CVE-2026-90139 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
fuse: check for NULL root inode in fuse_fill_super_submount
fuse_iget() can return NULL when its inode allocation fails, but fuse_fill_super_submount() passed the result straight to get_fuse_inode() and decremented fi->nlookup without checking it:
root = fuse_iget(sb, parent_fi->nodeid, ...); fi = get_fuse_inode(root); fi->nlookup--;
Inside fuse_iget() the inode allocation can fail and return NULL. The submount root takes the iget5_locked() path, whose alloc_inode() can fail under memory pressure (the auto-submount branch can fail the same way in new_inode() or fuse_alloc_submount_lookup()):
inode = iget5_locked(sb, nodeid, fuse_inode_eq, fuse_inode_set, &nodeid); if (!inode) return NULL;
A NULL root makes get_fuse_inode() a container_of() on NULL and the nlookup decrement a write to a bogus address, oopsing the mount. With CONFIG_KASAN the following null pointer dereference is reported when the root inode allocation of an auto-submount fails (e.g. under memory pressure):
================================================================== BUG: KASAN: null-ptr-deref in fuse_get_tree_submount+0x656/0x8b0 Read of size 8 at addr 00000000000002b0 by task ls/942 CPU: 0 PID: 942 Comm: ls Tainted: G W 6.6 #15 Call Trace: <TASK> fuse_get_tree_submount+0x656/0x8b0 vfs_get_tree+0x48/0x140 fc_mount+0x13/0x50 fuse_dentry_automount+0x7a/0xb0 __traverse_mounts+0xca/0x330 step_into+0x339/0xac0 path_lookupat+0xc5/0x2f0 filename_lookup+0x163/0x2a0 vfs_statx+0xd5/0x200 do_statx+0x83/0xd0 __x64_sys_statx+0xa0/0xc0 do_syscall_64+0x37/0x90 entry_SYSCALL_64_after_hwframe+0x78/0xe2 </TASK> ==================================================================
Return -ENOMEM instead; the caller tears down the partially built superblock on error, matching the other error returns in this function.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel's FUSE (Filesystem in Userspace) subsystem contains a critical null pointer dereference vulnerability within the fuse_fill_super_submount function that can lead to system instability or denial of service conditions under specific memory pressure scenarios. This flaw arises from an insufficient validation check when handling inode allocation for auto-submounted filesystems. Specifically, the fuse_iget() function is responsible for retrieving or creating inodes and may return NULL if its internal alloc_inode operation fails due to resource exhaustion. In the affected code path, the result of this call was passed directly into get_fuse_inode without verifying whether the pointer was valid. This lack of defensive programming allows a null reference to propagate further down the execution chain where it triggers undefined behavior during subsequent operations on the inode structure.
The technical root cause lies in the interaction between fuse_fill_super_submount and its dependency on fuse_iget(). When an auto-submount is initiated, typically triggered by accessing a directory that requires mounting a new FUSE instance, the kernel attempts to allocate memory for the root inode of this new filesystem context. The underlying mechanism uses iget5_locked which calls alloc_inode to reserve space for the inode structure. Under conditions such as high system load or low available memory, this allocation can fail and return NULL. However, fuse_fill_super_submount proceeds to call get_fuse_inode with this null pointer. Inside get_fuse_inode, a container_of macro is used to derive the FUSE-specific inode information from the generic VFS inode structure. Passing NULL into this macro results in arithmetic on a zero address, leading to an invalid memory access when attempting to decrement the nlookup counter associated with the file system instance.
The operational impact of this vulnerability manifests as a kernel panic or oops, effectively causing a denial of service for the affected node and potentially destabilizing the entire host system depending on configuration settings such as CONFIG_KASAN which aids in detecting these memory errors during development but does not prevent them in production kernels. The stack trace indicates that this error occurs during standard file status operations like statx when traversing mounts, meaning any user with permission to access directories triggering auto-submounts can potentially exploit this condition if they can induce sufficient memory pressure or race conditions against the allocation routine. This represents a significant reliability issue for systems relying on FUSE-based storage solutions that utilize automatic submounting features.
From a classification perspective, this vulnerability aligns with CWE-476 which denotes NULL Pointer Dereference, indicating improper handling of null pointers by software developers. In terms of attack vectors and techniques, it relates to ATT&CK technique T1053 Scheduled Task/Job as the trigger mechanism often involves system calls initiated by user processes or automated tasks that traverse directory structures requiring mount resolution. The vulnerability highlights a common pattern in kernel development where error paths are not fully sanitized before proceeding with resource manipulation.
To mitigate this risk, immediate patching of the Linux kernel is required to include checks for NULL return values from fuse_iget() within fuse_fill_super_submount. If the inode allocation fails, the function should return an appropriate error code such as -ENOMEM rather than continuing execution. This allows the caller to properly tear down any partially constructed superblock structures and handle the failure gracefully without crashing the kernel system. Administrators running vulnerable versions of the Linux kernel should apply security updates provided by their distribution vendors promptly. Additionally, monitoring for memory pressure events and optimizing FUSE daemon configurations can help reduce the likelihood of triggering this specific allocation failure path in production environments until patches are fully deployed across all affected systems.