memory leak in ppp_write

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    d8778f13 Merge tag 'xtensa-20190803' of git://github.com/j..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17a953d6600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=30cef20daf3e9977
dashboard link: https://syzkaller.appspot.com/bug?extid=d9c8bf24e56416d7ce2c
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16da002c600000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d9c8bf24e56416d7ce2c@xxxxxxxxxxxxxxxxxxxxxxxxx

2019/08/04 10:45:32 executed programs: 5
2019/08/04 10:45:38 executed programs: 7
2019/08/04 10:45:44 executed programs: 9
2019/08/04 10:45:51 executed programs: 11
BUG: memory leak
unreferenced object 0xffff88811b943200 (size 224):
  comm "syz-executor.0", pid 7102, jiffies 4294951426 (age 8.020s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000612bb18c>] kmemleak_alloc_recursive  
include/linux/kmemleak.h:43 [inline]
    [<00000000612bb18c>] slab_post_alloc_hook mm/slab.h:522 [inline]
    [<00000000612bb18c>] slab_alloc_node mm/slab.c:3262 [inline]
    [<00000000612bb18c>] kmem_cache_alloc_node+0x163/0x2f0 mm/slab.c:3574
    [<00000000f510d7dd>] __alloc_skb+0x6e/0x210 net/core/skbuff.c:197
    [<0000000064f35f9b>] alloc_skb include/linux/skbuff.h:1055 [inline]
    [<0000000064f35f9b>] ppp_write+0x48/0x120  
drivers/net/ppp/ppp_generic.c:502
    [<000000007d5732a9>] __vfs_write+0x43/0xa0 fs/read_write.c:494
    [<00000000b490138e>] vfs_write fs/read_write.c:558 [inline]
    [<00000000b490138e>] vfs_write+0xee/0x210 fs/read_write.c:542
    [<00000000d20d33e5>] ksys_write+0x7c/0x130 fs/read_write.c:611
    [<000000007b61e45c>] __do_sys_write fs/read_write.c:623 [inline]
    [<000000007b61e45c>] __se_sys_write fs/read_write.c:620 [inline]
    [<000000007b61e45c>] __x64_sys_write+0x1e/0x30 fs/read_write.c:620
    [<0000000067600a9b>] do_syscall_64+0x76/0x1a0  
arch/x86/entry/common.c:296
    [<000000007e48b83c>] entry_SYSCALL_64_after_hwframe+0x44/0xa9



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