The Linux kernel received a small but important correction to its DRM/TTM subsystem that eliminates an instance of
undefined behavior triggered by a signed left-shift of a 32‑bit integer — a fix tracked as CVE‑2022‑50390 that changes a flag definition from a signed shift to an unsigned shift to satisfy UBSAN and prevent architecture-dependent misbehavior.
Background
The Trusted Translation Memory (TTM) helpers inside the Linux DRM (Direct Rendering Manager) stack use a set of bit flags to represent state for page-based buffer objects. One of those flags,
TTM_TT_FLAG_PRIV_POPULATED, was originally defined using a literal left-shift expression that relies on the default type of the literal
1. On 32‑bit platforms (or more generally where
int is a 32‑bit signed type) the expression
1 << 31 attempts to set the sign bit of a signed
int, which the C standard defines as
undefined behavior. Running kernels built with the Undefined Behavior Sanitizer (UBSAN) produced a runtime warning and stack trace pointing to that header definition as the immediate source of the problem. This class of issue is not conceptual — it’s practical. When a left-shift expression produces a value that does not fit the source type, compilers and CPUs may produce platform-dependent results, sign‑extend unexpectedly, or trigger sanitizer traps. The kernel commit that resolved CVE‑2022‑50390 corrected the macro to force the constant to be
unsigned before shifting, removing the undefined behavior without changing the intended bit value.
Overview of the fix
What changed in the source
- Before:
#define TTM_TT_FLAG_PRIV_POPULATED (1 << 31)
- After:
#define TTM_TT_FLAG_PRIV_POPULATED (1U << 31)
The patch replaces
1 with
1U so the literal is an unsigned int and the left-shift operation produces an unsigned value that is well-defined when shifting into the top bit of a 32‑bit unsigned container. The change is intentionally minimal: it preserves semantics (the same bit position) and removes the UBSAN warning while keeping downstream ABI and behavior unchanged. The upstream commit and diff show this single-line edit.
Why the change matters
Left‑shifting a signed value into the sign bit is undefined in C. On some compilers or architectures that could, for example, produce a negative value, cause sign-extension when later assigned to a wider type, or — when the kernel is instrumented with UBSAN — cause a runtime diagnostic that may destabilize test/CI systems or reveal a potential crash vector in certain corner cases. Changing the literal to unsigned guarantees the operation is defined and consistent across compilers and architectures.
Technical analysis
The root cause in detail
- Integer literal types: In C, the literal
1 is of type int (signed) unless suffixed. On platforms where int is 32 bits, the expression 1 << 31 attempts to form 0x80000000 in a signed 32‑bit type, which cannot be represented as a positive int and thus yields undefined behavior.
- Sanitizer evidence: UBSAN emitted a shift-out-of-bounds warning, with a call trace leading into DRM/TTM call paths. The stack trace made the header definition and the shift the proximate cause. Distributions and vulnerability trackers reproduced this UBSAN trace in their advisories.
Why (1U << 31) is correct and safe
- The
U suffix changes the literal's type to unsigned int. Shifting an unsigned value into its highest bit is well-defined: for a 32-bit unsigned int, (1U << 31) yields 0x80000000 as intended.
- The flag value is used in a 32‑bit page_flags field; retaining the exact bit position is necessary for compatibility. Converting the literal to unsigned keeps the mask identical while guaranteeing defined behavior on all platforms and under sanitizers.
Alternatives and why the chosen fix is preferred
- Using
BIT(31) or an explicit UINT32_C(0x80000000) would also be acceptable. The upstream choice of 1U << 31 is the minimal surgical edit that follows long‑standing kernel style patterns for such constants and is easier to backport to stable trees.
- The fix is intentionally non-invasive: it addresses the undefined behavior without changing semantics or introducing new helper macros or conditional logic, which reduces the risk of regressions.
Impact and exploitability
Severity and classification
Multiple vendor trackers and the NVD summarize this as a correctness/UBSAN fix. The practical impact classified by distributors is
medium in many postings, primarily because the observable consequence is an availability or stability issue (kernel oops or diagnostic trap under UBSAN), rather than an immediate privilege escalation vector.
Attack surface and exploitation model
- Vector: Local. Triggering the affected paths requires code paths that exercise the DRM/TTM machinery (for example, memory movement, populating/unpopulating page arrays, or display modesetting).
- Privileges: In many desktop Linux setups unprivileged processes indirectly reach DRM code via compositors, video players, or sandboxed GPU workloads; therefore, the effective privilege barrier can be low in some configurations.
- Realistic outcome: The primary risk is denial-of-service (kernel oops, driver reset, or instability). There is no public evidence that this shift problem directly yields arbitrary code execution or privilege escalation by itself. Vendors and trackers treat it as a correctness bug whose exploitability is limited to causing instability, though local DoS primitives can be attractive for attackers seeking to disrupt shared infrastructure.
Evidence of in‑the‑wild exploitation
As of the public advisories and cross‑references at disclosure time, there were no verified reports of active exploitation that leveraged this specific shift‑based undefined behavior. Absence of proof of exploit does not equal absence of risk — kernel oopses and deterministic crash primitives can be repurposed — but the public record does not show active weaponization. Advisories accordingly prioritize patching for host stability and multi‑tenant safety.
