf_offset, &val);
>         mdelay(10);
>         if (!ret) {
> -               pos = snprintf(buf, len, "RF[0x%x] = 0x%08x\n",
> +               pos = scnprintf(buf, len, "RF[0x%x] = 0x%08x\n",
>                                 priv->rf_offset, val);
>                 ret = simple_read_from_buffer(userbuf, count, ppos, buf, pos);
>         }
> --
> 2.19.2
>


-- 
Kees Cook

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