ppp_async: limit MRU to 64K

In the Linux kernel, the following vulnerability has been resolved:

ppp_async: limit MRU to 64K

syzbot triggered a warning [1] in __alloc_pages():

WARN_ON_ONCE_GFP(order > MAX_PAGE_ORDER, gfp)

Willem fixed a similar issue in commit c0a2a1b0d631 ("ppp: limit MRU to 64K")

Adopt the same sanity check for ppp_async_ioctl(PPPIOCSMRU)

[1]:

WARNING: CPU: 1 PID: 11 at mm/page_alloc.c:4543 __alloc_pages+0x308/0x698 mm/page_alloc.c:4543
Modules linked in:
CPU: 1 PID: 11 Comm: kworker/u4:0 Not tainted 6.8.0-rc2-syzkaller-g41bccc98fb79 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023
Workqueue: events_unbound flush_to_ldisc
pstate: 204000c5 (nzCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : __alloc_pages+0x308/0x698 mm/page_alloc.c:4543
lr : __alloc_pages+0xc8/0x698 mm/page_alloc.c:4537
sp : ffff800093967580
x29: ffff800093967660 x28: ffff8000939675a0 x27: dfff800000000000
x26: ffff70001272ceb4 x25: 0000000000000000 x24: ffff8000939675c0
x23: 0000000000000000 x22: 0000000000060820 x21: 1ffff0001272ceb8
x20: ffff8000939675e0 x19: 0000000000000010 x18: ffff800093967120
x17: ffff800083bded5c x16: ffff80008ac97500 x15: 0000000000000005
x14: 1ffff0001272cebc x13: 0000000000000000 x12: 0000000000000000
x11: ffff70001272cec1 x10: 1ffff0001272cec0 x9 : 0000000000000001
x8 : ffff800091c91000 x7 : 0000000000000000 x6 : 000000000000003f
x5 : 00000000ffffffff x4 : 0000000000000000 x3 : 0000000000000020
x2 : 0000000000000008 x1 : 0000000000000000 x0 : ffff8000939675e0
Call trace:
__alloc_pages+0x308/0x698 mm/page_alloc.c:4543
__alloc_pages_node include/linux/gfp.h:238 [inline]
alloc_pages_node include/linux/gfp.h:261 [inline]
__kmalloc_large_node+0xbc/0x1fc mm/slub.c:3926
__do_kmalloc_node mm/slub.c:3969 [inline]
__kmalloc_node_track_caller+0x418/0x620 mm/slub.c:4001
kmalloc_reserve+0x17c/0x23c net/core/skbuff.c:590
__alloc_skb+0x1c8/0x3d8 net/core/skbuff.c:651
__netdev_alloc_skb+0xb8/0x3e8 net/core/skbuff.c:715
netdev_alloc_skb include/linux/skbuff.h:3235 [inline]
dev_alloc_skb include/linux/skbuff.h:3248 [inline]
ppp_async_input drivers/net/ppp/ppp_async.c:863 [inline]
ppp_asynctty_receive+0x588/0x186c drivers/net/ppp/ppp_async.c:341
tty_ldisc_receive_buf+0x12c/0x15c drivers/tty/tty_buffer.c:390
tty_port_default_receive_buf+0x74/0xac drivers/tty/tty_port.c:37
receive_buf drivers/tty/tty_buffer.c:444 [inline]
flush_to_ldisc+0x284/0x6e4 drivers/tty/tty_buffer.c:494
process_one_work+0x694/0x1204 kernel/workqueue.c:2633
process_scheduled_works kernel/workqueue.c:2706 [inline]
worker_thread+0x938/0xef4 kernel/workqueue.c:2787
kthread+0x288/0x310 kernel/kthread.c:388
ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:860