Improve code coverage of syzkaller

This patch series is intended to improve code coverage of syzkaller on the early receive path, specifically including flow dissector, GRO, and GRO with frags parts of the networking stack. Syzkaller exercises the stack through the TUN driver and this is therefore where changes reside. Current coverage through netif_receive_skb() is limited as it does not touch on any of the aforementioned code paths. Furthermore, for full coverage, it is necessary to have more flexibility over the linear and non-linear data of the skbs.

The following patches address this by providing the user(syzkaller) with the ability to send via napi_gro_receive() and napi_gro_frags(). Additionally, syzkaller can specify how many fragments there are and how much data per fragment there is. This is done by exploiting the convenient structure of iovecs. Finally, this patch series adds support for exercising the flow dissector during fuzzing.

The code path including napi_gro_receive() can be enabled via the IFF_NAPI flag. The remainder of the changes in this patch series give the user significantly more control over packets entering the kernel. To avoid potential security vulnerabilities, hide the ability to send custom skbs and the flow dissector code paths behind a capable(CAP_NET_ADMIN) check to require special user privileges.

Changes since v2 based on feedback from Willem de Bruijn and Mahesh Bandewar:

Patch 1/ No changes.
Patch 2/ Check if the preconditions for IFF_NAPI_FRAGS (IFF_NAPI and
IFF_TAP) are met before opening/attaching rather than after. If they are not, change the behavior from discarding the flag to rejecting the command with EINVAL.

Petar Penkov (2):
tun: enable NAPI for TUN/TAP driver
tun: enable napi_gro_frags() for TUN/TAP driver

 drivers/net/tun.c           | 261 +++++++++++++++++++++++++++++++++++++++++---
 include/uapi/linux/if_tun.h |   2 +
 2 files changed, 245 insertions(+), 18 deletions(-)

2.11.0