CVE-2026-74576 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
mm/slab: prevent unbounded recursion in free path with new kmalloc type
Commit 280ea9c3154b ("mm/slab: avoid allocating slabobj_ext array from its own slab") avoided recursive allocation of obj_exts from kmalloc caches of the same size, by bumping the obj_exts array's allocation size whenever the array size equals the size of the object being allocated.
However, as reported by Danielle Costantino and Shakeel Butt, even slabs from kmalloc caches of different sizes can form a cycle by allocating obj_exts arrays from each other [1]:
What happened: a KMALLOC_NORMAL slab's obj_exts array (used by allocation profiling / memcg accounting) is itself kmalloc()'d from a KMALLOC_NORMAL cache, so the "slab holds another slab's obj_exts array" relation can form cycles. With sizeof(struct slabobj_ext) == 16 and the host's geometry:
- kmalloc-512 has 64 objects/slab -> array is 64*16 == 1024 bytes, served from kmalloc-1k; - kmalloc-1k has 32 objects/slab -> array is 32*16 == 512 bytes, served from kmalloc-512.
A kmalloc-512 slab and a kmalloc-1k slab therefore hold each other's obj_exts array. Discarding one frees the other's array, which empties and discards that slab, which frees the first's array, and so on: __free_slab() -> free_slab_obj_exts() -> kfree() -> discard_slab() -> __free_slab() recurses along the cycle until the stack is exhausted.
With memory allocation profiling, this allows unbounded recursion in the free path and led to a stack overflow on a production host in the Meta fleet [1]:
BUG: TASK stack guard page was hit Oops: stack guard page RIP: 0010:kfree+0x8/0x5d0 Call Trace: __free_slab+0x66/0xc0 kfree+0x3f0/0x5d0 ... ( ~125x __free_slab <-> kfree ) ... <kernel driver freeing a resource> do_syscall_64
It is proposed [1] to resolve this issue by always serving the obj_exts
array allocation from kmalloc caches (or large kmalloc) of sizes larger than the object size. However, as pointed out by Vlastimil Babka [2],
this can waste an excessive amount of memory as slabs from large kmalloc sizes (e.g. kmalloc-8k) generally need obj_exts arrays much smaller than the object size.
Therefore, rather than bumping the size, let us take a different approach; disallow formation of cycles between kmalloc types when allocating obj_exts arrays. Currently, all obj_exts arrays are served from normal kmalloc caches. Cycles cannot be created if obj_exts arrays of normal kmalloc caches are served from a special kmalloc type that can never have obj_exts arrays.
To achieve this, create a new kmalloc type called KMALLOC_NO_OBJ_EXT. KMALLOC_NO_OBJ_EXT caches are created with SLAB_NO_OBJ_EXT flag when either 1) memory allocation profiling is not permanently disabled, or 2) kmalloc types with a priority higher than KMALLOC_CGROUP are aliased with KMALLOC_NORMAL.
Sheaf bootstrapping for KMALLOC_NO_OBJ_EXT caches now must be deferred because allocation of a barn can trigger obj_exts array allocation of normal kmalloc caches when the KMALLOC_NO_OBJ_EXT cache for that size is not ready yet. For simplicity, perform bootstrapping of sheaves for all kmalloc caches later.
Introduce a new slab alloc flag, SLAB_ALLOC_NO_OBJ_EXT, to prevent allocation of obj_exts arrays, and let kmalloc_slab() override the type to KMALLOC_NO_OBJ_EXT when specified. Note that kmalloc_type() remains unchanged because kmalloc_flags() bypasses the kmalloc fastpath.
Do not pass SLAB_ALLOC_NO_RECURSE to kmalloc_flags() in alloc_slab_obj_exts() and instead use SLAB_ALLOC_NO_OBJ_EXT only when the objects are allocated from normal kmalloc caches. While this prevents unbounded recursive allocation of obj_exts, it allows KMALLOC_NO_OBJ_EXT caches to have sheaves.
Since sheaf allocations specify SLAB_ALLOC_NO_RECURSE that prevents allocation of both sheaves and obj_exts arrays, the recursion depth is bounded.
obj_exts arrays for non- ---truncated---
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described involves a critical flaw in the Linux kernel's memory management subsystem, specifically within the slab allocator mechanism. This issue arises from improper handling of object extension arrays during memory deallocation processes. The core problem manifests as unbounded recursion in the free path when attempting to release memory objects, leading to stack overflow conditions that can crash production systems. The vulnerability is particularly dangerous because it can be triggered through normal memory allocation operations, making it a latent threat in well-established kernel environments.
The technical root cause stems from how the kernel handles slab object extensions used for memory allocation profiling and memcg accounting. When slabs from different kmalloc caches attempt to allocate their object extension arrays from each other's caches, cycles can form that lead to infinite recursion. This occurs because the system allows cross-cache allocations of obj_exts arrays without proper cycle detection mechanisms. The specific scenario involves kmalloc-512 and kmalloc-1k caches where each slab holds the obj_exts array of the other, creating a recursive loop when one slab is freed.
The operational impact of this vulnerability extends beyond simple stack overflow conditions to potentially destabilize entire production systems. In real-world deployments, such as those in Meta's fleet, this vulnerability has already been observed causing system crashes with explicit stack guard page violations. The recursion pattern follows a predictable sequence: __free_slab() calls free_slab_obj_exts() which invokes kfree(), leading to discard_slab() and ultimately back to __free_slab(), repeating until the kernel stack is exhausted. This behavior aligns with common attack patterns documented in the ATT&CK framework under privilege escalation techniques that leverage kernel memory corruption.
The proposed solution addresses this through a novel approach that introduces a new kmalloc type called KMALLOC_NO_OBJ_EXT, which serves as an isolation mechanism to prevent cycle formation during obj_exts array allocations. This approach is more sophisticated than simple size bumping because it fundamentally changes how allocation paths are constructed by ensuring that normal kmalloc caches cannot have their obj_exts arrays allocated from other normal caches. The implementation involves creating a dedicated slab flag SLAB_ALLOC_NO_OBJ_EXT that prevents object extension array allocation during specific operations while maintaining the integrity of the overall memory management system.
The mitigation strategy requires careful bootstrapping of new cache types to avoid triggering recursive allocation issues during kernel initialization phases. This is particularly important because early allocation events could still reference uninitialized caches, creating a chicken-and-egg problem. The solution also includes refactoring how allocation flags are handled, ensuring that kmalloc_flags() bypasses the fast path while still maintaining proper recursion bounds through the SLAB_ALLOC_NO_RECURSE flag. This approach aligns with CWE-674 principles of controlled recursion and proper resource management in kernel space operations.
The broader implications of this fix extend to memory efficiency considerations, as simply increasing allocation sizes would waste significant memory resources. Instead, the solution maintains optimal memory usage while providing robust protection against recursive allocation patterns. The implementation follows established kernel development practices and provides a scalable mechanism that can be extended to handle similar issues in other subsystems. This vulnerability demonstrates the complexity of modern kernel memory management systems and highlights why such low-level components require rigorous testing and analysis to prevent subtle but catastrophic failures in production environments.