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SubjectRe: [PATCH v4 8/9] vsprintf: Prevent crash when dereferencing invalid pointers
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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

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