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Date:   Sat, 23 Apr 2022 14:26:12 +0300
From:   Nikolay Aleksandrov <razor@...ckwall.org>
To:     netdev@...r.kernel.org
Cc:     kuba@...nel.org, davem@...emloft.net,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        stable@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH net] virtio_net: fix wrong buf address calculation when using xdp

We received a report[1] of kernel crashes when Cilium is used in XDP
mode with virtio_net after updating to newer kernels. After
investigating the reason it turned out that when using mergeable bufs
with an XDP program which adjusts xdp.data or xdp.data_meta page_to_buf()
calculates the build_skb address wrong because the offset can become less
than the headroom so it gets the address of the previous page (-X bytes
depending on how lower offset is):
 page_to_skb: page addr ffff9eb2923e2000 buf ffff9eb2923e1ffc offset 252 headroom 256

This is a pr_err() I added in the beginning of page_to_skb which clearly
shows offset that is less than headroom by adding 4 bytes of metadata
via an xdp prog. The calculations done are:
 receive_mergeable():
 headroom = VIRTIO_XDP_HEADROOM; // VIRTIO_XDP_HEADROOM == 256 bytes
 offset = xdp.data - page_address(xdp_page) -
          vi->hdr_len - metasize;

 page_to_skb():
 p = page_address(page) + offset;
 ...
 buf = p - headroom;

Now buf goes -4 bytes from the page's starting address as can be seen
above which is set as skb->head and skb->data by build_skb later. Depending
on what's done with the skb (when it's freed most often) we get all kinds
of corruptions and BUG_ON() triggers in mm[2]. The story of the faulty
commit is interesting because the patch was sent and applied twice (it
seems the first one got lost during merge back in 5.13 window). The
first version of the patch that was applied as:
 commit 7bf64460e3b2 ("virtio-net: get build_skb() buf by data ptr")
was actually correct because it calculated the page starting address
without relying on offset or headroom, but then the second version that
was applied as:
 commit 8fb7da9e9907 ("virtio_net: get build_skb() buf by data ptr")
was wrong and added the above calculation.
An example xdp prog[3] is below.

[1] https://github.com/cilium/cilium/issues/19453

[2] Two of the many traces:
 [   40.437400] BUG: Bad page state in process swapper/0  pfn:14940
 [   40.916726] BUG: Bad page state in process systemd-resolve  pfn:053b7
 [   41.300891] kernel BUG at include/linux/mm.h:720!
 [   41.301801] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
 [   41.302784] CPU: 1 PID: 1181 Comm: kubelet Kdump: loaded Tainted: G    B   W         5.18.0-rc1+ #37
 [   41.304458] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1.fc35 04/01/2014
 [   41.306018] RIP: 0010:page_frag_free+0x79/0xe0
 [   41.306836] Code: 00 00 75 ea 48 8b 07 a9 00 00 01 00 74 e0 48 8b 47 48 48 8d 50 ff a8 01 48 0f 45 fa eb d0 48 c7 c6 18 b8 30 a6 e8 d7 f8 fc ff <0f> 0b 48 8d 78 ff eb bc 48 8b 07 a9 00 00 01 00 74 3a 66 90 0f b6
 [   41.310235] RSP: 0018:ffffac05c2a6bc78 EFLAGS: 00010292
 [   41.311201] RAX: 000000000000003e RBX: 0000000000000000 RCX: 0000000000000000
 [   41.312502] RDX: 0000000000000001 RSI: ffffffffa6423004 RDI: 00000000ffffffff
 [   41.313794] RBP: ffff993c98823600 R08: 0000000000000000 R09: 00000000ffffdfff
 [   41.315089] R10: ffffac05c2a6ba68 R11: ffffffffa698ca28 R12: ffff993c98823600
 [   41.316398] R13: ffff993c86311ebc R14: 0000000000000000 R15: 000000000000005c
 [   41.317700] FS:  00007fe13fc56740(0000) GS:ffff993cdd900000(0000) knlGS:0000000000000000
 [   41.319150] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 [   41.320152] CR2: 000000c00008a000 CR3: 0000000014908000 CR4: 0000000000350ee0
 [   41.321387] Call Trace:
 [   41.321819]  <TASK>
 [   41.322193]  skb_release_data+0x13f/0x1c0
 [   41.322902]  __kfree_skb+0x20/0x30
 [   41.343870]  tcp_recvmsg_locked+0x671/0x880
 [   41.363764]  tcp_recvmsg+0x5e/0x1c0
 [   41.384102]  inet_recvmsg+0x42/0x100
 [   41.406783]  ? sock_recvmsg+0x1d/0x70
 [   41.428201]  sock_read_iter+0x84/0xd0
 [   41.445592]  ? 0xffffffffa3000000
 [   41.462442]  new_sync_read+0x148/0x160
 [   41.479314]  ? 0xffffffffa3000000
 [   41.496937]  vfs_read+0x138/0x190
 [   41.517198]  ksys_read+0x87/0xc0
 [   41.535336]  do_syscall_64+0x3b/0x90
 [   41.551637]  entry_SYSCALL_64_after_hwframe+0x44/0xae
 [   41.568050] RIP: 0033:0x48765b
 [   41.583955] Code: e8 4a 35 fe ff eb 88 cc cc cc cc cc cc cc cc e8 fb 7a fe ff 48 8b 7c 24 10 48 8b 74 24 18 48 8b 54 24 20 48 8b 44 24 08 0f 05 <48> 3d 01 f0 ff ff 76 20 48 c7 44 24 28 ff ff ff ff 48 c7 44 24 30
 [   41.632818] RSP: 002b:000000c000a2f5b8 EFLAGS: 00000212 ORIG_RAX: 0000000000000000
 [   41.664588] RAX: ffffffffffffffda RBX: 000000c000062000 RCX: 000000000048765b
 [   41.681205] RDX: 0000000000005e54 RSI: 000000c000e66000 RDI: 0000000000000016
 [   41.697164] RBP: 000000c000a2f608 R08: 0000000000000001 R09: 00000000000001b4
 [   41.713034] R10: 00000000000000b6 R11: 0000000000000212 R12: 00000000000000e9
 [   41.728755] R13: 0000000000000001 R14: 000000c000a92000 R15: ffffffffffffffff
 [   41.744254]  </TASK>
 [   41.758585] Modules linked in: br_netfilter bridge veth netconsole virtio_net

