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SubjectRe: [PATCH v5 2/3] net: add kcov handle to skb extensions
+ 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?

Thanks
[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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