Re: [PATCH 3/8] ocfs2: change snprintf to scnprintf for possible overflow

[Kees Cook]

On Sat, Jan 12, 2019 at 7:28 AM Willy Tarreau <w@xxxxxx> wrote:
>
> From: Silvio Cesare <silvio.cesare@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> Cc: Mark Fasheh <mark@xxxxxxxxxx>
> Cc: Joel Becker <jlbec@xxxxxxxxxxxx>
> Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Greg KH <greg@xxxxxxxxx>
> Signed-off-by: Willy Tarreau <w@xxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

>
> ---
>  fs/ocfs2/cluster/heartbeat.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/fs/ocfs2/cluster/heartbeat.c b/fs/ocfs2/cluster/heartbeat.c
> index 9b2ed62dd638..2a0af0887ba0 100644
> --- a/fs/ocfs2/cluster/heartbeat.c
> +++ b/fs/ocfs2/cluster/heartbeat.c
> @@ -1324,7 +1324,7 @@ static int o2hb_debug_open(struct inode *inode, struct file *file)
>
>         case O2HB_DB_TYPE_REGION_NUMBER:
>                 reg = (struct o2hb_region *)db->db_data;
> -               out += snprintf(buf + out, PAGE_SIZE - out, "%d\n",
> +               out += scnprintf(buf + out, PAGE_SIZE - out, "%d\n",
>                                 reg->hr_region_num);
>                 goto done;
>
> @@ -1334,12 +1334,12 @@ static int o2hb_debug_open(struct inode *inode, struct file *file)
>                 /* If 0, it has never been set before */
>                 if (lts)
>                         lts = jiffies_to_msecs(jiffies - lts);
> -               out += snprintf(buf + out, PAGE_SIZE - out, "%lu\n", lts);
> +               out += scnprintf(buf + out, PAGE_SIZE - out, "%lu\n", lts);
>                 goto done;
>
>         case O2HB_DB_TYPE_REGION_PINNED:
>                 reg = (struct o2hb_region *)db->db_data;
> -               out += snprintf(buf + out, PAGE_SIZE - out, "%u\n",
> +               out += scnprintf(buf + out, PAGE_SIZE - out, "%u\n",
>                                 !!reg->hr_item_pinned);
>                 goto done;
>
> @@ -1348,8 +1348,8 @@ static int o2hb_debug_open(struct inode *inode, struct file *file)
>         }
>
>         while ((i = find_next_bit(map, db->db_len, i + 1)) < db->db_len)
> -               out += snprintf(buf + out, PAGE_SIZE - out, "%d ", i);
> -       out += snprintf(buf + out, PAGE_SIZE - out, "\n");
> +               out += scnprintf(buf + out, PAGE_SIZE - out, "%d ", i);
> +       out += scnprintf(buf + out, PAGE_SIZE - out, "\n");
>
>  done:
>         i_size_write(inode, out);
> --
> 2.19.2
>


-- 
Kees Cook