[PATCH] net: xfrm: fix bug in ipcomp_free_scratches

[PATCH] net: xfrm: fix bug in ipcomp_free_scratches

[Dongliang Mu]

In ipcomp_alloc_scratches, if the vmalloc fails, there leaves a NULL
pointer. However, ipcomp_free_scratches does not check the per_pcu_ptr
pointer when invoking vfree.

Fix this by adding a sanity check before vfree.

Call Trace:
 ipcomp_free_scratches+0xbc/0x160 net/xfrm/xfrm_ipcomp.c:203
 ipcomp_free_data net/xfrm/xfrm_ipcomp.c:312 [inline]
 ipcomp_init_state+0x77c/0xa40 net/xfrm/xfrm_ipcomp.c:364
 ipcomp6_init_state+0xc2/0x700 net/ipv6/ipcomp6.c:154
 __xfrm_init_state+0x995/0x15c0 net/xfrm/xfrm_state.c:2648
 xfrm_init_state+0x1a/0x70 net/xfrm/xfrm_state.c:2675
 pfkey_msg2xfrm_state net/key/af_key.c:1287 [inline]
 pfkey_add+0x1a64/0x2cd0 net/key/af_key.c:1504
 pfkey_process+0x685/0x7e0 net/key/af_key.c:2837
 pfkey_sendmsg+0x43a/0x820 net/key/af_key.c:3676
 sock_sendmsg_nosec net/socket.c:703 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:723

Reported-by: syzbot+b9cfd1cc5d57ee0a09ab@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Fixes: 6fccab671f2f ("ipsec: ipcomp - Merge IPComp impl")
Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
---
 net/xfrm/xfrm_ipcomp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
index cb40ff0ff28d..9588ac05ab27 100644
--- a/net/xfrm/xfrm_ipcomp.c
+++ b/net/xfrm/xfrm_ipcomp.c
@@ -199,8 +199,11 @@ static void ipcomp_free_scratches(void)
 	if (!scratches)
 		return;
 
-	for_each_possible_cpu(i)
-		vfree(*per_cpu_ptr(scratches, i));
+	for_each_possible_cpu(i) {
+		void *scratch = *per_cpu_ptr(scratches, i);
+		if (!scratch)
+			vfree(scratch);
+	}
 
 	free_percpu(scratches);
 }
-- 
2.25.1

Re: [PATCH] net: xfrm: fix bug in ipcomp_free_scratches

[Herbert Xu]

On Mon, Aug 16, 2021 at 03:38:29PM +0800, Dongliang Mu wrote:
>
> -	for_each_possible_cpu(i)
> -		vfree(*per_cpu_ptr(scratches, i));
> +	for_each_possible_cpu(i) {
> +		void *scratch = *per_cpu_ptr(scratches, i);
> +		if (!scratch)
> +			vfree(scratch);
> +	}

This patch is unnecessary.  Please check the implementation of
vfree, it already checks for NULL pointers just like most of our
free primitives.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] net: xfrm: fix bug in ipcomp_free_scratches

[Dongliang Mu]

On Mon, Aug 16, 2021 at 3:53 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Mon, Aug 16, 2021 at 03:38:29PM +0800, Dongliang Mu wrote:
> >
> > -     for_each_possible_cpu(i)
> > -             vfree(*per_cpu_ptr(scratches, i));
> > +     for_each_possible_cpu(i) {
> > +             void *scratch = *per_cpu_ptr(scratches, i);
> > +             if (!scratch)
> > +                     vfree(scratch);
> > +     }
>
> This patch is unnecessary.  Please check the implementation of
> vfree, it already checks for NULL pointers just like most of our
> free primitives.

Yes, you're right. Let me double-check the code and find out where the
problem is.

>
> Cheers,
> --
> Email: Herbert Xu <herbert@gondor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] net: xfrm: fix bug in ipcomp_free_scratches

[Dongliang Mu]

On Mon, Aug 16, 2021 at 3:53 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Mon, Aug 16, 2021 at 03:38:29PM +0800, Dongliang Mu wrote:
> >
> > -     for_each_possible_cpu(i)
> > -             vfree(*per_cpu_ptr(scratches, i));
> > +     for_each_possible_cpu(i) {
> > +             void *scratch = *per_cpu_ptr(scratches, i);
> > +             if (!scratch)
> > +                     vfree(scratch);
> > +     }
>
> This patch is unnecessary.  Please check the implementation of
> vfree, it already checks for NULL pointers just like most of our
> free primitives.

Hi Herbert,

since there is no reproducer in the syzbot, I guess the problem might be:

if vmalloc_node in the ipcomp_alloc_scratches returns a NULL pointer,
it directly returns NULL without updating the per_cpu_ptr(scratches,
i).

Therefore, in the ipcomp_free_scratches, vfree will take an invalid
and outdated per_cpu_ptr as its argument, leading to the bug - BUG:
unable to handle kernel paging request in ipcomp_free_scratches.

Any idea?

>
> Cheers,
> --
> Email: Herbert Xu <herbert@gondor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt