net: disable preemption in dev_core_stats_XXX_inc() helpers

syzbot was kind enough to remind us that dev->{tx_dropped|rx_dropped}
could be increased in process context.

BUG: using smp_processor_id() in preemptible [00000000] code: syz-executor413/3593
caller is netdev_core_stats_alloc+0x98/0x110 net/core/dev.c:10298
CPU: 1 PID: 3593 Comm: syz-executor413 Not tainted 5.17.0-rc7-syzkaller-02426-g97aeb877 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 check_preemption_disabled+0x16b/0x170 lib/smp_processor_id.c:49
 netdev_core_stats_alloc+0x98/0x110 net/core/dev.c:10298
 dev_core_stats include/linux/netdevice.h:3855 [inline]
 dev_core_stats_rx_dropped_inc include/linux/netdevice.h:3866 [inline]
 tun_get_user+0x3455/0x3ab0 drivers/net/tun.c:1800
 tun_chr_write_iter+0xe1/0x200 drivers/net/tun.c:2015
 call_write_iter include/linux/fs.h:2074 [inline]
 new_sync_write+0x431/0x660 fs/read_write.c:503
 vfs_write+0x7cd/0xae0 fs/read_write.c:590
 ksys_write+0x12d/0x250 fs/read_write.c:643
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f2cf4f887e3
Code: 5d 41 5c 41 5d 41 5e e9 9b fd ff ff 66 2e 0f 1f 84 00 00 00 00 00 90 64 8b 04 25 18 00 00 00 85 c0 75 14 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 55 c3 0f 1f 40 00 48 83 ec 28 48 89 54 24 18
RSP: 002b:00007ffd50dd5fd8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007ffd50dd6000 RCX: 00007f2cf4f887e3
RDX: 000000000000002a RSI: 0000000000000000 RDI: 00000000000000c8
RBP: 0000000000000003 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffd50dd5ff0 R14: 00007ffd50dd5fe8 R15: 00007ffd50dd5fe4
 </TASK>

Fixes: 625788b5 ("net: add per-cpu storage and net->core_stats")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: jeffreyji <jeffreyji@google.com>
Cc: Brian Vazquez <brianvv@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20220312214505.3294762-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -3858,10 +3858,14 @@ static inline struct net_device_core_stats *dev_core_stats(struct net_device *de
 #define DEV_CORE_STATS_INC(FIELD)						\
 static inline void dev_core_stats_##FIELD##_inc(struct net_device *dev)		\
 {										\
-	struct net_device_core_stats *p = dev_core_stats(dev);			\
+	struct net_device_core_stats *p;					\
+										\
+	preempt_disable();							\
+	p = dev_core_stats(dev);						\
 										\
 	if (p)									\
 		local_inc(&p->FIELD);						\
+	preempt_enable();							\
 }
 DEV_CORE_STATS_INC(rx_dropped)
 DEV_CORE_STATS_INC(tx_dropped)