Re: [for-next][PATCH 09/24] seq_buf: Make seq_buf_puts() null-terminate the buffer

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 21 Dec 2018 12:56:27 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Michael Ellerman <mpe@ellerman.id.au>
> 
> Currently seq_buf_puts() will happily create a non null-terminated
> string for you in the buffer. This is particularly dangerous if the
> buffer is on the stack.
> 
> For example:
> 
>   char buf[8];
>   char secret = "secret";
>   struct seq_buf s;
> 
>   seq_buf_init(&s, buf, sizeof(buf));
>   seq_buf_puts(&s, "foo");
>   printk("Message is %s\n", buf);
> 
> Can result in:
> 
>   Message is fooªªªªªsecret

Sending this via quilt, and that we have non UTF8 characters causes
LKML to blow up.

There's a couple more patches with this issue. I'm going to fix up the
change logs and rebase them.

-- Steve

> 
> We could require all users to memset() their buffer to zero before
> use. But that seems likely to be forgotten and lead to bugs.
> 
> Instead we can change seq_buf_puts() to always leave the buffer in a
> null-terminated state.
> 
> The only downside is that this makes the buffer 1 character smaller
> for seq_buf_puts(), but that seems like a good trade off.
> 
> Link: http://lkml.kernel.org/r/20181019042109.8064-1-mpe@ellerman.id.au
> 
> Acked-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
>  lib/seq_buf.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/seq_buf.c b/lib/seq_buf.c
> index 11f2ae0f9099..6aabb609dd87 100644
> --- a/lib/seq_buf.c
> +++ b/lib/seq_buf.c
> @@ -144,9 +144,13 @@ int seq_buf_puts(struct seq_buf *s, const char *str)
>  
>  	WARN_ON(s->size == 0);
>  
> +	/* Add 1 to len for the trailing null byte which must be there */
> +	len += 1;
> +
>  	if (seq_buf_can_fit(s, len)) {
>  		memcpy(s->buffer + s->len, str, len);
> -		s->len += len;
> +		/* Don't count the trailing null byte against the capacity */
> +		s->len += len - 1;
>  		return 0;
>  	}
>  	seq_buf_set_overflow(s);