EUVD-2026-39839
GHSA-cx67-j66h-cm23
Severity by source
AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Writing sysfs module parameters typically requires root/CAP_SYS_ADMIN, so PR:H is more accurate than the provided PR:L; impact is availability-only with no scope change.
Primary rating from NVD.
CVSS VectorNVD
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Lifecycle Timeline
5DescriptionNVD
In the Linux kernel, the following vulnerability has been resolved:
scsi: sg: Resolve soft lockup issue when opening /dev/sgX
The parameter def_reserved_size defines the default buffer size reserved for each Sg_fd and should be restricted to a range between 0 and 1,048,576 (see https://tldp.org/HOWTO/SCSI-Generic-HOWTO/proc.html). Although the function sg_proc_write_dressz enforces this limit, it is possible to bypass it by directly modifying the module parameter as shown below, which then causes a soft lockup:
echo -1 > /sys/module/sg/parameters/def_reserved_size exec 4<> /dev/sg0
watchdog: BUG: soft lockup - CPU#5 stuck for 26 seconds! [bash:537] Modules loaded: CPU: 5 UID: 0 PID: 537 Command: bash, kernel version 6.19.0-rc3+ #134, PREEMPT disabled Hardware: QEMU Standard PC (i440FX + PIIX, 1996), BIOS version 1.16.1-2.fc37 dated 04/01/2014 ... Call Trace:
sg_build_reserve+0x5c/0xa0 sg_add_sfp+0x168/0x270 sg_open+0x16e/0x340 chrdev_open+0xbe/0x230 do_dentry_open+0x175/0x480 vfs_open+0x34/0xf0 do_open+0x265/0x3d0 path_openat+0x110/0x290 do_filp_open+0xc3/0x170 do_sys_openat2+0x71/0xe0 __x64_sys_openat+0x6d/0xa0 do_syscall_64+0x62/0x310 entry_SYSCALL_64_after_hwframe+0x76/0x7e
The fix is to use module_param_cb to validate and reject invalid values assigned to def_reserved_size.
AnalysisAI
Soft CPU lockup in the Linux kernel SCSI generic (sg) driver allows a local user to trigger a denial of service by writing an out-of-range value directly to the def_reserved_size sysfs module parameter, bypassing the validation enforced by sg_proc_write_dressz. Setting def_reserved_size to -1 via /sys/module/sg/parameters/def_reserved_size causes sg_build_reserve to enter a non-terminating allocation loop on the next open() of any /dev/sgX device, hanging the affected CPU core for 26+ seconds until the kernel watchdog fires. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires all of the following: (1) local access to the target system with write permission to /sys/module/sg/parameters/def_reserved_size - on default Linux distributions this path is writable only by root or a process with CAP_SYS_ADMIN, making the effective privilege requirement higher than CVSS PR:L may imply; security teams should audit the actual permission bits on their systems; (2) the sg (SCSI generic) kernel module must be loaded, which occurs only on systems with SCSI generic hardware present or where the module is explicitly loaded - systems without SCSI devices running the module autoload path are not affected; (3) an open() call on a /dev/sgX node must occur after the parameter is poisoned, which the attacker can trigger with their own process. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 3.1 score of 5.5 (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) correctly scopes this as a local, availability-only issue with no confidentiality or integrity impact. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | A local user with sufficient privilege writes -1 to /sys/module/sg/parameters/def_reserved_size, bypassing the validation that sg_proc_write_dressz would otherwise enforce. The attacker (or any subsequent process) then opens /dev/sg0, causing sg_build_reserve to enter a non-terminating allocation loop with the corrupted parameter value; the kernel watchdog fires after approximately 26 seconds as the CPU hangs, degrading system availability. … |
| Remediation | The primary fix is to upgrade to a patched Linux kernel on the applicable stable branch: 5.10.258, 5.15.209, 6.1.175, 6.6.141, 6.12.91, 6.18.33, 7.0.10, or 7.1 (or any later release in those series). … Detailed patch versions, workarounds, and compensating controls in full report. |
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Same weakness CWE-667 – Improper Locking
View allSame technique Authentication Bypass
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External POC / Exploit Code
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