CVE-2026-90042 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
ceph: properly decrypt filenames in vmalloc() buffers
The fscrypt subsystem uses the scatterlist crypto API, inheriting its requirement that any buffers are in the linear mapping region. However, the messenger client uses kvmalloc() to create buffers for messages, which will occasionally place those buffers in the vmalloc() region when physical memory fragmentation doesn't permit a large enough kmalloc(). The various callers of ceph_fname_to_usr() directly pass (slices of) raw messages from the MDS without considering that the messages may be in vmalloc() buffers, resulting in oopses especially on non-x86 platforms (see 'Closes:' for more details and a reproducer).
Make ceph_fname_to_usr() explicitly tolerant of vmalloc()-allocated fname->ctext, fname->name, and/or oname->name buffers, using `tname` (which, when non-null, must be a linear address; when null, is briefly allocated as necessary) as a bounce buffer to avoid passing any inappropriate addresses to fscrypt_fname_disk_to_usr().
Additionally change parse_reply_info_readdir() -- the only function to supply its own `tname` -- to follow the new "tname must never come from vmalloc()" rule by passing NULL when the message is not in the linear region. Though this causes a per-dentry kmalloc()+kfree(), this overhead exists only when processing the minority of messages that spill into vmalloc(). My (crude) testing puts this at only about 1 in 8,000 readdir messages. Still, if the overhead proves unreasonable in the future, it is easy enough to mitigate: a future change could allocate a bounce buffer in parse_reply_info_readdir() and use that as `tname` instead.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel's Ceph filesystem implementation contained a critical memory handling vulnerability related to cryptographic operations on filename data structures. The fscrypt subsystem, which handles file encryption within the kernel, relies on the scatterlist crypto API for performing decryption tasks. This specific API imposes strict constraints regarding memory layout, requiring that all buffers involved in cryptographic operations reside within the linear mapping region of physical memory. This requirement ensures direct accessibility and proper alignment for hardware or software-based cipher implementations. However, the Ceph messenger client utilizes kvmalloc() to allocate buffers for incoming messages from the Metadata Server (MDS). While kvmalloc() attempts to use kmalloc(), which guarantees a linear address, it falls back to vmalloc() when physical memory fragmentation prevents the allocation of large contiguous blocks. This fallback mechanism places message buffers in the vmalloc region, which is not directly mapped into the kernel's linear address space and cannot be safely passed to functions expecting linear addresses.
The core technical flaw arises because various callers within the Ceph codebase invoke ceph_fname_to_usr() by passing slices of raw messages retrieved from the MDS without verifying whether these buffers reside in vmalloc or kmalloc regions. When a buffer allocated via kvmalloc falls back to vmalloc due to memory pressure, and this non-linear address is subsequently passed through ceph_fname_to_usr() into fscrypt_fname_disk_to_usr(), it violates the linear mapping requirement of the scatterlist API. This violation leads to kernel oopses or panics, particularly on architectures that do not support direct access to vmalloc regions as seamlessly as x86 does. The vulnerability effectively allows a denial-of-service condition triggered by specific network conditions where memory fragmentation forces message buffers into non-linear memory spaces during directory operations or file name processing.
To remediate this issue, the kernel developers modified ceph_fname_to_usr() to explicitly handle and tolerate vmalloc-allocated buffers for fname->ctext, fname->name, and oname->name fields. The solution introduces a bounce buffer mechanism using a variable named tname. When tname is non-null, it must point to a linear address; when null, the function temporarily allocates memory as necessary to create this safe copy. This approach ensures that fscrypt_fname_disk_to_usr() always receives addresses from the linear mapping region, thereby preventing illegal memory accesses and subsequent kernel crashes. The implementation carefully manages these temporary allocations to maintain system stability while preserving data integrity during decryption operations.
A secondary adjustment was made to parse_reply_info_readdir(), which is the only function in this context that supplies its own tname buffer. This function was updated to adhere strictly to the new rule requiring linear addresses by passing NULL when the incoming message resides outside the linear region. Although this change introduces a minor performance overhead due to per-dentry kmalloc and kfree operations, empirical testing indicates that such cases are rare, occurring in approximately one out of eight thousand readdir messages. This minimal impact ensures that security is maintained without significantly degrading filesystem performance under normal operating conditions. If future profiling reveals unacceptable latency, the architecture allows for optimization by pre-allocating a bounce buffer within parse_reply_info_readdir() to reuse memory and eliminate repeated allocation overheads.
From a vulnerability classification perspective, this issue aligns with CWE-120 Buffer Copy without Checking Size of Input when considering the improper handling of input data boundaries relative to expected memory layouts, though more accurately it reflects CWE-789 Uncontrolled Memory Allocation leading to instability in cryptographic contexts. In terms of attack vectors and detection, while primarily a stability flaw rather than an exploit for privilege escalation or remote code execution, it impacts availability consistent with ATT&CK technique T1499 Endpoint Denial of Service. The vulnerability highlights the importance of strict adherence to memory model constraints within kernel subsystems that interface with hardware-specific cryptographic APIs. System administrators and developers should ensure their kernels are updated to include this fix to prevent potential denial-of-service scenarios triggered by high-load conditions or specific network traffic patterns involving Ceph filesystem operations.