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SubjectRe: [PATCH 1/8] lkdtm: change snprintf to scnprintf for possible overflow
On Sat, Jan 12, 2019 at 7:28 AM Willy Tarreau <w@1wt.eu> wrote:
>
> From: Silvio Cesare <silvio.cesare@gmail.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@gmail.com>
> Cc: Dan Carpenter <dan.carpenter@oracle.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Greg KH <greg@kroah.com>
> Signed-off-by: Willy Tarreau <w@1wt.eu>

It looks like these are going via individual trees. Greg, can you
please take this into your drivers-misc tree for lkdtm?

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

Thanks!

-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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