CVE-2026-52923

In the Linux kernel, the following vulnerability has been resolved:

ipc: limit next_id allocation to the valid ID range

The checkpoint/restore sysctl path can request the next SysV IPC id

through ids->next_id. ipc_idr_alloc() currently forwards that request to

idr_alloc() with an open-ended upper bound.

If the valid tail of the SysV IPC id space is full, the allocation can

spill beyond ipc_mni. The returned SysV IPC id still uses the normal

index encoding, so later lookup and removal can target the wrong slot.

This leaves the real IDR entry behind and breaks the IDR state for the

object.

The bug is in ipc_idr_alloc() in the checkpoint/restore path.

1. ids->next_id is passed to:

idr_alloc(&ids->ipcs_idr, new, ipcid_to_idx(next_id), 0, ...)

2. The zero upper bound makes the allocation effectively open-ended.

Once the valid SysV IPC tail is occupied, idr_alloc() can spill past

ipc_mni and allocate an entry beyond the valid IPC id range.

3. The new object id is still encoded with the narrower SysV IPC index

width:

new->id = (new->seq << ipcmni_seq_shift()) + idx

4. Later removal goes through ipc_rmid(), which uses:

ipcid_to_idx(ipcp->id)

That truncates the real IDR index. An object actually stored at a

high index can then be removed as if it lived at a low in-range

index.

5. For shared memory, shm_destroy() frees the current object anyway, but

the real high IDR slot is left behind as a dangling pointer.

6. A subsequent walk of /proc/sysvipc/shm reaches the stale IDR entry

and dereferences freed memory.

Prevent this by bounding the requested allocation to ipc_mni so the

checkpoint/restore path fails once the valid range is exhausted.