Vendor and distribution response
- Upstream kernel commit: The single-line fix was applied in upstream kernel trees; the commit message and diff are publicly available and clearly show the change from
(1 << 31) to (1U << 31).
- NVD and distribution trackers: The NVD entry authors and distributions (Ubuntu/SUSE/others) recorded CVE‑2022‑50390 with a summary of the fix and reproduced the UBSAN trace. Ubuntu's security tracker and OSV mirrors list the CVE and map it to kernel package updates in their respective release timelines.
- Red Hat / enterprise trackers: Bugzilla entries and vendor advisories mirror the same diagnosis and map the upstream commit into stable backports where appropriate; vendors advise installing patched kernel packages when available.
Detection and triage for operators
How to tell if you're exposed
- Kernel version mapping: Check your distribution’s security advisory to determine whether the kernel package version you run includes the stable commit that fixes CVE‑2022‑50390. For many distributions this mapping is published in their security trackers.
- Kernel logs: On systems instrumented with UBSAN (typical in test/CI kernels), look for messages like UBSAN: shift-out-of-bounds with a call trace referencing drm/ttm and include/drm/ttm/ttm_tt.h as shown in advisories.
- Module / device exposure: Identify hosts where GPU device nodes are exposed to unprivileged users or to containers (check /dev/dri/* and udev/group permissions) — these hosts should be prioritized.
Short triage checklist
- Run uname -r and list kernel package versions across fleets.
- Query your distro security tracker for the CVE and fixed package mapping.
- Inspect dmesg/journal logs for UBSAN shift warnings or DRM/TTM stack traces.
- Identify hosts exposing /dev/dri to untrusted or multi‑tenant workloads and escalate patching priority for those systems.
Remediation and mitigations
Definitive fix
- Install the vendor-supplied kernel update that contains the upstream commit (or rebuild kernels that include the commit) and reboot. Kernel-level fixes only take effect after booting into the patched kernel.
Interim mitigations (if patching is delayed)
- Restrict access to DRM device nodes:
- Use udev rules and group membership to limit who can open /dev/dri/*.
- Remove device passthrough or --device bindings from untrusted containers.
- Increase telemetry and monitoring:
- Add alerting for kernel oopses mentioning DRM/TTM symbols.
- Collect and preserve full oops stack traces (kdump, persistent journal, serial console) to aid analysis.
- For embedded or vendor kernels: engage vendors for backport guidance or updated images if you cannot rebuild kernels yourself.
Critical assessment: strengths of the remediation and residual risk
Strengths
- The upstream change is minimal and low-risk: a one-line edit that removes undefined behavior without changing ABI or logic.
- The fix follows established kernel practice (make constants unsigned or use safe macros), which simplifies backporting and validation in stable trees.
- Multiple independent trackers, upstream commit logs, and distribution advisories converge on the same diagnosis and remediation path, increasing confidence the root cause was correctly identified and addressed.
Residual and systemic risks
- Long-tail vendor kernels: Embedded devices, SoC vendor trees, Android OEM kernels, and other downstream forks may lag or omit the upstream fix, leaving many devices exposed for longer periods.
- Access control misconfigurations: Systems that intentionally grant unprivileged or containerized access to DRM devices (for GPU acceleration in CI or multi‑tenant hosting) remain more exposed than locked-down hosts.
- Combinatorial bugs: While this fix addresses one undefined behavior instance, DRM and driver subsystems historically contain other mixed-width arithmetic and assignment pitfalls; a single surgical patch does not eliminate the general class of arithmetic/UB issues. Security teams should remain vigilant and monitor advisories for related fixes.
Practical recommendations (prioritized)
- Inventory: Identify kernels and hosts that load DRM modules or expose GPU devices to untrusted users.
- Patch: Apply vendor kernel updates that include the upstream stable commit and reboot at the earliest maintenance window.
- Hardening: If patching will be delayed, restrict /dev/dri access and remove GPU passthrough from untrusted containers.
- Monitoring: Add SIEM/telemetry rules to detect DRM-related oopses, and capture full kernel traces for any crashes.
- Long-term: For teams that ship or maintain vendor kernels, adopt a policy of scanning for UB-sanitizer and integer-width issues (or enabling UBSAN in test builds) to catch similar issues earlier in the cycle.
Conclusion
CVE‑2022‑50390 is an instructive example of how a tiny arithmetic detail — a literal type on a shift operation — can produce undefined behavior visible under sanitizers and potentially destabilize kernel components. The upstream remediation was appropriately narrow: change the literal to unsigned and remove the UBSAN trap while preserving the intended semantics. Operators should treat this as a stability/availability fix and prioritize kernel package updates on hosts that expose DRM devices to untrusted code, while vendors maintaining custom kernels must backport the commit to their trees. The lesson for developers is timeless: mixed-width arithmetic and implicit signedness assumptions are a perennial source of subtle kernel bugs, and sanitizers like UBSAN remain valuable tools to find and eliminate such issues before they become reliability or security problems.
Source: MSRC
Security Update Guide - Microsoft Security Response Center