CVE-2026-99155 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
scsi: xen/scsifront: Check for a NULL shadow entry on a backend response
scsifront_do_response() validates the rqid in a backend response against VSCSIIF_MAX_REQS before using it to index info->shadow[], but then
dereferences the entry to test ->inflight without checking that the slot is populated. Shadow slots are NULL before a command is submitted (the host private area is zeroed at allocation) and are reset to NULL in _scsifront_put_rqid() once a request completes, so an in-range rqid that does not correspond to an outstanding request makes the frontend dereference a NULL pointer.
A malicious or buggy backend can thus crash the guest by returning a response whose rqid is in range but not in flight -- for example a spurious response before any command has been issued, or a duplicate of one already completed. The ring has only VSCSIIF_MAX_REQS (16) slots, so before the first command every in-range rqid selects a NULL slot.
Reject a response whose shadow slot is not populated.
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Analysis
by VulDB Data Team • 10/28/2026
The vulnerability identified within the Linux kernel's SCSI Xen frontend driver represents a critical null pointer dereference flaw that can be exploited to cause a denial of service against virtualized guest systems. This issue resides in the scsifront_do_response function, which is responsible for processing incoming responses from the backend storage device. The core technical failure stems from an incomplete validation sequence during request ID handling. While the code correctly validates that the received request identifier (rqid) falls within the acceptable range defined by VSCSIIF_MAX_REQS before using it as an index into the info->shadow array, it fails to verify whether the specific slot at that index actually contains a valid pointer. The shadow entries are initialized to NULL when allocated and are explicitly reset to NULL in _scsifront_put_rqid once a request completes or is discarded. Consequently, any rqid that is numerically within bounds but does not correspond to an currently outstanding command will result in the kernel attempting to dereference a null pointer when checking the ->inflight flag.
From an operational perspective, this flaw allows a malicious or buggy backend device to crash the guest operating system by sending spurious responses. For instance, if the backend sends a response for a request ID that has not yet been used, or one that corresponds to a previously completed command, the frontend driver will attempt to access memory at address zero. This triggers an immediate kernel panic, effectively terminating the virtual machine and disrupting any services running within it. Given that the ring buffer contains only sixteen slots defined by VSCSIIF_MAX_REQS, the window of opportunity for such attacks is significant during periods where no commands are actively being issued or immediately after command completion before new requests are queued. This scenario highlights a classic race condition-like vulnerability exacerbated by insufficient state verification in asynchronous communication protocols between virtualization components.
This vulnerability aligns with CWE-476, which describes the NULL Pointer Dereference weakness class, indicating that the software performs an operation using a pointer that is expected to be valid but is actually null. In terms of attack classification under the MITRE ATT&CK framework for cloud infrastructure and virtualization environments, this flaw facilitates Denial of Service (DoS) attacks against hypervisor resources or guest instances through improper input validation of inter-vm communication channels. The exploitation does not require privilege escalation within the guest but rather relies on the backend's ability to inject malformed packets into the shared ring buffer, a capability that should be strictly controlled by the virtualization layer security policies.
To mitigate this risk, developers must implement strict null checks before dereferencing any pointer retrieved from dynamic arrays or shadow buffers in kernel space. The fix involves verifying that info->shadow[rqid] is not NULL prior to accessing its members such as ->inflight. If the slot is empty, indicating no active request corresponds to the provided rqid, the response should be rejected immediately with an appropriate error code rather than proceeding with memory access. This defensive programming practice ensures that only valid, in-flight requests are processed, thereby preventing unauthorized or erroneous state transitions. System administrators managing Xen-based virtualized environments should ensure their kernels are updated to include this patch and monitor backend device behavior for anomalies that might indicate exploitation attempts of similar inter-vm communication flaws.