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Date:   Mon, 14 Jan 2019 17:02:51 -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>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Will Deacon <will.deacon@....com>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH 1/8] lkdtm: 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.

(I didn't find a 0/8 cover letter, so I'm replying here...)

Many of these fixes are just robustness updates (e.g. the lkdtm case
below is not current a problem: the size of the static array getting
displayed is less than PAGE_SIZE). It might be worth noting which are
actually problems (and include the appropriate Cc: and Fixes: lines).

>
> 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: 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>

Are these changes going into someone's single tree, or are they
intended for individual maintainers to pick up?

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

-Kees

>
> ---
>  drivers/misc/lkdtm/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index 2837dc77478e..610aa3bfe630 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -347,9 +347,9 @@ static ssize_t lkdtm_debugfs_read(struct file *f, char __user *user_buf,
>         if (buf == NULL)
>                 return -ENOMEM;
>
> -       n = snprintf(buf, PAGE_SIZE, "Available crash types:\n");
> +       n = scnprintf(buf, PAGE_SIZE, "Available crash types:\n");
>         for (i = 0; i < ARRAY_SIZE(crashtypes); i++) {
> -               n += snprintf(buf + n, PAGE_SIZE - n, "%s\n",
> +               n += scnprintf(buf + n, PAGE_SIZE - n, "%s\n",
>                               crashtypes[i].name);
>         }
>         buf[n] = '\0';
> --
> 2.19.2
>


-- 
Kees Cook

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