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[34.145.139.141]) by smtp.gmail.com with UTF8SMTPSA id 6a1803df08f44-79356b3e5cesm13504216d6.73.2025.09.18.07.30.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 18 Sep 2025 07:30:19 -0700 (PDT) Date: Thu, 18 Sep 2025 10:30:19 -0400 From: Willem de Bruijn To: Wang Liang , willemdebruijn.kernel@gmail.com, jasowang@redhat.com, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, sdf@fomichev.me, lorenzo@kernel.org, toke@redhat.com Cc: yuehaibing@huawei.com, zhangchangzhong@huawei.com, wangliang74@huawei.com, linux-kernel@vger.kernel.org, netdev@vger.kernel.org Message-ID: In-Reply-To: <20250917113919.3991267-1-wangliang74@huawei.com> References: <20250917113919.3991267-1-wangliang74@huawei.com> Subject: Re: [PATCH net] net: tun: Update napi->skb after XDP process Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit 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: > > 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 > > > 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@syzkaller.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 > --- > 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.