Re: printk()s of user-supplied strings

[Solar Designer <solar@openwall.com>]

On Sun, Aug 27, 2006 at 10:04:40PM +0200, Willy Tarreau wrote:
> I didn't want to affect anything but "%s". Unfortunately, it seems that it
> was already too much because there are several users in the kernel which
> first construct their multi-line strings then call printk("%s") with it :-(

I've tried looking up the printk() calls corresponding to the example
output that you provided.  Those that I've checked do not use "%s" on
multi-line strings; rather, they merely embed the final '\n' that is meant
to end the line within the last "%s" string.  Perhaps we could allow for
this special case (that is, not escape the '\n' if it is the last
character to be output by the printk() call).  Yes, there will be a few
printk("%s", ...) calls that we won't protect - namely, those that are
meant to output user-supplied strings in the middle of a line, yet are
invoked separately from the printk() that is meant to complete the line.
Those would need to be spotted and fixed individually.

> Serial driver version 5.05c (2001-07-08) with MANY_PORTS SHARE_IRQ SERIAL_PCI ISAPNP enabled^J<6>ttyS00 at 0x03f8 (irq = 4) is a 16550A

The first line (before the ^J) is output with:

	printk(KERN_INFO "%s version %s%s (%s) with%s", serial_name,
	       serial_version, LOCAL_VERSTRING, serial_revdate,
	       serial_options);

The linefeed is embedded in serial_options.

> IP Protocols: ICMP, UDP, TCP, IGMP^J<6>IP: routing cache hash table of 8192 buckets, 64Kbytes

	printk(KERN_INFO "IP Protocols: ");
	for (p = inet_protocol_base; p != NULL;) {
		struct inet_protocol *tmp = (struct inet_protocol *) p->next;
		inet_add_protocol(p);
		printk("%s%s",p->name,tmp?", ":"\n");
		p = tmp;
	}

As you can see, this one is similar - a single line is formed with
multiple printk()s and the linefeed is embedded in the last "%s" string.

> Another idea I had was to add a new format specifier to vsnprintf()
> to explicitly escape the string (eg: "%S"). But there are so many
> users of printk() to fix then that I'm not sure we would find them
> all. However, it would be the real fix and not a hack because what
> we're trying to do is to enforce controls on some data type, which
> is exactly the point of this solution.

Yes, I had this thought, too.  This would be the cleanest solution, but
I'm afraid that it will fail in practice.  People will continue
introducing printk() calls with plain "%s" where the new "%S" would be
needed.  Another problem is that there will be cases where it is
difficult to make a determination whether a given string is untrusted
user input.  A solution would be to normally use "%S" and only use
"%s" where "%S" wouldn't work.  In that case, we could as well swap "%s"
and "%S", though - hardening the existing "%s" and introducing "%S" for
those callers that depend on the old behavior.

Alexander
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