CVE-2026-74576
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
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Exploitation outlook
- Low exploitation risk0.18% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 7% of all EPSS-scored CVEs.
Advisory coverage (1)
External references
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