linux-kernel - Re: [PATCH v4 8/9] vsprintf: Prevent crash when dereferencing invalid pointers

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Date:   Thu, 5 Apr 2018 16:46:23 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Petr Mladek <pmladek@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 8/9] vsprintf: Prevent crash when dereferencing invalid
 pointers

On 2018-04-04 10:58, Petr Mladek wrote:

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3551b7957d9e..1a080a75a825 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -599,12 +599,46 @@ char *__string(char *buf, char *end, const char *s, struct printf_spec spec)
>  	return widen_string(buf, len, end, spec);
>  }
>  
> + /*
> +  * This is not a fool-proof test. 99% of the time that this will fault is
> +  * due to a bad pointer, not one that crosses into bad memory. Just test
> +  * the address to make sure it doesn't fault due to a poorly added printk
> +  * during debugging.
> +  */
> +static const char *check_pointer_access(const void *ptr)
> +{
> +	char byte;
> +
> +	if (!ptr)
> +		return "(null)";
> +
> +	if (probe_kernel_address(ptr, byte))
> +		return "(efault)";
> +
> +	return NULL;
> +}

So while I think the WARNings are mostly pointless for the bad format
specifiers, I'm wondering why an averted crash is not worth a WARN_ONCE?
This means there's an actual bug somewhere, probably even exploitable,
but we're just silently producing some innocent string...

Also, I'd still prefer to insist on ptr being a kernel pointer. Sure,
for %ph userspace gets to print their own memory, but for a lot of the
others, we're chasing pointers another level, so if an attacker can feed
a user pointer to one of those, there's a trivial arbitrary read gadget.
We have lots of printks in untested error paths, and I find it quite
likely that one of those uses a garbage pointer.

I know you're mostly phrasing this in terms of preventing a crash, but
it seems silly not to close that when it only costs a pointer comparison.

You're also missing the %pD (struct file*) case, which is one of those
double-pointer chasing cases.

Rasmus