CVE-2026-74506 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
afs: Fix UAF when sending a message
In afs_make_call(), there's a race with async call reception and destruction. If a call is dispatched that doesn't have call->write_iter set (used to specify the data content for FS.StoreData), then the first rxrpc_kernel_send_data() will not set MSG_MORE in the msghdr.
Once rxrpc_send_data() queues the last request packet, the response could come in at any time and cause the call to be completed and put. However, afs_make_call() will look at the call again to see it ->write_iter should be handled - something it's only allowed to do if it has its own ref on the call. Whilst this is the case for synchronous calls, it isn't true for async calls such as FS.FetchData.
There's also a potential UAF in afs_make_call() in the event that an asynchronous call is being sent, but the call fails in some way (e.g. it gets aborted from the server). The problem there is that afs_make_call() tries to abort a call if the rxrpc send fails, but the asynchronous notification from rxrpc may have caused the afs_call to be torn down.
generic/650 plays games with randomly taking CPUs offline, and can interject a significant delay such that the call is deallocated before afs_make_call() gets to check call->write_iter - and a UAF ensues (caught by KASAN).
BUG: KASAN: slab-use-after-free in afs_make_call+0x1c90/0x2210 [kafs]
Read of size 8 at addr ffff888035e050e8 by task fsstress/1409
Fix this by making afs_make_op_call() give the op->call its own ref rather than transferring the caller's ref to it and then dropping the ref when afs_make_call() returns.
This also means that the afs_make_call() func never loses its ref on the call now.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a use-after-free condition in the AFS (Andrew File System) implementation within the Linux kernel, specifically occurring in the afs_make_call() function. This flaw arises from improper handling of reference counting during asynchronous call processing, creating a race condition between call dispatch and destruction operations. The issue manifests when asynchronous calls are processed without proper reference management, leading to potential memory corruption scenarios that can be exploited by malicious actors.
The technical root cause involves the interaction between asynchronous call reception and destruction mechanisms within the AFS subsystem. When afs_make_call() processes a call where call->write_iter is not set, the initial rxrpc_kernel_send_data() operation omits the MSG_MORE flag in the msghdr structure. This creates a timing window where response packets can arrive and trigger call completion and deallocation before the calling function has finished processing. The race condition becomes particularly problematic with asynchronous operations like FS.FetchData, where the caller's reference is not properly maintained throughout the operation lifecycle.
The vulnerability demonstrates characteristics consistent with CWE-416 Use After Free, where memory allocated to a call structure is accessed after it has been freed. The problem extends beyond simple memory corruption into potential privilege escalation scenarios since kernel memory corruption can be leveraged to execute arbitrary code. The KASAN (Kernel Address Sanitizer) detection confirms the use-after-free behavior with specific memory access patterns indicating that afs_make_call() attempts to read from a deallocated structure address. This vulnerability impacts the generic/650 stress test which deliberately introduces CPU offline scenarios and timing delays, amplifying the race condition probability.
The fix implemented addresses this by modifying afs_make_op_call() to maintain its own reference to the call object rather than transferring the caller's reference and subsequently dropping it during afs_make_call() return. This approach ensures that the afs_make_call() function never loses its reference to the call structure, providing consistent reference management throughout the operation lifecycle. The solution directly addresses the race condition by preventing premature deallocation of call structures before all processing is complete, thereby eliminating the use-after-free scenario.
This vulnerability aligns with ATT&CK technique T1068 Exploitation for Privilege Escalation, where kernel-level memory corruption can be exploited to gain elevated privileges. The fix demonstrates proper reference counting practices as recommended in kernel security best practices, ensuring that asynchronous operations maintain proper object lifetime management. The solution also reflects principles of defensive programming by preventing race conditions through consistent reference handling rather than relying on timing-sensitive operations that could be disrupted by system stress or concurrent operations.