CVE-2026-63810 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
block: Avoid mounting the bdev pseudo-filesystem in userspace
The bdev pseudo-filesystem is an internal kernel filesystem with which userspace should not interfere. Unregister it so that userspace cannot even attempt to mount it.
This fixes a bug [1] that occurs when attempting to access files,
because the system call move_mount() uses pointers declared in the inode_operations structure, which for the bdev pseudo-filesystem are always equal to 0. `inode->i_op = &empty_iops;`
[1]
BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor instruction fetch in kernel mode #PF: error_code(0x0010) - not-present page PGD 23380067 P4D 23380067 PUD 23381067 PMD 0 Oops: 0010 [#1] PREEMPT SMP KASAN NOPTI
CPU: 2 PID: 17125 Comm: syz-executor.0 Not tainted 6.1.155-syzkaller-00350-g84221fde2681 #0 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 RIP: 0010:0x0
Call Trace: <TASK> lookup_open.isra.0+0x700/0x1180 fs/namei.c:3460 open_last_lookups fs/namei.c:3550 [inline]
path_openat+0x953/0x2700 fs/namei.c:3780 do_filp_open+0x1c5/0x410 fs/namei.c:3810 do_sys_openat2+0x171/0x4d0 fs/open.c:1318 do_sys_open fs/open.c:1334 [inline]
__do_sys_openat fs/open.c:1350 [inline]
__se_sys_openat fs/open.c:1345 [inline]
__x64_sys_openat+0x13c/0x1f0 fs/open.c:1345 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x35/0x80 arch/x86/entry/common.c:81 entry_SYSCALL_64_after_hwframe+0x6e/0xd8
Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability represents a critical kernel-level flaw in the Linux kernel's block device handling mechanism that allows unauthorized userspace processes to interact with the bdev pseudo-filesystem in ways that trigger kernel NULL pointer dereferences. The bdev pseudo-filesystem serves as an internal kernel construct designed to provide information about block devices without exposing direct filesystem interfaces to user applications. When userspace attempts to mount this pseudo-filesystem, it triggers a cascade of operations that ultimately leads to kernel memory corruption through improper handling of inode operations structures.
The technical root cause stems from the improper exposure of the bdev pseudo-filesystem to user space environments. Specifically, when system calls such as move_mount() attempt to process operations on this filesystem, they reference inode_operations structures that contain null pointers due to the declaration `inode->i_op = &empty_iops`. This occurs because the bdev filesystem's inode operations are intentionally left empty but not properly protected from user space access. The kernel's name resolution and file opening mechanisms, particularly in fs/namei.c where lookup_open and path_openat functions execute, attempt to dereference these null pointers during mount operations, resulting in immediate kernel crashes with NULL pointer dereference errors.
The operational impact of this vulnerability extends beyond simple system instability to represent a potential privilege escalation vector within kernel space. When the move_mount() system call processes requests against the bdev pseudo-filesystem, it accesses memory locations that are not properly initialized or protected, causing supervisor instruction fetch failures and kernel oops conditions that result in complete system crashes. This behavior aligns with CWE-476 which addresses NULL pointer dereferences in software systems, and represents a classic example of improper access control in kernel-level filesystem interfaces. The vulnerability manifests as a kernel panic condition with error code 0x0010 indicating page faults during instruction fetch operations, suggesting that the kernel attempted to execute instructions from memory locations containing null values.
The fix implemented addresses this by unregistering the bdev pseudo-filesystem entirely from userspace mount capabilities, preventing any attempt to access it through standard filesystem mounting interfaces. This approach directly resolves the underlying issue identified in the Linux Verification Center testing environment using Syzkaller, which is a systematic fuzzer designed to discover kernel-level vulnerabilities. The solution follows established security principles by eliminating the attack surface rather than attempting to patch the specific execution path that leads to the crash. This vulnerability demonstrates the importance of maintaining strict separation between kernel internal filesystems and user space interfaces, aligning with ATT&CK technique T1068 which covers local privilege escalation through kernel vulnerabilities.
This type of vulnerability represents a fundamental flaw in kernel security architecture where internal kernel data structures are improperly exposed to user space contexts. The bdev pseudo-filesystem should never have been accessible to userspace processes, yet the improper handling allowed for direct interaction that led to memory corruption during normal system call processing. The fix ensures that when userspace attempts to mount or otherwise interact with this filesystem, it will fail gracefully rather than causing kernel-level crashes. This vulnerability affects all Linux kernel versions where the bdev pseudo-filesystem was improperly exposed and represents a critical security concern for systems running unpatched kernel versions. The specific error pattern observed indicates that traditional kernel debugging mechanisms were insufficient to prevent the execution path that led to the null pointer dereference, highlighting the need for more robust kernel-level access controls and proper filesystem isolation mechanisms.
The resolution of this vulnerability through unregistration of the bdev pseudo-filesystem from userspace mount capabilities represents a fundamental architectural fix that prevents the root cause rather than merely patching symptoms. This approach ensures that no user space process can ever attempt to interact with this internal kernel filesystem in ways that would trigger memory corruption, thereby maintaining kernel stability and security. The vulnerability's discovery through systematic fuzzing tools like Syzkaller demonstrates the effectiveness of automated testing in identifying subtle kernel-level flaws that might otherwise remain undetected until exploited in production environments. This fix maintains system integrity by preventing unauthorized access to kernel internal structures while preserving normal block device functionality for legitimate user space applications.