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

[Andrew Kanner <andrew.kanner@gmail.com>]

Syzkaller reported the following issue:
=======================================
Too BIG xdp->frame_sz = 131072
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.

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().

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>

 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);
+
 	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);
-- 
2.39.3