CVE-2025-21719

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

ipmr: do not call mr_mfc_uses_dev() for unres entries

syzbot found that calling mr_mfc_uses_dev() for unres entries
would crash [1], because c->mfc_un.res.minvif / c->mfc_un.res.maxvif
alias to "struct sk_buff_head unresolved", which contain two pointers.

This code never worked, lets remove it.

[1]
Unable to handle kernel paging request at virtual address ffff5fff2d536613
KASAN: maybe wild-memory-access in range [0xfffefff96a9b3098-0xfffefff96a9b309f]
Modules linked in:
CPU: 1 UID: 0 PID: 7321 Comm: syz.0.16 Not tainted 6.13.0-rc7-syzkaller-g1950a0af2d55 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : mr_mfc_uses_dev net/ipv4/ipmr_base.c:290 [inline]
 pc : mr_table_dump+0x5a4/0x8b0 net/ipv4/ipmr_base.c:334
 lr : mr_mfc_uses_dev net/ipv4/ipmr_base.c:289 [inline]
 lr : mr_table_dump+0x694/0x8b0 net/ipv4/ipmr_base.c:334
Call trace:
  mr_mfc_uses_dev net/ipv4/ipmr_base.c:290 [inline] (P)
  mr_table_dump+0x5a4/0x8b0 net/ipv4/ipmr_base.c:334 (P)
  mr_rtm_dumproute+0x254/0x454 net/ipv4/ipmr_base.c:382
  ipmr_rtm_dumproute+0x248/0x4b4 net/ipv4/ipmr.c:2648
  rtnl_dump_all+0x2e4/0x4e8 net/core/rtnetlink.c:4327
  rtnl_dumpit+0x98/0x1d0 net/core/rtnetlink.c:6791
  netlink_dump+0x4f0/0xbc0 net/netlink/af_netlink.c:2317
  netlink_recvmsg+0x56c/0xe64 net/netlink/af_netlink.c:1973
  sock_recvmsg_nosec net/socket.c:1033 [inline]
  sock_recvmsg net/socket.c:1055 [inline]
  sock_read_iter+0x2d8/0x40c net/socket.c:1125
  new_sync_read fs/read_write.c:484 [inline]
  vfs_read+0x740/0x970 fs/read_write.c:565
  ksys_read+0x15c/0x26c fs/read_write.c:708