Message-ID: <willemdebruijn.kernel.1aca4aa96eb20@gmail.com>
Date: Thu, 18 Sep 2025 10:30:19 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Wang Liang <wangliang74@...wei.com>, 
 willemdebruijn.kernel@...il.com, 
 jasowang@...hat.com, 
 andrew+netdev@...n.ch, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 ast@...nel.org, 
 daniel@...earbox.net, 
 hawk@...nel.org, 
 john.fastabend@...il.com, 
 sdf@...ichev.me, 
 lorenzo@...nel.org, 
 toke@...hat.com
Cc: yuehaibing@...wei.com, 
 zhangchangzhong@...wei.com, 
 wangliang74@...wei.com, 
 linux-kernel@...r.kernel.org, 
 netdev@...r.kernel.org
Subject: Re: [PATCH net] net: tun: Update napi->skb after XDP process

Wang Liang wrote:
> The syzbot report a UAF issue:
> 
>   BUG: KASAN: slab-use-after-free in skb_reset_mac_header include/linux/skbuff.h:3150 [inline]
>   BUG: KASAN: slab-use-after-free in napi_frags_skb net/core/gro.c:723 [inline]
>   BUG: KASAN: slab-use-after-free in napi_gro_frags+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>
>    dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>    print_address_description mm/kasan/report.c:378 [inline]
>    print_report+0xca/0x240 mm/kasan/report.c:482
>    kasan_report+0x118/0x150 mm/kasan/report.c:595
>    skb_reset_mac_header include/linux/skbuff.h:3150 [inline]
>    napi_frags_skb net/core/gro.c:723 [inline]
>    napi_gro_frags+0x6e/0x1030 net/core/gro.c:758
>    tun_get_user+0x28cb/0x3e20 drivers/net/tun.c:1920
>    tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:1996
>    new_sync_write fs/read_write.c:593 [inline]
>    vfs_write+0x5c9/0xb30 fs/read_write.c:686
>    ksys_write+0x145/0x250 fs/read_write.c:738
>    do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>    do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>    entry_SYSCALL_64_after_hwframe+0x77/0x7f
>    </TASK>
> 
>   Allocated by task 6079:
>    kasan_save_stack mm/kasan/common.c:47 [inline]
>    kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
>    unpoison_slab_object mm/kasan/common.c:330 [inline]
>    __kasan_mempool_unpoison_object+0xa0/0x170 mm/kasan/common.c:558
>    kasan_mempool_unpoison_object include/linux/kasan.h:388 [inline]
>    napi_skb_cache_get+0x37b/0x6d0 net/core/skbuff.c:295
>    __alloc_skb+0x11e/0x2d0 net/core/skbuff.c:657
>    napi_alloc_skb+0x84/0x7d0 net/core/skbuff.c:811
>    napi_get_frags+0x69/0x140 net/core/gro.c:673
>    tun_napi_alloc_frags drivers/net/tun.c:1404 [inline]
>    tun_get_user+0x77c/0x3e20 drivers/net/tun.c:1784
>    tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:1996
>    new_sync_write fs/read_write.c:593 [inline]
>    vfs_write+0x5c9/0xb30 fs/read_write.c:686
>    ksys_write+0x145/0x250 fs/read_write.c:738
>    do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>    do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>    entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
>   Freed by task 6079:
>    kasan_save_stack mm/kasan/common.c:47 [inline]
>    kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
>    kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:576
>    poison_slab_object mm/kasan/common.c:243 [inline]
>    __kasan_slab_free+0x5b/0x80 mm/kasan/common.c:275
>    kasan_slab_free include/linux/kasan.h:233 [inline]
>    slab_free_hook mm/slub.c:2422 [inline]
>    slab_free mm/slub.c:4695 [inline]
>    kmem_cache_free+0x18f/0x400 mm/slub.c:4797
>    skb_pp_cow_data+0xdd8/0x13e0 net/core/skbuff.c:969
>    netif_skb_check_for_xdp net/core/dev.c:5390 [inline]
>    netif_receive_generic_xdp net/core/dev.c:5431 [inline]
>    do_xdp_generic+0x699/0x11a0 net/core/dev.c:5499
>    tun_get_user+0x2523/0x3e20 drivers/net/tun.c:1872
>    tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:1996
>    new_sync_write fs/read_write.c:593 [inline]
>    vfs_write+0x5c9/0xb30 fs/read_write.c:686
>    ksys_write+0x145/0x250 fs/read_write.c:738
>    do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>    do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>    entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> After commit e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in
> generic mode"), the original skb may be freed in skb_pp_cow_data() when
> XDP program was attached, which was allocated in tun_napi_alloc_frags().
> However, the napi->skb still point to the original skb, update it after
> XDP process.
> 
> Reported-by: syzbot+64e24275ad95a915a313@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=64e24275ad95a915a313
> Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
> Signed-off-by: Wang Liang <wangliang74@...wei.com>
> ---
>  drivers/net/tun.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index cc6c50180663..47ddcb4b9a78 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1875,6 +1875,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  				local_bh_enable();
>  				goto unlock_frags;
>  			}
> +
> +			if (frags && skb != tfile->napi.skb)
> +				tfile->napi.skb = skb;

This is observed with tun because syzkaller can fuzz napi with that.
That unfortunately added fuzz test coverage to a combination that is
not intended for real use: XDP generic before napi frags.

Tun is the only driver that calls do_xdp_generic on a napi.skb and
later passes this napi to napi_gro_frags.

But this is no longer a napi frags skb on which napi_gro_frags
(and napi_frags_skb and gro_pull_from_frag0) should be called? As the
skb now has a linear part. Not sure that the frag0 is still correct.

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