net: cdc_ncm: Allow for dwNtbOutMaxSize to be unset or zero

Currently, due to the sequential use of min_t() and clamp_t() macros,
in cdc_ncm_check_tx_max(), if dwNtbOutMaxSize is not set, the logic
sets tx_max to 0.  This is then used to allocate the data area of the
SKB requested later in cdc_ncm_fill_tx_frame().

This does not cause an issue presently because when memory is
allocated during initialisation phase of SKB creation, more memory
(512b) is allocated than is required for the SKB headers alone (320b),
leaving some space (512b - 320b = 192b) for CDC data (172b).

However, if more elements (for example 3 x u64 = [24b]) were added to
one of the SKB header structs, say 'struct skb_shared_info',
increasing its original size (320b [320b aligned]) to something larger
(344b [384b aligned]), then suddenly the CDC data (172b) no longer
fits in the spare SKB data area (512b - 384b = 128b).

Consequently the SKB bounds checking semantics fails and panics:

  skbuff: skb_over_panic: text:ffffffff830a5b5f len:184 put:172   \
     head:ffff888119227c00 data:ffff888119227c00 tail:0xb8 end:0x80 dev:<NULL>

  ------------[ cut here ]------------
  kernel BUG at net/core/skbuff.c:110!
  RIP: 0010:skb_panic+0x14f/0x160 net/core/skbuff.c:106
  <snip>
  Call Trace:
   <IRQ>
   skb_over_panic+0x2c/0x30 net/core/skbuff.c:115
   skb_put+0x205/0x210 net/core/skbuff.c:1877
   skb_put_zero include/linux/skbuff.h:2270 [inline]
   cdc_ncm_ndp16 drivers/net/usb/cdc_ncm.c:1116 [inline]
   cdc_ncm_fill_tx_frame+0x127f/0x3d50 drivers/net/usb/cdc_ncm.c:1293
   cdc_ncm_tx_fixup+0x98/0xf0 drivers/net/usb/cdc_ncm.c:1514

By overriding the max value with the default CDC_NCM_NTB_MAX_SIZE_TX
when not offered through the system provided params, we ensure enough
data space is allocated to handle the CDC data, meaning no crash will
occur.

Cc: Oliver Neukum <oliver@neukum.org>
Fixes: 289507d3 ("net: cdc_ncm: use sysfs for rx/tx aggregation tuning")
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Bjørn Mork <bjorn@mork.no>
Link: https://lore.kernel.org/r/20211202143437.1411410-1-lee.jones@linaro.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>