CVE-2026-98900 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
xen/gntalloc: validate grant count before allocation
gntalloc_ioctl_alloc() allocates the grant-id array before checking whether the requested count fits within the global grant limit. Counts above that limit cannot succeed, so reject them before the user-controlled allocation reaches kcalloc().
Use a subtraction-based check while holding gref_mutex so adding the requested count cannot wrap. Also cast the count before advancing the per-file index so the page-size multiplication is performed in 64-bit arithmetic.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel vulnerability identified within the xen/gntalloc subsystem represents a critical flaw in resource management and input validation logic, specifically affecting how grant IDs are allocated for Xen virtualization environments. The core issue resides in the gntalloc_ioctl_alloc function, which is responsible for handling allocation requests from user space applications interacting with the guest-to-host memory sharing mechanism. In its vulnerable state, the implementation performed the actual memory allocation of a grant-id array using kcalloc before verifying whether the requested number of grants exceeded the global system limit defined by xen_grant_table_size. This inversion of validation and execution order creates a window where untrusted user input directly influences kernel heap operations without adequate boundary checks, leading to potential denial of service conditions or resource exhaustion attacks that can destabilize the host system.
From a technical perspective, this flaw is classified under CWE-787: Out-of-bounds Write due to improper validation of array indices and sizes, as well as CWE-20: Improper Input Validation because the kernel failed to sanitize the count parameter against established policy limits before proceeding with allocation. The vulnerability allows an attacker who has access to the gntalloc device node to trigger a kcalloc call with a size derived from user-controlled input. If the requested grant count is excessively large, it may result in a massive memory allocation that consumes significant kernel heap space or triggers an out-of-memory condition within the kernel context. Furthermore, because this operation occurs while holding the gref_mutex, a successful exploitation could lead to thread contention and system-wide latency spikes, effectively creating a denial of service scenario for other Xen guests running on the same physical host.
The operational impact of this vulnerability extends beyond simple resource exhaustion. By forcing the allocation of large arrays in kernel space, an attacker can degrade the performance of critical hypervisor functions that rely on grant table operations. In severe cases, if the memory pressure becomes extreme enough to trigger out-of-memory killers or cause stack overflows during subsequent processing, it could lead to a complete system crash (kernel panic). This undermines the reliability and availability guarantees provided by virtualization platforms like Xen, where stability is paramount for multi-tenant environments. The lack of proper arithmetic checks also introduces risks related to integer overflow if not handled correctly in future iterations or similar code paths, although the primary immediate risk here is the unbounded allocation based on unchecked input magnitude.
Mitigation strategies focus primarily on applying the upstream kernel patch that reorders these operations and strengthens type safety. The fix involves moving the validation of the grant count before any memory allocation takes place, ensuring that requests exceeding xen_grant_table_size are rejected immediately with an appropriate error code such as EINVAL or E2BIG. Additionally, the implementation now employs a subtraction-based check while holding gref_mutex to prevent integer wraparound issues when calculating total resource usage. It is also crucial to ensure that arithmetic operations involving page sizes and counts use 64-bit casting to avoid truncation errors on systems where pointer sizes might differ from standard integer widths. Administrators should apply the latest security updates for their Linux distribution, which include this corrected gntalloc logic, thereby closing the gap between user input validation and kernel resource allocation in Xen grant table management routines aligned with ATT&CK techniques related to resource hijacking and denial of service via local exploitation.