CVE-2025-21709 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
kernel: be more careful about dup_mmap() failures and uprobe registering
If a memory allocation fails during dup_mmap(), the maple tree can be left in an unsafe state for other iterators besides the exit path. All the locks are dropped before the exit_mmap() call (in mm/mmap.c), but the incomplete mm_struct can be reached through (at least) the rmap finding the vmas which have a pointer back to the mm_struct.
Up to this point, there have been no issues with being able to find an mm_struct that was only partially initialised. Syzbot was able to make the incomplete mm_struct fail with recent forking changes, so it has been proven unsafe to use the mm_struct that hasn't been initialised, as referenced in the link below.
Although 8ac662f5da19f ("fork: avoid inappropriate uprobe access to invalid mm") fixed the uprobe access, it does not completely remove the race.
This patch sets the MMF_OOM_SKIP to avoid the iteration of the vmas on the oom side (even though this is extremely unlikely to be selected as an oom victim in the race window), and sets MMF_UNSTABLE to avoid other potential users from using a partially initialised mm_struct.
When registering vmas for uprobe, skip the vmas in an mm that is marked unstable. Modifying a vma in an unstable mm may cause issues if the mm isn't fully initialised.
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability described in CVE-2025-21709 represents a critical race condition and memory management flaw within the Linux kernel's memory management subsystem. This issue specifically affects the dup_mmap() function which is responsible for duplicating memory mappings during process forking operations. The vulnerability stems from insufficient handling of memory allocation failures during the duplication process, creating a scenario where partially initialized memory management structures can persist in an unsafe state. The flaw occurs when memory allocation fails during dup_mmap() execution, leaving the maple tree data structure in an inconsistent state that can be accessed by other kernel iterators beyond the normal exit path. This creates a potential attack surface where malicious actors could exploit the incomplete memory management structure to gain unauthorized access or cause system instability.
The technical implementation of this vulnerability involves the improper handling of locks and memory structure initialization during process forking operations. When dup_mmap() encounters allocation failures, it drops all locks before calling exit_mmap() but leaves the mm_struct in a partially initialized state. This incomplete structure can be accessed through reverse mapping (rmap) operations that traverse virtual memory areas (vmas) which maintain pointers back to the mm_struct. The vulnerability was discovered through automated testing by Syzbot, which demonstrated that recent forking changes could trigger the condition where an incomplete mm_struct would fail during memory allocation. This represents a fundamental flaw in the kernel's memory management error handling, where the system fails to properly abort or clean up partially constructed memory management structures, creating a persistent security risk.
The operational impact of this vulnerability extends beyond simple system instability to potentially enable privilege escalation and arbitrary code execution. The race condition allows for scenarios where the mm_struct can be accessed by multiple kernel threads simultaneously, creating opportunities for data corruption, information disclosure, or denial of service attacks. The vulnerability affects the uprobe subsystem which is used for kernel debugging and profiling, making it particularly dangerous as it can be exploited to bypass security controls or gain elevated privileges. The attack vector requires careful exploitation but is feasible through process forking operations, making it a significant concern for systems running vulnerable kernel versions. This vulnerability directly relates to CWE-362, which describes race conditions in concurrent programming, and could potentially map to ATT&CK techniques involving privilege escalation through kernel exploitation.
The mitigation strategy implemented in the patch involves setting specific memory management flags to prevent access to partially initialized structures. The MMF_OOM_SKIP flag prevents the iteration of virtual memory areas during out-of-memory conditions, while MMF_UNSTABLE flag marks memory management structures that are in an inconsistent state. These flags ensure that other kernel subsystems cannot inadvertently access or modify partially constructed memory management structures. Additionally, the patch modifies the uprobe registration process to skip virtual memory areas within memory management structures marked as unstable. This comprehensive approach addresses both the immediate race condition and the broader implications of accessing incomplete memory structures. The solution follows established kernel security practices by ensuring proper cleanup and validation of memory management structures before they become accessible to other kernel components, effectively closing the security gap that could be exploited by malicious actors to gain unauthorized system access or execute arbitrary code.