CVE-2026-43139

In the Linux kernel, the following vulnerability has been resolved:

xfrm6: fix uninitialized saddr in xfrm6_get_saddr()

xfrm6_get_saddr() does not check the return value of
ipv6_dev_get_saddr(). When ipv6_dev_get_saddr() fails to find a suitable
source address (returns -EADDRNOTAVAIL), saddr->in6 is left
uninitialized, but xfrm6_get_saddr() still returns 0 (success).

This causes the caller xfrm_tmpl_resolve_one() to use the uninitialized
address in xfrm_state_find(), triggering KMSAN warning:

=====================================================
BUG: KMSAN: uninit-value in xfrm_state_find+0x2424/0xa940
 xfrm_state_find+0x2424/0xa940
 xfrm_resolve_and_create_bundle+0x906/0x5a20
 xfrm_lookup_with_ifid+0xcc0/0x3770
 xfrm_lookup_route+0x63/0x2b0
 ip_route_output_flow+0x1ce/0x270
 udp_sendmsg+0x2ce1/0x3400
 inet_sendmsg+0x1ef/0x2a0
 __sock_sendmsg+0x278/0x3d0
 __sys_sendto+0x593/0x720
 __x64_sys_sendto+0x130/0x200
 x64_sys_call+0x332b/0x3e70
 do_syscall_64+0xd3/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Local variable tmp.i.i created at:
 xfrm_resolve_and_create_bundle+0x3e3/0x5a20
 xfrm_lookup_with_ifid+0xcc0/0x3770
=====================================================

Fix by checking the return value of ipv6_dev_get_saddr() and propagating
the error.