[PATCH 1/2] drivers: net: use xdp only inside tun_build_skb()

[Andrew Kanner]

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

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 to exceed XDP limits
for packet size. The way to mitigate this is to reduce xdp to be used
only in tun_build_skb(), without falling in tun_alloc_skb(), etc.

Reported-and-tested-by: syzbot+f817490f5bd20541b90a@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@xxxxxxxxxx/T/
Link: https://syzkaller.appspot.com/bug?id=5335c7c62bfff89bbb1c8f14cdabebe91909060f
Signed-off-by: Andrew Kanner <andrew.kanner@xxxxxxxxx>
---
 drivers/net/tun.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d75456adc62a..18ccbbe9830a 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1580,8 +1580,14 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
 }
 
 static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
-			      int len, int noblock, bool zerocopy)
+			      int len, int noblock, bool zerocopy, int *skb_xdp)
 {
+	if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
+	    SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE) {
+		*skb_xdp = 0;
+		return false;
+	}
+
 	if ((tun->flags & TUN_TYPE_MASK) != IFF_TAP)
 		return false;
 
@@ -1594,10 +1600,6 @@ 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;
 }
 
@@ -1809,7 +1811,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 			zerocopy = true;
 	}
 
-	if (!frags && tun_can_build_skb(tun, tfile, len, noblock, zerocopy)) {
+	if (tun_can_build_skb(tun, tfile, len, noblock, zerocopy, &skb_xdp) && !frags) {
 		/* For the packet that is not easy to be processed
 		 * (e.g gso or jumbo packet), we will do it at after
 		 * skb was created with generic XDP routine.
-- 
2.39.3