[PATCH v2 1/3] net: wwan: mhi_wwan_mbim: guard against a cyclic NDP chain

[Guanglei Zhu <zhugl3@xiaopeng.com>]

The NDP traversal in mhi_mbim_rx() only stops when wNextNdpIndex is
zero.  Nothing requires the offsets to advance, so a modem that
points an NDP at itself, or at an earlier NDP, keeps the loop
spinning forever on one CPU.

Break out when the next NDP offset is not larger than the current
one.

Fixes: aa730a9905b7 ("net: wwan: Add MHI MBIM network driver")
Cc: stable@vger.kernel.org
Suggested-by: Loic Poulain <loic.poulain@oss.qualcomm.com>
Signed-off-by: Guanglei Zhu <zhugl3@xiaopeng.com>

Verified in a QEMU guest with a fault injector feeding the driver's
receive callback an NTB whose single NDP points at itself: the
unpatched driver spins in mhi_mbim_rx() with one CPU pinned at 100%
and the thread never returns.  With this check the loop terminates
within one iteration.

Changes in v2: move the non-increasing check to the wNextNdpIndex
retrieval site, as suggested by Loic Poulain, instead of tracking
the previous offset in a separate variable.
---
 drivers/net/wwan/mhi_wwan_mbim.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/net/wwan/mhi_wwan_mbim.c b/drivers/net/wwan/mhi_wwan_mbim.c
index a949987125..5679948546 100644
--- a/drivers/net/wwan/mhi_wwan_mbim.c
+++ b/drivers/net/wwan/mhi_wwan_mbim.c
@@ -349,9 +349,13 @@ static void mhi_mbim_rx(struct mhi_mbim_context *mbim, struct sk_buff *skb)
 unlock:
 		rcu_read_unlock();
 next_ndp:
-		/* Other NDP to process? */
-		ndpoffset = (int)le16_to_cpu(ndp16.wNextNdpIndex);
-		if (!ndpoffset)
+		/* Other NDP to process?  The offsets must advance, or a
+		 * self-referencing NDP keeps the loop spinning forever.
+		 */
+		n = (int)le16_to_cpu(ndp16.wNextNdpIndex);
+		if (n > ndpoffset)
+			ndpoffset = n;
+		else
 			break;
 	}
 
-- 
2.43.0