From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63910][MODERATE 7.0] dma-buf: fix UAF in dma_buf_fd() tracepoint Date: Sun, 19 Jul 2026 22:49:57 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63910 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: b569f86e2f8dbf6f11d31d3de794d22e18098b23 List-Id: CVE: CVE-2026-63910 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: dma-buf: fix UAF in dma_buf_fd() tracepoint Commit: b569f86e2f8dbf6f11d31d3de794d22e18098b23 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b569f86e2f8dbf6f11d31d3de794d22e18098b23 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63910 Analysis date: Sun, 19 Jul 2026 22:49:57 -0400 ActionableScore: 5 ActionableScore lower bound: 4 Actionable bucket: Actionable Moderate at minimum Manual review required: YES Summary: A local race in dma_buf_fd() can publish an fd before DMA_BUF_TRACE() runs, allowing a racing close() to free the dma_buf and causing a tracepoint slab use-after-free with realistic DoS impact and manual-review-worthy memory corruption risk. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63910 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63910 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: MODERATE 7.0 Manual review required: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-63910 MODERATE CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-367;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in dma_buf_fd() can expose a live file descriptor before DMA_BUF_TRACE() runs. A second thread sharing the same file descriptor table can close the new fd in the small window after FD_ADD() and before the tracepoint, dropping the last reference and freeing the dma_buf while the tracepoint still dereferences it. This is a slab use-after-free in the dma-buf tracepoint path and can cause a kernel crash. Because the tracepoint dereference also touches synchronization state such as dmabuf name_lock, memory corruption and privilege escalation cannot be excluded for manual review. For the CVSS the PR:L is used because the high risk scenario assumes a local user or process can create dma-buf fds and race close() on a shared fd table, while tracepoint availability is an environmental precondition. The issue is not network reachable.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) SKIP CVE-2026-63910 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 281a226314238e8adb08d11b765347687ffb38b9 YES NO NO unknown MAYBE DANGER UAF SYZBOT KPANIC MEMORY - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=4 ## 1. ActionableScore * Conservative score: 4 * Paranoid score: 5 * Final recommended bucket: **Actionable Moderate at minimum**:: ## 2. Signal breakdown * Local unprivileged trigger: +1. A local process that can obtain a dma-buf fd through an available exporter may race another thread sharing the same fd table. * Memory corruption, weak primitive: +1. This is a race-dependent slab UAF dereference, but the patch does not show attacker-controlled reclaim, overwrite, callback control, or type confusion. * Real lifetime corruption: +1. The fd becomes visible before the tracepoint, allowing close() to drop the last reference and free the dma_buf before DMA_BUF_TRACE() dereferences it. * Reliable kernel crash / strong DoS: +1. The commit explicitly describes slab-use-after-free and syzbot reported the issue. * Broad/common subsystem exposure: +1. dma-buf is common on graphics, media, GPU, and Android-like systems, although practical exposure depends on accessible exporters. * Hard race / timing window: -1 for conservative score. The close must occur between fd publication and tracepoint execution. * Paranoid adjustment: +1 relative to conservative by not applying the hard-race penalty, because tracing can widen the window and syzbot indicates practical reproducibility. ## 3. Reachability analysis The bug is local, not network reachable. Triggering requires a process to obtain a dma-buf fd and use another thread sharing the same file descriptor table to close it during the narrow window after fd publication and before DMA_BUF_TRACE(). Namespace or container impact depends on whether the containerized workload has access to dma-buf exporters such as GPU, DRM, media, or other device interfaces. A key environmental condition is tracepoint activity, because the UAF occurs in the tracepoint path. Call-site confidence: high. The changed function and lifetime ordering are directly present in the patch and explained by the commit. ## 4. Severity interpretation This is more than an ordinary Moderate because it is a real kernel UAF lifetime race in a common subsystem. Realistic demonstrated impact is primarily local kernel crash or DoS. Privilege escalation is not strongly demonstrated because there is no shown reclaim, stale object replacement, write-after-free, type confusion, callback dispatch, or refcount takeover. The paranoid score keeps it actionable for manual review because kernel UAFs in shared fd lifetime paths are historically easy to underestimate. ## 5. One-sentence report phrase A local race in dma_buf_fd() can publish an fd before DMA_BUF_TRACE() runs, allowing a racing close() to free the dma_buf and causing a tracepoint slab use-after-free with realistic DoS impact and manual-review-worthy memory corruption risk. ## 6. Manual review recommendation MANUAL CHECK REQUIRED. This is a syzbot-reported race-based UAF in a common kernel subsystem, and while the visible primitive is a stale dereference rather than a demonstrated LPE primitive, it should not be auto-closed. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: dma-buf: fix UAF in dma_buf_fd() tracepoint Commit: b569f86e2f8dbf6f11d31d3de794d22e18098b23 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b569f86e2f8dbf6f11d31d3de794d22e18098b23 Commit description: Once FD_ADD() returns, the fd is live in the file descriptor table and a thread sharing that table can close() it before DMA_BUF_TRACE() runs. The close drops the last reference, __fput() frees the dma_buf, and the tracepoint then dereferences dmabuf to take dmabuf->name_lock -- slab-use-after-free. Split FD_ADD() back into get_unused_fd_flags() + fd_install() and emit the tracepoint between them. While the fdtable slot is reserved with a NULL file pointer, a racing close() returns -EBADF without entering __fput(), so the dma_buf stays alive across the trace. Same approach as commit 2d76319 ("dma-buf: fix UAF in dma_buf_put() tracepoint"). This undoes the FD_ADD() conversion done in commit 34dfce5 ("dma: convert dma_buf_fd() to FD_ADD()"); FD_ADD() has no place to hook the tracepoint safely. Reported-by: syzbot+7f4987d0afb97dd090cb@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=7f4987d0afb97dd090cb Fixes: 281a226 ("dma-buf: add some tracepoints to debug.") Cc: stable@vger.kernel.org # 7.0.x Signed-off-by: David Carlier Reviewed-by: Christian König Signed-off-by: Sumit Semwal Link: https://patch.msgid.link/20260523181446.69525-1-devnexen@gmail.com Signed-off-by: Greg Kroah-Hartman Changed files: drivers/dma-buf/dma-buf.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b569f86e2f8dbf6f11d31d3de794d22e18098b23 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63910 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63910 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 7 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: 4 Paranoid ActionableScore: 5 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: MODERATE KPANIC detected for this report: YES Published priority for this report (same as in Subject): MODERATE 7.0 These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).