Re: [PATCH v5 2/3] net: add kcov handle to skb extensions

[Ido Schimmel <idosch@idosch.org>]

On Mon, Nov 30, 2020 at 05:52:48PM -0800, Jakub Kicinski wrote:
> On Sat, 21 Nov 2020 18:09:41 +0200 Ido Schimmel wrote:
> > + Florian
> > 
> > On Thu, Oct 29, 2020 at 05:36:19PM +0000, Aleksandr Nogikh wrote:
> > > From: Aleksandr Nogikh <nogikh@google.com>
> > > 
> > > Remote KCOV coverage collection enables coverage-guided fuzzing of the
> > > code that is not reachable during normal system call execution. It is
> > > especially helpful for fuzzing networking subsystems, where it is
> > > common to perform packet handling in separate work queues even for the
> > > packets that originated directly from the user space.
> > > 
> > > Enable coverage-guided frame injection by adding kcov remote handle to
> > > skb extensions. Default initialization in __alloc_skb and
> > > __build_skb_around ensures that no socket buffer that was generated
> > > during a system call will be missed.
> > > 
> > > Code that is of interest and that performs packet processing should be
> > > annotated with kcov_remote_start()/kcov_remote_stop().
> > > 
> > > An alternative approach is to determine kcov_handle solely on the
> > > basis of the device/interface that received the specific socket
> > > buffer. However, in this case it would be impossible to distinguish
> > > between packets that originated during normal background network
> > > processes or were intentionally injected from the user space.
> > > 
> > > Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
> > > Acked-by: Willem de Bruijn <willemb@google.com>  
> > 
> > [...]
> > 
> > > @@ -249,6 +249,9 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> > >  
> > >  		fclones->skb2.fclone = SKB_FCLONE_CLONE;
> > >  	}
> > > +
> > > +	skb_set_kcov_handle(skb, kcov_common_handle());  
> > 
> > Hi,
> > 
> > This causes skb extensions to be allocated for the allocated skb, but
> > there are instances that blindly overwrite 'skb->extensions' by invoking
> > skb_copy_header() after __alloc_skb(). For example, skb_copy(),
> > __pskb_copy_fclone() and skb_copy_expand(). This results in the skb
> > extensions being leaked [1].
> > 
> > One possible solution is to try to patch all these instances with
> > skb_ext_put() before skb_copy_header().
> > 
> > Another possible solution is to convert skb_copy_header() to use
> > skb_ext_copy() instead of __skb_ext_copy(). It will first drop the
> > reference on the skb extensions of the new skb, but it assumes that
> > 'skb->active_extensions' is valid. This is not the case in the
> > skb_clone() path so we should probably zero this field in __skb_clone().
> > 
> > Other suggestions?
> 
> Looking at the patch from Marco to move back to a field now I'm
> wondering how you run into this, Ido :D
> 
> AFAIU the extension is only added if process as a KCOV handle.
> 
> Are you using KCOV?

Hi Jakub,

Yes. We have an internal syzkaller instance where this is enabled. See
"syz-executor.0" in the trace below.

> 
> > [1]
> > BUG: memory leak
> > unreferenced object 0xffff888027f9a490 (size 16):
> >   comm "syz-executor.0", pid 1155, jiffies 4295996826 (age 66.927s)
> >   hex dump (first 16 bytes):
> >     01 00 00 00 01 02 6b 6b 01 00 00 00 00 00 00 00  ......kk........
> >   backtrace:
> >     [<0000000005a5f2c4>] kmemleak_alloc_recursive include/linux/kmemleak.h:43 [inline]
> >     [<0000000005a5f2c4>] slab_post_alloc_hook mm/slab.h:528 [inline]
> >     [<0000000005a5f2c4>] slab_alloc_node mm/slub.c:2891 [inline]
> >     [<0000000005a5f2c4>] slab_alloc mm/slub.c:2899 [inline]
> >     [<0000000005a5f2c4>] kmem_cache_alloc+0x173/0x800 mm/slub.c:2904
> >     [<00000000c5e43ea9>] __skb_ext_alloc+0x22/0x90 net/core/skbuff.c:6173
> >     [<000000000de35e81>] skb_ext_add+0x230/0x4a0 net/core/skbuff.c:6268
> >     [<000000003b7efba4>] skb_set_kcov_handle include/linux/skbuff.h:4622 [inline]
> >     [<000000003b7efba4>] skb_set_kcov_handle include/linux/skbuff.h:4612 [inline]
> >     [<000000003b7efba4>] __alloc_skb+0x47f/0x6a0 net/core/skbuff.c:253
> >     [<000000007f789b23>] skb_copy+0x151/0x310 net/core/skbuff.c:1512
> >     [<000000001ce26864>] mlxsw_emad_transmit+0x4e/0x620 drivers/net/ethernet/mellanox/mlxsw/core.c:585
> >     [<000000005c732123>] mlxsw_emad_reg_access drivers/net/ethernet/mellanox/mlxsw/core.c:829 [inline]
> >     [<000000005c732123>] mlxsw_core_reg_access_emad+0xda8/0x1770 drivers/net/ethernet/mellanox/mlxsw/core.c:2408
> >     [<00000000c07840b3>] mlxsw_core_reg_access+0x101/0x7f0 drivers/net/ethernet/mellanox/mlxsw/core.c:2583
> >     [<000000007c47f30f>] mlxsw_reg_write+0x30/0x40 drivers/net/ethernet/mellanox/mlxsw/core.c:2603
> >     [<00000000675e3fc7>] mlxsw_sp_port_admin_status_set+0x8a7/0x980 drivers/net/ethernet/mellanox/mlxsw/spectrum.c:300
> >     [<00000000fefe35a4>] mlxsw_sp_port_stop+0x63/0x70 drivers/net/ethernet/mellanox/mlxsw/spectrum.c:537
> >     [<00000000c41390e8>] __dev_close_many+0x1c7/0x300 net/core/dev.c:1607
> >     [<00000000628c5987>] __dev_close net/core/dev.c:1619 [inline]
> >     [<00000000628c5987>] __dev_change_flags+0x2b9/0x710 net/core/dev.c:8421
> >     [<000000008cc810c6>] dev_change_flags+0x97/0x170 net/core/dev.c:8494
> >     [<0000000053274a78>] do_setlink+0xa5b/0x3b80 net/core/rtnetlink.c:2706
> >     [<00000000e4085785>] rtnl_group_changelink net/core/rtnetlink.c:3225 [inline]
> >     [<00000000e4085785>] __rtnl_newlink+0xe06/0x17d0 net/core/rtnetlink.c:3379
>