Re: [PATCH v3] drivers: net: prevent tun_get_user() to exceed xdp size limits

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

Cc. John and Ahern

On 26/07/2023 04.09, Jason Wang wrote:
> On Tue, Jul 25, 2023 at 11:54 PM Andrew Kanner <andrew.kanner@gmail.com> wrote:
>>
>> Syzkaller reported the following issue:
>> =======================================
>> Too BIG xdp->frame_sz = 131072

Is this a contiguous physical memory allocation?

131072 bytes equal order 5 page.

Looking at tun.c code I cannot find a code path that could create
order-5 skb->data, but only SKB with order-0 fragments.  But I guess it
is the netif_receive_generic_xdp() what will realloc to make this linear
(via skb_linearize())

>> WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
>>    ____bpf_xdp_adjust_tail net/core/filter.c:4121 [inline]
>> WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
>>    bpf_xdp_adjust_tail+0x466/0xa10 net/core/filter.c:4103
>> ...
>> Call Trace:
>>   <TASK>
>>   bpf_prog_4add87e5301a4105+0x1a/0x1c
>>   __bpf_prog_run include/linux/filter.h:600 [inline]
>>   bpf_prog_run_xdp include/linux/filter.h:775 [inline]
>>   bpf_prog_run_generic_xdp+0x57e/0x11e0 net/core/dev.c:4721
>>   netif_receive_generic_xdp net/core/dev.c:4807 [inline]
>>   do_xdp_generic+0x35c/0x770 net/core/dev.c:4866
>>   tun_get_user+0x2340/0x3ca0 drivers/net/tun.c:1919
>>   tun_chr_write_iter+0xe8/0x210 drivers/net/tun.c:2043
>>   call_write_iter include/linux/fs.h:1871 [inline]
>>   new_sync_write fs/read_write.c:491 [inline]
>>   vfs_write+0x650/0xe40 fs/read_write.c:584
>>   ksys_write+0x12f/0x250 fs/read_write.c:637
>>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>   do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
>>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>
>> xdp->frame_sz > PAGE_SIZE check was introduced in commit c8741e2bfe87
>> ("xdp: Allow bpf_xdp_adjust_tail() to grow packet size"). But
>> tun_get_user() still provides an execution path with do_xdp_generic()
>> and exceed XDP limits for packet size.

I added this check and maybe it is too strict. XDP can work on higher
order pages, as long as this is contiguous physical memory (e.g. a 
page).  And

An order 5 page (131072 bytes) seems excessive, but maybe TUN have a 
use-case for having such large packets? (Question to Ahern?)

I'm considering we should change the size-limit to order-2 (16384) or 
order-3 (32768).

Order-3 because netstack have:
   #define SKB_FRAG_PAGE_ORDER get_order(32768)

And order-2 because netstack have: SKB_MAX_ALLOC (16KiB)
  - See discussion in commit 6306c1189e77 ("bpf: Remove MTU check in 
__bpf_skb_max_len").
  - https://git.kernel.org/torvalds/c/6306c1189e77


>>
>> Using the syzkaller repro with reduced packet size it was also
>> discovered that XDP_PACKET_HEADROOM is not checked in
>> tun_can_build_skb(), although pad may be incremented in
>> tun_build_skb().
>>
>> If we move the limit check from tun_can_build_skb() to tun_build_skb()
>> we will make xdp to be used only in tun_build_skb(), without falling
>> in tun_alloc_skb(), etc. And moreover we will drop the packet which
>> can't be processed in tun_build_skb().

Looking at tun_build_skb() is uses the page_frag system, and can thus 
create up-to SKB_FRAG_PAGE_ORDER (size 32768 / order-3).

>>
>> Reported-and-tested-by: syzbot+f817490f5bd20541b90a@syzkaller.appspotmail.com
>> Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@google.com/T/
>> Link: https://syzkaller.appspot.com/bug?id=5335c7c62bfff89bbb1c8f14cdabebe91909060f
>> Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
>> Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
>> ---
>>
>> Notes:
>>      V2 -> V3:
>>      * attach the forgotten changelog
>>      V1 -> V2:
>>      * merged 2 patches in 1, fixing both issues: WARN_ON_ONCE with
>>        syzkaller repro and missing XDP_PACKET_HEADROOM in pad
>>      * changed the title and description of the execution path, suggested
>>        by Jason Wang <jasowang@redhat.com>
>>      * move the limit check from tun_can_build_skb() to tun_build_skb() to
>>        remove duplication and locking issue, and also drop the packet in
>>        case of a failed check - noted by Jason Wang <jasowang@redhat.com>
> 
> Acked-by: Jason Wang <jasowang@redhat.com>
> 
> Thanks
> 
>>
>>   drivers/net/tun.c | 7 +++----
>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index d75456adc62a..7c2b05ce0421 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1594,10 +1594,6 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
>>          if (zerocopy)
>>                  return false;
>>
>> -       if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
>> -           SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
>> -               return false;
>> -
>>          return true;
>>   }
>>
>> @@ -1673,6 +1669,9 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>          buflen += SKB_DATA_ALIGN(len + pad);
>>          rcu_read_unlock();
>>
>> +       if (buflen > PAGE_SIZE)
>> +               return ERR_PTR(-EFAULT);

Concretely I'm saying maybe use SKB_FRAG_PAGE_ORDER "size" here?

e.g. create SKB_FRAG_PAGE_SIZE define as below.
  if (buflen > SKB_FRAG_PAGE_SIZE)

diff --git a/include/net/sock.h b/include/net/sock.h
index 656ea89f60ff..4c4b3c257b52 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2886,7 +2886,8 @@ extern int sysctl_optmem_max;
  extern __u32 sysctl_wmem_default;
  extern __u32 sysctl_rmem_default;

-#define SKB_FRAG_PAGE_ORDER    get_order(32768)
+#define SKB_FRAG_PAGE_SIZE     32768
+#define SKB_FRAG_PAGE_ORDER    get_order(SKB_FRAG_PAGE_SIZE)
  DECLARE_STATIC_KEY_FALSE(net_high_order_alloc_disable_key);

>> +
>>          alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);
>>          if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL)))
>>                  return ERR_PTR(-ENOMEM);

--Jesper