If skb_put_padto() returns an error, skb has been freed.
Better not touch it anymore, as reported by syzbot [1]

Note to qrtr maintainers : this suggests qrtr_sendmsg()
should adjust sock_alloc_send_skb() second parameter
to account for the potential added alignment to avoid
reallocation.

[1]

BUG: KASAN: use-after-free in __skb_insert include/linux/skbuff.h:1907 [inline]
BUG: KASAN: use-after-free in __skb_queue_before include/linux/skbuff.h:2016 [inline]
BUG: KASAN: use-after-free in __skb_queue_tail include/linux/skbuff.h:2049 [inline]
BUG: KASAN: use-after-free in skb_queue_tail+0x6b/0x120 net/core/skbuff.c:3146
Write of size 8 at addr ffff88804d8ab3c0 by task syz-executor.4/4316

CPU: 1 PID: 4316 Comm: syz-executor.4 Not tainted 5.9.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1d6/0x29e lib/dump_stack.c:118
 print_address_description+0x66/0x620 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report+0x132/0x1d0 mm/kasan/report.c:530
 __skb_insert include/linux/skbuff.h:1907 [inline]
 __skb_queue_before include/linux/skbuff.h:2016 [inline]
 __skb_queue_tail include/linux/skbuff.h:2049 [inline]
 skb_queue_tail+0x6b/0x120 net/core/skbuff.c:3146
 qrtr_tun_send+0x1a/0x40 net/qrtr/tun.c:23
 qrtr_node_enqueue+0x44f/0xc00 net/qrtr/qrtr.c:364
 qrtr_bcast_enqueue+0xbe/0x140 net/qrtr/qrtr.c:861
 qrtr_sendmsg+0x680/0x9c0 net/qrtr/qrtr.c:960
 sock_sendmsg_nosec net/socket.c:651 [inline]
 sock_sendmsg net/socket.c:671 [inline]
 sock_write_iter+0x317/0x470 net/socket.c:998
 call_write_iter include/linux/fs.h:1882 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0xa96/0xd10 fs/read_write.c:578
 ksys_write+0x11b/0x220 fs/read_write.c:631
 do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45d5b9
Code: 5d b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 2b b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f84b5b81c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000038b40 RCX: 000000000045d5b9
RDX: 0000000000000055 RSI: 0000000020001240 RDI: 0000000000000003
RBP: 00007f84b5b81ca0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000000f
R13: 00007ffcbbf86daf R14: 00007f84b5b829c0 R15: 000000000118cf4c

Allocated by task 4316:
 kasan_save_stack mm/kasan/common.c:48 [inline]
 kasan_set_track mm/kasan/common.c:56 [inline]
 __kasan_kmalloc+0x100/0x130 mm/kasan/common.c:461
 slab_post_alloc_hook+0x3e/0x290 mm/slab.h:518
 slab_alloc mm/slab.c:3312 [inline]
 kmem_cache_alloc+0x1c1/0x2d0 mm/slab.c:3482
 skb_clone+0x1b2/0x370 net/core/skbuff.c:1449
 qrtr_bcast_enqueue+0x6d/0x140 net/qrtr/qrtr.c:857
 qrtr_sendmsg+0x680/0x9c0 net/qrtr/qrtr.c:960
 sock_sendmsg_nosec net/socket.c:651 [inline]
 sock_sendmsg net/socket.c:671 [inline]
 sock_write_iter+0x317/0x470 net/socket.c:998
 call_write_iter include/linux/fs.h:1882 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0xa96/0xd10 fs/read_write.c:578
 ksys_write+0x11b/0x220 fs/read_write.c:631
 do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 4316:
 kasan_save_stack mm/kasan/common.c:48 [inline]
 kasan_set_track+0x3d/0x70 mm/kasan/common.c:56
 kasan_set_free_info+0x17/0x30 mm/kasan/generic.c:355
 __kasan_slab_free+0xdd/0x110 mm/kasan/common.c:422
 __cache_free mm/slab.c:3418 [inline]
 kmem_cache_free+0x82/0xf0 mm/slab.c:3693
 __skb_pad+0x3f5/0x5a0 net/core/skbuff.c:1823
 __skb_put_padto include/linux/skbuff.h:3233 [inline]
 skb_put_padto include/linux/skbuff.h:3252 [inline]
 qrtr_node_enqueue+0x62f/0xc00 net/qrtr/qrtr.c:360
 qrtr_bcast_enqueue+0xbe/0x140 net/qrtr/qrtr.c:861
 qrtr_sendmsg+0x680/0x9c0 net/qrtr/qrtr.c:960
 sock_sendmsg_nosec net/socket.c:651 [inline]
 sock_sendmsg net/socket.c:671 [inline]
 sock_write_iter+0x317/0x470 net/socket.c:998
 call_write_iter include/linux/fs.h:1882 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0xa96/0xd10 fs/read_write.c:578
 ksys_write+0x11b/0x220 fs/read_write.c:631
 do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The buggy address belongs to the object at ffff88804d8ab3c0
 which belongs to the cache skbuff_head_cache of size 224
The buggy address is located 0 bytes inside of
 224-byte region [ffff88804d8ab3c0, ffff88804d8ab4a0)
The buggy address belongs to the page:
page:00000000ea8cccfb refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff88804d8abb40 pfn:0x4d8ab
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea0002237ec8 ffffea00029b3388 ffff88821bb66800
raw: ffff88804d8abb40 ffff88804d8ab000 000000010000000b 0000000000000000
page dumped because: kasan: bad access detected

Fixes: ce57785b ("net: qrtr: fix len of skb_put_padto in qrtr_node_enqueue")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Carl Huang <cjhuang@codeaurora.org>
Cc: Wen Gong <wgong@codeaurora.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>