Date:   Mon, 30 Nov 2020 17:52:48 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     Aleksandr Nogikh <aleksandrnogikh@...il.com>, fw@...len.de,
        davem@...emloft.net, johannes@...solutions.net,
        edumazet@...gle.com, andreyknvl@...gle.com, dvyukov@...gle.com,
        elver@...gle.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, linux-wireless@...r.kernel.org,
        willemdebruijn.kernel@...il.com,
        Aleksandr Nogikh <nogikh@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v5 2/3] net: add kcov handle to skb extensions

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@...gle.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@...gle.com>
> > Acked-by: Willem de Bruijn <willemb@...gle.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?

> [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

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