CVE-2026-99136 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
dm-crypt: fix a tiny race condition in crypt_dec_pending
crypt_dec_pending reads io->error before calling atomic_dec_and_test. Another context, for example crypt_endio called from an interrupt, may set io->error and drop its reference between the read and the decrement. crypt_dec_pending then drops the last reference and completes the bio with the stale status - so a read that failed and was never decrypted, or a write that failed, is reported as successful.
The read was placed before the decrement by commit b35f8caa0890 ("dm crypt: wait for endio to complete before destruction"), because that commit freed dm_crypt_io before calling bio_endio. This is no longer the case, dm_crypt_io lives in the per-bio data now.
Read io->error after atomic_dec_and_test instead. atomic_dec_and_test is fully ordered, so no additional barrier is needed.
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Analysis
by VulDB Data Team • 10/28/2026
The Linux kernel's device-mapper crypt subsystem contains a race condition within the crypt_dec_pending function that can lead to incorrect reporting of I/O operation status. This vulnerability arises from an improper ordering of operations when handling reference counts and error states for block layer bio structures. Specifically, the original implementation reads the io->error field before performing an atomic decrement on the pending count via atomic_dec_and_test. This sequence creates a window where another execution context, such as crypt_endio invoked from interrupt context, can modify the error state and release its reference between the initial read of the error flag and the subsequent atomic decrement.
The operational impact of this race condition is significant because it allows failed I/O operations to be reported as successful to upper layers of the kernel stack. When a read operation fails or data cannot be decrypted due to an underlying device error, or when a write operation encounters a failure during submission, the affected bio structure may still have pending references held by other contexts. If those contexts complete their work and set io->error while crypt_dec_pending has already read a zeroed or clean error state, the final completion of the bio will use that stale status. Consequently, applications and filesystems receive an indication that data was successfully retrieved or written when it actually failed, leading to potential data corruption, silent data loss, or system instability depending on how higher-level software handles such discrepancies.
This flaw originated from a previous optimization commit b35f8caa0890 titled dm crypt wait for endio to complete before destruction which altered the lifecycle management of dm_crypt_io structures. That change moved the bio_endio call after the freeing of dm_crypt_io, assuming that waiting for endio completion was necessary during object destruction. However subsequent refactoring changed this model so that dm_crypt_io is now stored in per-bio data rather than being a separate allocated structure freed independently. This architectural shift means the previous synchronization logic became redundant and introduced unnecessary latency while also creating the race condition described above since the error state check no longer needed to precede the reference count decrement for safety reasons related to object lifetime.
The resolution involves reordering the operations within crypt_dec_pending so that io->error is read after atomic_dec_and_test has been executed. Since atomic_dec_and_test provides full memory ordering guarantees, it ensures that all prior writes including those setting io->error in other contexts are visible before proceeding with bio completion logic. This eliminates the race window without requiring additional explicit memory barriers which would add performance overhead. By aligning the error check with the final reference drop decision, the kernel ensures that any concurrent modifications to the error state by interrupt handlers or other threads are properly captured and reflected in the bio status passed up the stack.
From a vulnerability classification perspective this issue maps to CWE-362 which describes concurrent execution race conditions where shared resources are accessed without proper synchronization leading to unpredictable behavior. In terms of MITRE ATT&CK framework this could be categorized under T1499 endpoint denial of service if exploited to cause system instability or potentially data integrity violations though it is primarily a reliability and correctness issue rather than an intentional exploitation vector for privilege escalation. The fix demonstrates the importance of maintaining synchronization primitives consistent with current memory management models in kernel subsystems especially when dealing with asynchronous I/O paths that span multiple execution contexts including softirq, hardirq, and process context.
System administrators should ensure their Linux kernels are updated to include this patch which is typically available through standard distribution security updates or by applying upstream stable kernel releases. For environments running custom compiled kernels the specific change involves modifying drivers/md/dm-crypt.c to swap the order of reading io->error and calling atomic_dec_and_test within crypt_dec_pending. Verification can be performed by monitoring I/O error rates in system logs such as dmesg for discrepancies between reported success and actual device errors particularly under high load conditions where race windows are more likely to manifest. Maintaining up-to-date kernel versions is critical to prevent silent data corruption scenarios that could compromise application integrity or database consistency relying on accurate block layer feedback.