On Mon, Jan 25, 2021 at 11:39:08AM -0800, Jakub Kicinski wrote:
> On Sun, 24 Jan 2021 11:33:01 +0100 Lukas Wunner wrote:
> > On Fri, Jan 22, 2021 at 10:40:05AM +0100, Eric Dumazet wrote:
> > > On Fri, Jan 22, 2021 at 9:55 AM Lukas Wunner <lukas@xxxxxxxxx> wrote:
> > > > sch_handle_egress() returns either the skb or NULL to signal to its
> > > > caller __dev_queue_xmit() whether a packet should continue to be
> > > > processed.
> > > >
> > > > The skb is always non-NULL, otherwise __dev_queue_xmit() would hit a
> > > > NULL pointer deref right at its top.
> > > >
> > > > But the compiler doesn't know that. So if sch_handle_egress() signals
> > > > success by returning the skb, the "if (!skb) goto out;" statement
> > > > results in a gratuitous NULL pointer check in the Assembler output.
> > > >
> > > > Avoid by telling the compiler that __dev_queue_xmit() is never passed a
> > > > NULL skb.
> > [...]
> > > > we're about to add a netfilter egress hook to __dev_queue_xmit()
> > > > and without the micro-optimization, it will result in a performance
> > > > degradation which is indeed measurable:
> > [...]
> > > > --- a/net/core/dev.c
> > > > +++ b/net/core/dev.c
> > > > +__attribute__((nonnull(1)))
> > > > static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
> > > > {
> > > > struct net_device *dev = skb->dev;
> > >
> > > It is a bit sad the compilers do not automatically get this knowledge
> > > from the very first instruction :
> > >
> > > struct net_device *dev = skb->dev;
> >
> > The compiler (gcc) is capable of doing that, but the feature was disabled by:
> >
> > commit a3ca86aea507904148870946d599e07a340b39bf
> > Author: Eugene Teo <eteo@xxxxxxxxxx>
> > Date: Wed Jul 15 14:59:10 2009 +0800
> >
> > Add '-fno-delete-null-pointer-checks' to gcc CFLAGS
> >
> > If -fno-delete-null-pointer-checks is dropped from the top-level Makefile
> > then the gratuitous NULL pointer checks disappear from the Assembler output,
> > obviating the need to litter hot paths with __attribute__((nonnull(1)))
> > annotations.
> >
> > Taking a closer look at that commit, its rationale appears questionable:
> > It says that broken code such as ...
> >
> > struct agnx_priv *priv = dev->priv;
> >
> > if (!dev)
> > return;
> >
> > ... would result in the NULL pointer check being optimized away.
> > The commit message claims that keeping the NULL pointer check in
> > "makes it harder to abuse" the broken code.
> >
> > I don't see how that's the case: If dev is NULL, the NULL pointer
> > dereference at the function's top causes termination of the task
> > in kernel/exit.c:do_exit(). So the NULL pointer check is never
> > reached by the task. If on the other hand dev is non-NULL,
> > the task isn't terminated but then the NULL pointer check is
> > unnecessary as well.
> >
> > So the point of the commit remains elusive to me. I could submit
> > an RFC patch which drops -fno-delete-null-pointer-checks and see
> > if any security folks cry foul. Thoughts?
>
This was a famous tun.c bug back in the day. In those days we weren't
careful to disallow remapping NULL to a different pointer. See
/proc/sys/vm/mmap_min_addr. The exploit was to remap NULL to be a valid
user controlled pointer. It should have been impossible to exploit
because the code had a check for NULL, but the compiler optimized it
away.
https://lwn.net/Articles/342330/
> I wonder if modern compilers can't simply warn about this particular
> case. Not to mention our static checkers..
>
>
> Dan, do you think the concern from the above-quoted commit is still
> valid? Is this something that smatch flags these days? We're apparently
> paying a real performance price in networking for tying compiler's hands
> with -fno-delete-null-pointer-checks
If I had to guess why GCC doesn't warn about this I would say that
probably it's because a lot of macros have NULL checks built in. Most
static analysis tools have a warning about inconsistent NULL checks but
Smatch won't warn about it unless it can lead to a NULL dereference.
The fact that pointless NULL checks slow down the code has never
bothered anyone up to now.
regards,
dan carpenter
