CVE-2026-98324 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
dmaengine: pxa: fix double counting of the hw descriptors
pxad_alloc_desc() was converted from
kzalloc(struct_size(sw_desc, hw_desc, nb_hw_desc), GFP_NOWAIT)
to kzalloc_flex(), which sets the __counted_by() counter sw_desc->nb_desc itself - but only where the compiler has __builtin_counted_by_ref(), so from gcc 15.1 or clang 22.1 on. The loop below it still increments nb_desc, which makes it come out doubled there and correct elsewhere.
nb_desc is what pxad_free_desc() iterates over and what set_updater_desc() indexes from, so set it explicitly and drop the increment. The error path has to lower it to the number of descriptors allocated so far, otherwise pxad_free_desc() would free entries that were never allocated.
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Analysis
by VulDB Data Team • 10/06/2026
The vulnerability identified in the Linux kernel's DMA engine driver for PXA architectures stems from a subtle implementation error during a refactoring effort aimed at improving memory safety and flexibility. The core issue resides within the pxad_alloc_desc function, which is responsible for allocating software descriptors that wrap hardware descriptor arrays. This allocation process was recently updated to utilize kzalloc_flex(), a helper designed to manage flexible array members more robustly by automatically setting the __counted_by() counter associated with the structure member sw_desc->nb_desc. However, this automatic counting mechanism relies on compiler support for the __builtin_counted_by_ref feature, which is only available in GCC version 15.1 and later or Clang version 22.1 and newer. In environments utilizing older compilers that lack this specific builtin support, the automatic counter remains unset by kzalloc_flex(), leaving it to rely on manual management within the driver code itself.
The technical flaw arises because the subsequent loop in pxad_alloc_desc continues to increment nb_desc manually under all conditions, regardless of whether the compiler automatically handled the counting or not. This results in a double-counting scenario where the value stored in sw_desc->nb_desc becomes twice the actual number of hardware descriptors allocated when running on older compilers. Since this counter serves as the authoritative source for memory management operations within the driver, its incorrect state propagates through subsequent function calls. Specifically, pxad_free_desc() iterates over nb_desc to release previously allocated resources, and set_updater_desc() uses it to index into descriptor arrays. Consequently, an inflated count leads to out-of-bounds access patterns or attempts to free memory that was never successfully allocated during the current operation.
The operational impact of this vulnerability is significant for system stability and security integrity. When pxad_free_desc() executes with a doubled nb_desc value, it may attempt to deallocate entries beyond the bounds of the actual allocation, leading to heap corruption or use-after-free conditions if those memory regions have been reallocated to other purposes. Similarly, set_updater_desc() indexing based on this erroneous count could result in reading or writing to invalid memory locations, potentially causing kernel panics, data corruption, or arbitrary code execution depending on the specific memory layout and attacker control over adjacent structures. The error path handling further exacerbates the risk; if an allocation fails partway through, the cleanup routine must correctly adjust nb_desc down to the number of descriptors actually allocated so far. Failure to do so would cause pxad_free_desc() to attempt freeing entries that were never initialized or reserved, compounding the memory safety violations and increasing the likelihood of a denial-of-service condition or exploitation vector for privilege escalation.
To mitigate this vulnerability, explicit assignment of nb_desc is required rather than relying solely on loop increments when compiler support for __builtin_counted_by_ref is absent. The fix involves setting sw_desc->nb_desc explicitly to reflect the true number of allocated hardware descriptors and removing the redundant increment operation that causes the double counting. Additionally, error handling paths must be updated to decrement nb_desc appropriately if an allocation failure occurs mid-process, ensuring it accurately reflects only the successfully allocated portion before triggering cleanup routines. From a broader security perspective, this issue aligns with CWE-120 Buffer Copy without Checking Size of Input and CWE-416 Use After Free, as the incorrect counter leads to buffer overruns and improper resource deallocation respectively. It also relates to ATT&CK technique T1583 Acquire Infrastructure if an attacker were to exploit this for persistent access via kernel compromise, though primarily it represents a critical reliability flaw in driver memory management that requires immediate patching through standard Linux kernel update channels or backporting the fix to affected stable releases.