CVE-2025-39984

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

net: tun: Update napi->skb after XDP process

The syzbot report a UAF issue:

BUG: KASAN: slab-use-after-free in skbresetmacheader include/linux/skbuff.h:3150 [inline] BUG: KASAN: slab-use-after-free in napifragsskb net/core/gro.c:723 [inline] BUG: KASAN: slab-use-after-free in napigrofrags+0x6e/0x1030 net/core/gro.c:758 Read of size 8 at addr ffff88802ef22c18 by task syz.0.17/6079 CPU: 0 UID: 0 PID: 6079 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) Call Trace: <TASK> dumpstacklvl+0x189/0x250 lib/dumpstack.c:120 printaddressdescription mm/kasan/report.c:378 [inline] printreport+0xca/0x240 mm/kasan/report.c:482 kasanreport+0x118/0x150 mm/kasan/report.c:595 skbresetmacheader include/linux/skbuff.h:3150 [inline] napifragsskb net/core/gro.c:723 [inline] napigrofrags+0x6e/0x1030 net/core/gro.c:758 tungetuser+0x28cb/0x3e20 drivers/net/tun.c:1920 tunchrwriteiter+0x113/0x200 drivers/net/tun.c:1996 newsyncwrite fs/readwrite.c:593 [inline] vfswrite+0x5c9/0xb30 fs/readwrite.c:686 ksyswrite+0x145/0x250 fs/readwrite.c:738 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xfa/0x3b0 arch/x86/entry/syscall64.c:94 entrySYSCALL64after_hwframe+0x77/0x7f </TASK>

Allocated by task 6079: kasansavestack mm/kasan/common.c:47 [inline] kasansavetrack+0x3e/0x80 mm/kasan/common.c:68 unpoisonslabobject mm/kasan/common.c:330 [inline] _kasanmempoolunpoisonobject+0xa0/0x170 mm/kasan/common.c:558 kasanmempoolunpoisonobject include/linux/kasan.h:388 [inline] napiskbcacheget+0x37b/0x6d0 net/core/skbuff.c:295 _allocskb+0x11e/0x2d0 net/core/skbuff.c:657 napiallocskb+0x84/0x7d0 net/core/skbuff.c:811 napigetfrags+0x69/0x140 net/core/gro.c:673 tunnapiallocfrags drivers/net/tun.c:1404 [inline] tungetuser+0x77c/0x3e20 drivers/net/tun.c:1784 tunchrwriteiter+0x113/0x200 drivers/net/tun.c:1996 newsyncwrite fs/readwrite.c:593 [inline] vfswrite+0x5c9/0xb30 fs/readwrite.c:686 ksyswrite+0x145/0x250 fs/readwrite.c:738 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xfa/0x3b0 arch/x86/entry/syscall64.c:94 entrySYSCALL64after_hwframe+0x77/0x7f

Freed by task 6079: kasansavestack mm/kasan/common.c:47 [inline] kasansavetrack+0x3e/0x80 mm/kasan/common.c:68 kasansavefreeinfo+0x46/0x50 mm/kasan/generic.c:576 poisonslabobject mm/kasan/common.c:243 [inline] _kasanslabfree+0x5b/0x80 mm/kasan/common.c:275 kasanslabfree include/linux/kasan.h:233 [inline] slabfreehook mm/slub.c:2422 [inline] slabfree mm/slub.c:4695 [inline] kmemcachefree+0x18f/0x400 mm/slub.c:4797 skbppcowdata+0xdd8/0x13e0 net/core/skbuff.c:969 netifskbcheckforxdp net/core/dev.c:5390 [inline] netifreceivegenericxdp net/core/dev.c:5431 [inline] doxdpgeneric+0x699/0x11a0 net/core/dev.c:5499 tungetuser+0x2523/0x3e20 drivers/net/tun.c:1872 tunchrwriteiter+0x113/0x200 drivers/net/tun.c:1996 newsyncwrite fs/readwrite.c:593 [inline] vfswrite+0x5c9/0xb30 fs/readwrite.c:686 ksyswrite+0x145/0x250 fs/readwrite.c:738 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xfa/0x3b0 arch/x86/entry/syscall64.c:94 entrySYSCALL64after_hwframe+0x77/0x7f

After commit e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode"), the original skb may be freed in skbppcowdata() when XDP program was attached, which was allocated in tunnapiallocfrags(). However, the napi->skb still point to the original skb, update it after XDP process.