Re: [PATCH 2/3] net: wwan: mhi_wwan_mbim: check skb_copy_bits() return value

mhi_mbim_rx() ignores the return value of skb_copy_bits() when it
copies each datagram out of the NTB.  The datagram offset and length
come from the DPE, which is only checked to lie within the NTB
itself, so a modem can point a datagram outside the received skb.
The copy then fails and the freshly allocated skbn is passed to
netif_rx() with its uninitialized contents still in place, leaking
kernel heap memory into the network stack.

Free the skb and account an error when the copy fails.

Fixes: aa730a9905b7 ("net: wwan: Add MHI MBIM network driver")
Cc: <redacted>
Signed-off-by: Guanglei Zhu <redacted>
---

Verified in a QEMU guest with a fault injector pointing a DPE
outside the received NTB: the copy fails, and the unpatched driver
hands the uninitialized skbn to the network stack (observed as
"unknown protocol" on bytes that were never written).  With this
check the failed datagram is dropped and counted as an rx error.
 drivers/net/wwan/mhi_wwan_mbim.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wwan/mhi_wwan_mbim.c b/drivers/net/wwan/mhi_wwan_mbim.c
index ef158edeb..71e700008 100644
--- a/drivers/net/wwan/mhi_wwan_mbim.c
+++ b/drivers/net/wwan/mhi_wwan_mbim.c

@@ -328,7 +328,13 @@ static void mhi_mbim_rx(struct mhi_mbim_context *mbim, struct sk_buff *skb)
                                continue;

                        skb_put(skbn, dgram_len);
-                       skb_copy_bits(skb, dgram_offset, skbn->data, dgram_len);
+                       if (skb_copy_bits(skb, dgram_offset, skbn->data, dgram_len)) {
+                               dev_kfree_skb_any(skbn);
+                               u64_stats_update_begin(&link->rx_syncp);
+                               u64_stats_inc(&link->rx_errors);
+                               u64_stats_update_end(&link->rx_syncp);
+                               continue;