CVE-2026-72422 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: fix use-after-free of conn->preauth_info in concurrent SMB2 NEGOTIATE
conn->preauth_info is shared connection state (struct preauth_integrity_info, kmalloc-96) that is allocated and freed by the SMB2 NEGOTIATE handler and read by the response send path.
smb2_handle_negotiate() allocates conn->preauth_info, and on a deassemble_neg_contexts() failure kfrees it and sets it to NULL. Both the allocation and the free/NULL happen under ksmbd_conn_lock(conn) (the connection srv_mutex), which is held across the whole handler body.
The response send path smb3_preauth_hash_rsp(), called from the send: block of __handle_ksmbd_work(), reads conn->preauth_info and dereferences conn->preauth_info->Preauth_HashValue (via ksmbd_gen_preauth_integrity_hash()) without taking conn_lock. When a client drives two SMB2 NEGOTIATE requests on the same connection, one worker can free conn->preauth_info on the failing-negotiate path while a concurrent send-path worker is reading it, producing a slab use-after-free read (KASAN-confirmed).
The send-path read tested conn->preauth_info for NULL but raced with the free that occurs between the NULL check and the dereference, so the NULL guard alone does not close the window.
Serialize the NEGOTIATE-branch read in smb3_preauth_hash_rsp() under ksmbd_conn_lock(conn) and re-check conn->preauth_info inside the lock. Because the negotiate handler holds conn_lock across its kfree + NULL assignment, a reader that also takes conn_lock either runs fully before the allocation or fully after the NULL store, and can never observe the freed-but-not-yet-NULLed pointer. ksmbd_gen_preauth_integrity_hash() takes no locks itself (it only computes a SHA-512 over the buffer), so no lock-ordering inversion is introduced, and conn_lock is a sleepable mutex which is safe on this send path (it already performs network I/O).
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Analysis
by VulDB Data Team • 08/16/2026
The vulnerability in question affects the Linux kernel's ksmbd implementation, specifically within the SMB2 NEGOTIATE handler where a use-after-free condition occurs due to improper synchronization of shared connection state. This flaw manifests when concurrent SMB2 NEGOTIATE requests are processed on the same connection, creating a race condition between the allocation and deallocation of the conn->preauth_info structure. The preauth_integrity_info structure is allocated using kmalloc-96 and represents shared connection state that is managed by the SMB2 NEGOTIATE handler while simultaneously being accessed by the response send path.
The technical implementation involves the smb2_handle_negotiate() function which allocates conn->preauth_info under the protection of ksmbd_conn_lock(conn) that encompasses the entire handler body. However, when deassemble_neg_contexts() fails, the structure gets freed via kfree and set to NULL within the same locked context. The problematic interaction occurs in smb3_preauth_hash_rsp() which is invoked from the send block of __handle_ksmbd_work(), where conn->preauth_info is read without acquiring conn_lock, creating a window where a reader can access the memory after it has been freed but before the NULL assignment occurs. This race condition results in a slab use-after-free read as confirmed by KASAN.
The operational impact of this vulnerability represents a serious security concern that could potentially allow an attacker to exploit the use-after-free condition through carefully crafted concurrent SMB2 NEGOTIATE requests on the same connection. The flaw is classified as a CWE-416 Use After Free vulnerability, which falls under the broader category of memory safety issues in kernel space operations. According to ATT&CK framework, this represents a privilege escalation vector through kernel memory corruption, potentially enabling adversaries to execute arbitrary code with kernel privileges.
The mitigation strategy involves serializing the NEGOTIATE-branch read in smb3_preauth_hash_rsp() by acquiring ksmbd_conn_lock(conn) before accessing conn->preauth_info and re-checking the pointer value inside the lock. This approach ensures that readers either execute entirely before or after the allocation/deallocation sequence, eliminating the race window. Since the negotiate handler maintains conn_lock across its entire kfree operation and NULL assignment, any reader that also takes conn_lock will observe a consistent state. The solution is architecturally sound as ksmbd_gen_preauth_integrity_hash() does not require additional locking since it only performs SHA-512 computation on buffers, avoiding potential lock-ordering inversions. The conn_lock being a sleepable mutex makes it safe for use in the send path where network I/O operations are already performed, maintaining system responsiveness while ensuring memory safety. This fix ensures that the NULL guard mechanism becomes effective by placing the check within the synchronized context where the pointer state is guaranteed to be stable throughout the access operation.