Re: [Linux-kernel-mentees] [PATCH v2] net: ipv6: fix use-after-free Read in __xfrm6_tunnel_spi_lookup

On Sun, Jul 26, 2020 at 11:05 AM Cong Wang [off-list ref] wrote:

On Sat, Jul 25, 2020 at 8:09 PM B K Karthik [off-list ref] wrote:

quoted

@@ -103,10 +103,10 @@ static int __xfrm6_tunnel_spi_check(struct net *net, u32 spi)
 {
        struct xfrm6_tunnel_net *xfrm6_tn = xfrm6_tunnel_pernet(net);
        struct xfrm6_tunnel_spi *x6spi;
-       int index = xfrm6_tunnel_spi_hash_byspi(spi);
+       int index = xfrm6_tunnel_spi_hash_byaddr((const xfrm_address_t *)spi);

        hlist_for_each_entry(x6spi,
-                            &xfrm6_tn->spi_byspi[index],
+                            &xfrm6_tn->spi_byaddr[index],
                             list_byspi) {
                if (x6spi->spi == spi)

How did you convince yourself this is correct? This lookup is still
using spi. :)

I'm sorry, but my intention behind writing this patch was not to fix
the UAF, but to fix a slab-out-of-bound.
If required, I can definitely change the subject line and resend the
patch, but I figured this was correct for
https://syzkaller.appspot.com/bug?id=058d05f470583ab2843b1d6785fa8d0658ef66ae
. since that particular report did not have a reproducer,
Dmitry Vyukov [off-list ref] suggested that I test this patch on
other reports for xfrm/spi .

Forgive me if this was the wrong way to send a patch for that
particular report, but I guessed since the reproducer did not trigger
the crash
for UAF, I would leave the subject line as 'fix UAF' :)

xfrm6_spi_hash_by_hash seemed more convincing because I had to prevent
a slab-out-of-bounds because it uses ipv6_addr_hash.
It would be of great help if you could help me understand how this was
able to fix a UAF.

More importantly, can you explain how UAF happens? Apparently
the syzbot stack traces you quote make no sense at all. I also
looked at other similar reports, none of them makes sense to me.

Forgive me, but I do not understand what you mean by the stack traces
(this or other similar reports) "make no sense".

I apologise if this message was hurtful / disrespectful in any manner.
thanks,

karthik
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