On 23/04/2022 17:36, Xuan Zhuo wrote:
> On Sat, 23 Apr 2022 17:30:11 +0300, Nikolay Aleksandrov <razor@xxxxxxxxxxxxx> wrote:
>> On 23/04/2022 17:16, Nikolay Aleksandrov wrote:
>>> On 23/04/2022 16:31, Xuan Zhuo wrote:
>>>> On Sat, 23 Apr 2022 14:26:12 +0300, Nikolay Aleksandrov <razor@xxxxxxxxxxxxx> wrote:
>>>>> 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@xxxxxxxxxxxxxxx
>>>>> CC: Jason Wang <jasowang@xxxxxxxxxx>
>>>>> CC: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
>>>>> CC: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
>>>>> CC: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
>>>>> CC: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
>>>>> Fixes: 8fb7da9e9907 ("virtio_net: get build_skb() buf by data ptr")
>>>>> Signed-off-by: Nikolay Aleksandrov <razor@xxxxxxxxxxxxx>
>>>>> ---
>>>>> 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);
>>>>
>>>> The reason for not doing this is that buf and p may not be on the same page, and
>>>> buf is probably not page-aligned.
>>>>
>>>> The implementation of virtio-net merge is add_recvbuf_mergeable(), which
>>>> allocates a large block of memory at one time, and allocates from it each time.
>>>> Although in xdp mode, each allocation is page_size, it does not guarantee that
>>>> each allocation is page-aligned .
>>>>
>>>> The problem here is that the value of headroom is wrong, the package is
>>>> structured like this:
>>>>
>>>> from device | headroom | virtio-net hdr | data |
>>>> after xdp | headroom | virtio-net hdr | meta | data |
>>>
>>> You're free to push data back (not necessarily through meta).
>>> You don't have virtio-net hdr for the xdp case (hdr_valid is false there).
>>>
>>>>
>>>> The page_address(page) + offset we pass to page_to_skb() points to the
>>>> virtio-net hdr.
>>>>
>>>> So I think it might be better to change it this way.
>>>>
>>>> Thanks.
>>>>
>>>>
>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>> index 87838cbe38cf..086ae835ec86 100644
>>>> --- a/drivers/net/virtio_net.c
>>>> +++ b/drivers/net/virtio_net.c
>>>> @@ -1012,7 +1012,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>>>> head_skb = page_to_skb(vi, rq, xdp_page, offset,
>>>> len, PAGE_SIZE, false,
>>>> metasize,
>>>> - VIRTIO_XDP_HEADROOM);
>>>> + VIRTIO_XDP_HEADROOM - metazie);
>>>> return head_skb;
>>>> }
>>>> break;
>>>
>>> That patch doesn't fix it, as I said with xdp you can move both data and data_meta.
>>> So just doing that would take care of the meta, but won't take care of moving data.
>>>
>>
>> Also it doesn't take care of the case where page_to_skb() is called with the original page
>> i.e. when we already have headroom, so we hit the next/standard page_to_skb() call (xdp_page == page).
>
> Yes, you are right.
>
>>
>> The above change guarantees that buf and p will be in the same page
>
>
> How can this be guaranteed?
>
> 1. For example, we applied for a 32k buffer first, and took away 1500 + hdr_len
> from the allocation.
> 2. set xdp
> 3. alloc for new buffer
>
p = page_address(page) + offset;
buf = p & PAGE_MASK; // whatever page p lands in is where buf is set
=> p and buf are always in the same page, no?
> The buffer allocated in the third step must be unaligned, and it is entirely
> possible that p and buf are not on the same page.
>
> I fixed my previous patch.
>
> Thanks.
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 87838cbe38cf..d95e82255b94 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1005,6 +1005,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> * xdp.data_meta were adjusted
> */
> len = xdp.data_end - xdp.data + vi->hdr_len + metasize;
> +
> + headroom = xdp.data - vi->hdr_len - metasize - (buf - headroom);
This is wrong, xdp.data isn't related to buf in the xdp_linearize_page() case.
> /* We can only create skb based on xdp_page. */
> if (unlikely(xdp_page != page)) {
> rcu_read_unlock();
> @@ -1012,7 +1014,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> head_skb = page_to_skb(vi, rq, xdp_page, offset,
> len, PAGE_SIZE, false,
> metasize,
> - VIRTIO_XDP_HEADROOM);
> + headroom);
> return head_skb;
> }
> break;
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