CVE-2023-54094 in Linux
Résumé
par VulDB • 03/07/2026
Based on the kernel crash log and description provided, here is an analysis of the bug, its root cause, and the fix.
### 1. Summary of the Issue The Linux kernel experienced a **general protection fault** (likely a NULL pointer dereference or invalid memory access at address `0xc0`) in the network stack when handling GRO (Generic Receive Offload) packets traversing a bridge to both local input and an egress TUN device.
The root cause is that **GRO-aggregated skbs are being segmented without proper cloning**, leading to unsafe writes on shared skb heads, which corrupts memory or causes invalid pointer dereferences during segmentation in the `inet_gso_segment` path.
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### 2. Detailed Analysis of the Call Trace
#### Key Functions Involved: 1. **`ixgbe_poll` → `napi_complete_done`**: The NIC driver (Intel IXGBE) receives packets and passes them up via NAPI polling with GRO enabled (`rx-gro-list`). 2. **`__netif_receive_skb_list_core` / `br_handle_frame`**: Packets are processed by the bridge subsystem. Since they are part of a GRO list, multiple frames may be aggregated into one large skb. 3. **`br_flood` → `maybe_deliver` → `br_dev_queue_push_xmit`**: The bridged packet is sent to: - Local input path (for local delivery). - Egress device (`tun`) via `br_dev_queue_push_xmit`. 4. **`__dev_queue_xmit` → `validate_xmit_skb_list` → `sch_direct_xmit`**: Before transmission, the packet goes through QoS validation and segmentation checks. 5. **`skb_mac_gso_segment` → `inet_gso_segment`**: The kernel attempts to segment the large GRO skb into MTU-sized chunks for transmission over the TUN interface (which may require smaller packets).
#### The Bug: Unsafe Segmentation of Shared Skb Heads - When a packet is cloned or shared between multiple paths (e.g., local delivery + bridge forwarding), its `skb->head` and data structures are **shared**. - During GSO segmentation (`inet_gso_segment`), the kernel may modify fields in the skb header or fragment descriptors. - If these modifications occur on a **non-cloned, shared skb**, it corrupts memory used by other references to that same skb (e.g., the local input path still expecting an unmodified version). - This leads to invalid pointer dereferences (`CR2: 0x00000000000000c0` suggests accessing a very low address, possibly due to corrupted pointers or offsets).
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### 3. Root Cause in Code Logic
In `net/ipv4/af_inet.c`, the function `inet_gso_segment()` is called during segmentation. If the incoming skb has been shared (e.g., via cloning for bridge forwarding) and not properly duplicated before modification, any write to its header or fragment list will affect all owners of that skb.
The issue arises because: - GRO aggregation creates large skbs. - Bridging may clone these skbs for multiple egress ports/local delivery. - When one path (e.g., TUN) requires segmentation, it modifies the shared structure without ensuring exclusive ownership via `skb_clone()` or similar mechanisms.
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### 4. Fix Explanation
The fix involves **ensuring that any skb to be segmented is uniquely owned** before modification. This is typically done by:
1. **Cloning the skb if necessary**: Before calling segmentation functions, check if the skb has multiple references (`skb->users > 1`) or shares data with other skbs. 2. **Uncloning as needed**: If shared, create a private copy using `skb_clone()` so that modifications during GSO segmentation do not affect other users of the original packet.
This ensures thread-safe and memory-safe handling of GRO packets in multi-path scenarios like bridging + TUN forwarding.
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### 5. Recommended Patch Strategy (Conceptual)
In `net/ipv4/af_inet.c` or related GSO segmentation code:
```c static struct sk_buff *inet_gso_segment(struct sk_buff *skb, ...) {
// Ensure exclusive ownership before modifying the skb if (!skb_cloned(skb)) {
goto do_segment; }
/* Clone the skb to avoid corrupting shared data */ struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC); if (unlikely(!nskb)) {
kfree_skb(skb); return NULL; }
consume_skb(skb); // Free original since we now own nskb skb = nskb;
do_segment: //
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