 and

 [   33.524802] BUG: Bad page state in process systemd-network  pfn:11e60
 [   33.528617] page ffffe05dc0147b00 ffffe05dc04e7a00 ffff8ae9851ec000 (1) len 82 offset 252 metasize 4 hroom 0 hdr_len 12 data ffff8ae9851ec10c data_meta ffff8ae9851ec108 data_end ffff8ae9851ec14e
 [   33.529764] page:000000003792b5ba refcount:0 mapcount:-512 mapping:0000000000000000 index:0x0 pfn:0x11e60
 [   33.532463] flags: 0xfffffc0000000(node=0|zone=1|lastcpupid=0x1fffff)
 [   33.532468] raw: 000fffffc0000000 0000000000000000 dead000000000122 0000000000000000
 [   33.532470] raw: 0000000000000000 0000000000000000 00000000fffffdff 0000000000000000
 [   33.532471] page dumped because: nonzero mapcount
 [   33.532472] Modules linked in: br_netfilter bridge veth netconsole virtio_net
 [   33.532479] CPU: 0 PID: 791 Comm: systemd-network Kdump: loaded Not tainted 5.18.0-rc1+ #37
 [   33.532482] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1.fc35 04/01/2014
 [   33.532484] Call Trace:
 [   33.532496]  <TASK>
 [   33.532500]  dump_stack_lvl+0x45/0x5a
 [   33.532506]  bad_page.cold+0x63/0x94
 [   33.532510]  free_pcp_prepare+0x290/0x420
 [   33.532515]  free_unref_page+0x1b/0x100
 [   33.532518]  skb_release_data+0x13f/0x1c0
 [   33.532524]  kfree_skb_reason+0x3e/0xc0
 [   33.532527]  ip6_mc_input+0x23c/0x2b0
 [   33.532531]  ip6_sublist_rcv_finish+0x83/0x90
 [   33.532534]  ip6_sublist_rcv+0x22b/0x2b0

[3] XDP program to reproduce(xdp_pass.c):
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>

 SEC("xdp_pass")
 int xdp_pkt_pass(struct xdp_md *ctx)
 {
          bpf_xdp_adjust_head(ctx, -(int)32);
          return XDP_PASS;
 }

 char _license[] SEC("license") = "GPL";

 compile: clang -O2 -g -Wall -target bpf -c xdp_pass.c -o xdp_pass.o
 load on virtio_net: ip link set enp1s0 xdpdrv obj xdp_pass.o sec xdp_pass

CC: stable@...r.kernel.org
CC: Jason Wang <jasowang@...hat.com>
CC: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
CC: Daniel Borkmann <daniel@...earbox.net>
CC: "Michael S. Tsirkin" <mst@...hat.com>
CC: virtualization@...ts.linux-foundation.org
Fixes: 8fb7da9e9907 ("virtio_net: get build_skb() buf by data ptr")
Signed-off-by: Nikolay Aleksandrov <razor@...ckwall.org>
---
 drivers/net/virtio_net.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 87838cbe38cf..0687dd88e97f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -434,9 +434,13 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 	 * Buffers with headroom use PAGE_SIZE as alloc size, see
 	 * add_recvbuf_mergeable() + get_mergeable_buf_len()
 	 */
-	truesize = headroom ? PAGE_SIZE : truesize;
+	if (headroom) {
+		truesize = PAGE_SIZE;
+		buf = (char *)((unsigned long)p & PAGE_MASK);
+	} else {
+		buf = p;
+	}
 	tailroom = truesize - headroom;
-	buf = p - headroom;
 
 	len -= hdr_len;
 	offset += hdr_padded_len;
-- 
2.35.1

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