CVE-2026-98989 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
namespace: check a recursive bind mount for mount namespace loops
do_loopback() refuses to bind mount the file of a mount namespace that is as old as the caller's or older because a mount namespace that holds a mount of its own file, or of an ancestor's, would cause a cycle. But it only checks the dentry the bind mount starts from. With MS_REC everything below it is copied and do_loopback() passes CL_COPY_MNT_NS_FILE so mount namespace files below the source are copied.
That's fine for a source in the caller's own mount namespace. Every mount namespace file in there passed the same check when it was bind-mounted. It isn't fine for a source in another mount namespace. may_copy_tree() accepts a bind mount of any nsfs or pidfs file no matter what mount namespace it lives in so that /proc/<pid>/ns/<ns> and pidfds can be bind mounted. If we create a bind-mount stack of mount namespaces file descriptors the kernel will copy them irrespective of their ancestoral relationship to the mount namespace in question:
151 146 0:7 net:[4026531833] /tmp/nrc/y rw - nsfs nsfs rw
152 151 0:7 mnt:[4026532293] /tmp/nrc/y rw - nsfs nsfs rw
Mount 152 is a mount of the mount namespace file of mount namespace 4026532293 inside mount namespace 4026532293. That namespace and every mount in it are leaked... All it takes is a process in an older mount namespace that stacks the file and repeating it leaks without limit:
after control: Shmem: 380 kB child: mount(/proc/self/fd/6, MS_BIND|MS_REC): ok after cycle: Shmem: 65916 kB
do_move_mount() runs check_for_nsfs_mounts() over a detached tree for exactly that reason. Do the same for the copy before it is grafted.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel contains a vulnerability in the mount namespace handling logic, specifically within the do_loopback function used during bind mounts with the MS_REC flag. This flaw allows for an uncontrolled memory leak and potential denial of service through recursive binding of mount namespace file descriptors. The core issue stems from insufficient validation when copying mount trees across different namespaces. While the kernel correctly prevents a process from creating a direct cycle by refusing to bind mount its own or older mount namespace files, this check is only applied to the initial dentry provided for the operation. When the MS_REC flag is used, which instructs the kernel to recursively copy all mounts below the source point, the validation logic fails to extend to these nested structures if they reside in a different mount namespace than the caller's own context.
In scenarios where a process operates within an older or distinct mount namespace and attempts to bind mount file descriptors representing other namespaces, such as those found under /proc/<pid>/ns/ or pidfds, the may_copy_tree function accepts these mounts without verifying their ancestral relationship to the target mount namespace. This permissiveness allows for the creation of a stack of mount namespace file descriptors that reference each other in a circular manner. For instance, if a process binds a mount namespace file descriptor inside its own namespace and then recursively copies it, the kernel may copy subsequent layers of these references without detecting the loop. Consequently, every layer added to this recursive bind mount structure results in new memory allocations for mount structures that are never properly released because they form an unbreakable cycle within the kernel's internal data structures.
The operational impact of this vulnerability is significant resource exhaustion leading to a denial of service condition. As demonstrated by empirical testing, repeatedly executing such operations causes rapid and substantial growth in shmem usage, indicating severe memory leakage. An attacker with local access can exploit this behavior to consume all available system memory or trigger kernel panics due to allocation failures. This effectively degrades the stability and availability of the host system for legitimate processes. The vulnerability highlights a critical gap in how mount namespace isolation is enforced during recursive operations across different security contexts, allowing attackers to bypass intended containment mechanisms through carefully crafted sequences of bind mounts involving nsfs entries.
To mitigate this risk, kernel developers have implemented checks similar to those used by do_move_mount() when handling detached trees. The fix involves running check_for_nsfs_mounts on the copy before it is grafted into the target mount namespace tree. This ensures that any recursive binding operation validates whether the source or its descendants belong to a mount namespace that would create a cycle with the destination's context, regardless of which specific dentry initiated the bind request. System administrators should apply kernel updates containing this patch immediately. Additionally, monitoring for unusual spikes in shmem consumption can serve as an early detection mechanism for potential exploitation attempts until all systems are fully patched.