CVE-2026-63799 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
sched/mmcid: Fix OOB clear_bit when CID is MM_CID_UNSET in fixup path
In mm_cid_fixup_cpus_to_tasks(), when rq->curr has the target mm and mm_cid.active is set, the CID is checked with cid_in_transit() before setting the transition bit. In per-CPU mode a newly forked or exec'd task can be running with mm_cid.cid == MM_CID_UNSET because CIDs are assigned lazily on schedule-in. With cid_in_transit() the guard passes for MM_CID_UNSET (no transit bit), converts it to MM_CID_UNSET | MM_CID_TRANSIT and stores it back; later mm_cid_schedout() feeds this to clear_bit() with MM_CID_UNSET as the bit number, triggering an out-of-bounds write.
Symptoms: this is genuine memory corruption, but a bounded out-of-bounds write, not an arbitrary one. MM_CID_UNSET is the fixed sentinel BIT(31), so once the bad value reaches mm_cid_schedout() the cid_from_transit_cid() strip leaves MM_CID_UNSET, which fails the "cid < max_cids" convergence test and falls into mm_drop_cid() -> clear_bit(MM_CID_UNSET, mm_cidmask(mm)). The cid bitmap is embedded in the mm_struct slab object (after cpu_bitmap and mm_cpus_allowed) and is only num_possible_cpus() bits wide, so clearing bit 31 is a deterministic OOB bit-clear at a fixed offset of 2^31 / 8 == 256 MiB past the bitmap base. The address is not attacker-influenced (fixed sentinel -> fixed offset) and the op only clears a single bit; what sits 256 MiB further along the direct map is whatever kernel object happens to live there, so this corrupts one bit of unpredictable kernel memory -- it is not an arbitrary-address or arbitrary-value write.
It triggers only in per-CPU CID mode, when a CPU is running an active task of the target mm whose cid is still MM_CID_UNSET -- the fork()/execve() window before that task's next schedule-in assigns it a real CID -- and a per-CPU -> per-task fixup walks over it (the mode fallback driven by a thread exit, sched_mm_cid_exit(), or by the deferred max_cids recompute in mm_cid_work_fn()).
In practice syzkaller surfaced it as a KASAN use-after-free reported in __schedule -> mm_cid_switch_to, where the offending clear_bit() is inlined via mm_cid_schedout() -> mm_drop_cid().
Guard the transition-bit assignment against MM_CID_UNSET, in addition to the existing cid_in_transit() check, so the bit is only set on a genuine task-owned CID. A CPU-owned (MM_CID_ONCPU) CID of a running active task is handled by the cid_on_cpu(pcp->cid) branch above and never reaches this path, so excluding MM_CID_UNSET (and the already-transitioning case) is sufficient.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described represents a critical out-of-bounds memory access condition within the Linux kernel's memory management subsystem, specifically affecting the multi-memory controller identification (MMCID) implementation. This flaw manifests in the scheduler's handling of memory controller identifiers during task migration and context switching operations. The issue occurs when processing tasks that have not yet received their assigned CID values, particularly in per-CPU mode configurations where CIDs are lazily allocated upon schedule-in events.
The technical root cause stems from an insufficient validation check within the `mm_cid_fixup_cpus_to_tasks()` function. When a task is running on a CPU with `rq->curr` having the target memory management context and `mm_cid.active` set, the system performs a `cid_in_transit()` check before setting transition bits. However, this existing guard fails to account for tasks that have not yet received their actual CID values, specifically those with `mm_cid.cid == MM_CID_UNSET`. The sentinel value `MM_CID_UNSET` is defined as `BIT(31)`, which represents a fixed bit position in the CID bitmap structure.
In per-CPU mode configurations, newly forked or exec'd tasks can execute with `mm_cid.cid == MM_CID_UNSET` because CIDs are assigned lazily upon schedule-in events. When the `cid_in_transit()` check passes for `MM_CID_UNSET`, the system converts it to `MM_CID_UNSET | MM_CID_TRANSIT` and stores this invalid combination back into the CID structure. Subsequently, `mm_cid_schedout()` processes this corrupted value and feeds it into `clear_bit()` with `MM_CID_UNSET` as the bit number, triggering an out-of-bounds write operation.
This vulnerability directly relates to CWE-129, which addresses Out-of-Bounds Read conditions, and specifically manifests as a bounded out-of-bounds write rather than an arbitrary memory corruption. The memory corruption occurs at a deterministic offset of 256 MiB past the bitmap base, as the CID bitmap is embedded within the mm_struct slab object and is only `num_possible_cpus()` bits wide. The operation only clears a single bit, making it a predictable but still dangerous form of memory corruption that can affect adjacent kernel objects in the direct map.
The operational impact of this vulnerability is significant, as it results in genuine memory corruption that can lead to system instability, data corruption, or potentially exploitable conditions depending on the specific memory layout at the corrupted offset. The issue only triggers under specific conditions involving per-CPU CID mode, when a CPU is running an active task with `mm_cid.cid == MM_CID_UNSET` and a per-CPU to per-task fixup operation occurs. These conditions are typically driven by thread exits (`sched_mm_cid_exit()`) or deferred maximum CID recomputation (`mm_cid_work_fn()`).
The vulnerability was detected through systematic testing using syzkaller, which reported it as a KASAN use-after-free condition in the `__schedule` function within `mm_cid_switch_to`. The offending `clear_bit()` operation is inlined via `mm_cid_schedout()` to `mm_drop_cid()`, making the attack surface more complex to analyze and mitigate. According to ATT&CK framework category T1068, this represents a privilege escalation vector through local kernel memory corruption.
The mitigation approach involves adding an additional guard condition against `MM_CID_UNSET` in addition to the existing `cid_in_transit()` check. This ensures that transition bits are only set on genuine task-owned CIDs rather than the sentinel value representing unassigned state. The solution is straightforward and targeted, specifically excluding `MM_CID_UNSET` (and already-transitioning cases) from the transition-bit assignment process, which prevents the corruption of the CID tracking mechanism while maintaining all existing functionality for legitimate CID operations.
This fix addresses the fundamental design flaw in the CID assignment logic and prevents the propagation of invalid CID states through the memory management subsystem. The solution aligns with secure coding practices by ensuring proper validation of input parameters before processing operations that modify kernel data structures, thereby preventing potential privilege escalation attacks and maintaining system stability under all expected operating conditions.