CVE-2025-37961 ipvs: fix uninit-value for saddr in do_output_route4
In the Linux kernel, the following vulnerability has been resolved:

ipvs: fix uninit-value for saddr in do_output_route4

syzbot reports for uninit-value for the saddr argument [1].
commit 4754957f04f5 ("ipvs: do not use random local source address for
tunnels") already implies that the input value of saddr
should be ignored but the code is still reading it which can prevent
to connect the route. Fix it by changing the argument to ret_saddr.

[1]
BUG: KMSAN: uninit-value in do_output_route4+0x42c/0x4d0 net/netfilter/ipvs/ip_vs_xmit.c:147
 do_output_route4+0x42c/0x4d0 net/netfilter/ipvs/ip_vs_xmit.c:147
 __ip_vs_get_out_rt+0x403/0x21d0 net/netfilter/ipvs/ip_vs_xmit.c:330
 ip_vs_tunnel_xmit+0x205/0x2380 net/netfilter/ipvs/ip_vs_xmit.c:1136
 ip_vs_in_hook+0x1aa5/0x35b0 net/netfilter/ipvs/ip_vs_core.c:2063
 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
 nf_hook_slow+0xf7/0x400 net/netfilter/core.c:626
 nf_hook include/linux/netfilter.h:269 [inline]
 __ip_local_out+0x758/0x7e0 net/ipv4/ip_output.c:118
 ip_local_out net/ipv4/ip_output.c:127 [inline]
 ip_send_skb+0x6a/0x3c0 net/ipv4/ip_output.c:1501
 udp_send_skb+0xfda/0x1b70 net/ipv4/udp.c:1195
 udp_sendmsg+0x2fe3/0x33c0 net/ipv4/udp.c:1483
 inet_sendmsg+0x1fc/0x280 net/ipv4/af_inet.c:851
 sock_sendmsg_nosec net/socket.c:712 [inline]
 __sock_sendmsg+0x267/0x380 net/socket.c:727
 ____sys_sendmsg+0x91b/0xda0 net/socket.c:2566
 ___sys_sendmsg+0x28d/0x3c0 net/socket.c:2620
 __sys_sendmmsg+0x41d/0x880 net/socket.c:2702
 __compat_sys_sendmmsg net/compat.c:360 [inline]
 __do_compat_sys_sendmmsg net/compat.c:367 [inline]
 __se_compat_sys_sendmmsg net/compat.c:364 [inline]
 __ia32_compat_sys_sendmmsg+0xc8/0x140 net/compat.c:364
 ia32_sys_call+0x3ffa/0x41f0 arch/x86/include/generated/asm/syscalls_32.h:346
 do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:83 [inline]
 __do_fast_syscall_32+0xb0/0x110 arch/x86/entry/syscall_32.c:306
 do_fast_syscall_32+0x38/0x80 arch/x86/entry/syscall_32.c:331
 do_SYSENTER_32+0x1f/0x30 arch/x86/entry/syscall_32.c:369
 entry_SYSENTER_compat_after_hwframe+0x84/0x8e

Uninit was created at:
 slab_post_alloc_hook mm/slub.c:4167 [inline]
 slab_alloc_node mm/slub.c:4210 [inline]
 __kmalloc_cache_noprof+0x8fa/0xe00 mm/slub.c:4367
 kmalloc_noprof include/linux/slab.h:905 [inline]
 ip_vs_dest_dst_alloc net/netfilter/ipvs/ip_vs_xmit.c:61 [inline]
 __ip_vs_get_out_rt+0x35d/0x21d0 net/netfilter/ipvs/ip_vs_xmit.c:323
 ip_vs_tunnel_xmit+0x205/0x2380 net/netfilter/ipvs/ip_vs_xmit.c:1136
 ip_vs_in_hook+0x1aa5/0x35b0 net/netfilter/ipvs/ip_vs_core.c:2063
 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
 nf_hook_slow+0xf7/0x400 net/netfilter/core.c:626
 nf_hook include/linux/netfilter.h:269 [inline]
 __ip_local_out+0x758/0x7e0 net/ipv4/ip_output.c:118
 ip_local_out net/ipv4/ip_output.c:127 [inline]
 ip_send_skb+0x6a/0x3c0 net/ipv4/ip_output.c:1501
 udp_send_skb+0xfda/0x1b70 net/ipv4/udp.c:1195
 udp_sendmsg+0x2fe3/0x33c0 net/ipv4/udp.c:1483
 inet_sendmsg+0x1fc/0x280 net/ipv4/af_inet.c:851
 sock_sendmsg_nosec net/socket.c:712 [inline]
 __sock_sendmsg+0x267/0x380 net/socket.c:727
 ____sys_sendmsg+0x91b/0xda0 net/socket.c:2566
 ___sys_sendmsg+0x28d/0x3c0 net/socket.c:2620
 __sys_sendmmsg+0x41d/0x880 net/socket.c:2702
 __compat_sys_sendmmsg net/compat.c:360 [inline]
 __do_compat_sys_sendmmsg net/compat.c:367 [inline]
 __se_compat_sys_sendmmsg net/compat.c:364 [inline]
 __ia32_compat_sys_sendmmsg+0xc8/0x140 net/compat.c:364
 ia32_sys_call+0x3ffa/0x41f0 arch/x86/include/generated/asm/syscalls_32.h:346
 do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:83 [inline]
 __do_fast_syscall_32+0xb0/0x110 arch/x86/entry/syscall_32.c:306
 do_fast_syscall_32+0x38/0x80 arch/x86/entry/syscall_32.c:331
 do_SYSENTER_32+0x1f/0x30 arch/x86/entry/syscall_32.c:369
 entry_SYSENTER_compat_after_hwframe+0x84/0x8e

CPU: 0 UID: 0 PID: 22408 Comm: syz.4.5165 Not tainted 6.15.0-rc3-syzkaller-00019-gbc3372351d0c #0 PREEMPT(undef)
Hardware name: Google Google Compute Engi
---truncated---
CVSS v3.1 score: 5.5