[syzbot] [net?] KMSAN: uninit-value in pptp_xmit (3)

[syzbot <syzbot+afad90ffc8645324afe5@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    038d61fd6422 Linux 6.16
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=139e9034580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=bc98b07dbc3d5a34
dashboard link: https://syzkaller.appspot.com/bug?extid=afad90ffc8645324afe5
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=163b84a2580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11a594a2580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/58944a2f9bd3/disk-038d61fd.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c9a849a27d33/vmlinux-038d61fd.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0de16772849a/bzImage-038d61fd.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+afad90ffc8645324afe5@syzkaller.appspotmail.com

=====================================================
BUG: KMSAN: uninit-value in pptp_xmit+0xc34/0x2720 drivers/net/ppp/pptp.c:193
 pptp_xmit+0xc34/0x2720 drivers/net/ppp/pptp.c:193
 ppp_channel_bridge_input drivers/net/ppp/ppp_generic.c:2290 [inline]
 ppp_input+0x1d6/0xe60 drivers/net/ppp/ppp_generic.c:2314
 pppoe_rcv_core+0x1e8/0x760 drivers/net/ppp/pppoe.c:379
 sk_backlog_rcv+0x142/0x420 include/net/sock.h:1148
 __release_sock+0x1d3/0x330 net/core/sock.c:3213
 release_sock+0x6b/0x270 net/core/sock.c:3767
 pppoe_sendmsg+0x15d/0xcb0 drivers/net/ppp/pppoe.c:904
 sock_sendmsg_nosec net/socket.c:712 [inline]
 __sock_sendmsg+0x330/0x3d0 net/socket.c:727
 ____sys_sendmsg+0x893/0xd80 net/socket.c:2566
 ___sys_sendmsg+0x271/0x3b0 net/socket.c:2620
 __sys_sendmmsg+0x2d9/0x7c0 net/socket.c:2709
 __do_sys_sendmmsg net/socket.c:2736 [inline]
 __se_sys_sendmmsg net/socket.c:2733 [inline]
 __x64_sys_sendmmsg+0xc6/0x150 net/socket.c:2733
 x64_sys_call+0x3ce7/0x3db0 arch/x86/include/generated/asm/syscalls_64.h:308
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd9/0x210 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
 slab_post_alloc_hook mm/slub.c:4154 [inline]
 slab_alloc_node mm/slub.c:4197 [inline]
 __do_kmalloc_node mm/slub.c:4327 [inline]
 __kmalloc_node_track_caller_noprof+0x96d/0x12f0 mm/slub.c:4347
 kmalloc_reserve+0x22f/0x4b0 net/core/skbuff.c:601
 pskb_expand_head+0x1fc/0x1610 net/core/skbuff.c:2241
 skb_realloc_headroom+0x152/0x2d0 net/core/skbuff.c:2321
 pptp_xmit+0x9d4/0x2720 drivers/net/ppp/pptp.c:181
 ppp_channel_bridge_input drivers/net/ppp/ppp_generic.c:2290 [inline]
 ppp_input+0x1d6/0xe60 drivers/net/ppp/ppp_generic.c:2314
 pppoe_rcv_core+0x1e8/0x760 drivers/net/ppp/pppoe.c:379
 sk_backlog_rcv+0x142/0x420 include/net/sock.h:1148
 __release_sock+0x1d3/0x330 net/core/sock.c:3213
 release_sock+0x6b/0x270 net/core/sock.c:3767
 pppoe_sendmsg+0x15d/0xcb0 drivers/net/ppp/pppoe.c:904
 sock_sendmsg_nosec net/socket.c:712 [inline]
 __sock_sendmsg+0x330/0x3d0 net/socket.c:727
 ____sys_sendmsg+0x893/0xd80 net/socket.c:2566
 ___sys_sendmsg+0x271/0x3b0 net/socket.c:2620
 __sys_sendmmsg+0x2d9/0x7c0 net/socket.c:2709
 __do_sys_sendmmsg net/socket.c:2736 [inline]
 __se_sys_sendmmsg net/socket.c:2733 [inline]
 __x64_sys_sendmmsg+0xc6/0x150 net/socket.c:2733
 x64_sys_call+0x3ce7/0x3db0 arch/x86/include/generated/asm/syscalls_64.h:308
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd9/0x210 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 0 UID: 0 PID: 5830 Comm: syz-executor110 Not tainted 6.16.0-syzkaller #0 PREEMPT(none) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
=====================================================


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