In the Linux kernel, the following vulnerability has been resolved:
selinux: preserve user SID across nested backing files
SELinux saves the user file SID in a backing-file security blob so it
remains available after mmap() replaces vma->vm_file with a backing file.
For nested backing files (overlayfs over overlayfs, or FUSE passthrough
backed by overlayfs), user_file may itself be a backing file. Its
fsec->sid is the SID of the mounter that opened it, rather than the user
that opened the top-level file. mprotect() then checks fd { use } against
the mounter SID. This can incorrectly deny access without a domain
transition, or check the wrong target SID after one.
Copy the saved user SID when user_file is a backing file. Keep using the
regular file SID for the first backing layer.
With two nested overlayfs mounts and SELinux enforcing,
mprotect(PROT_READ) returns EACCES with an fd { use } denial against the
mounter SID. With this change, mprotect() succeeds.
Tested on arm64 QEMU with a small BusyBox initramfs and a purpose-built
SELinux policy. The original test was also repeated with Fedora Cloud
Base 44 userspace and gave the same result.
selinux: preserve user SID across nested backing files
SELinux saves the user file SID in a backing-file security blob so it
remains available after mmap() replaces vma->vm_file with a backing file.
For nested backing files (overlayfs over overlayfs, or FUSE passthrough
backed by overlayfs), user_file may itself be a backing file. Its
fsec->sid is the SID of the mounter that opened it, rather than the user
that opened the top-level file. mprotect() then checks fd { use } against
the mounter SID. This can incorrectly deny access without a domain
transition, or check the wrong target SID after one.
Copy the saved user SID when user_file is a backing file. Keep using the
regular file SID for the first backing layer.
With two nested overlayfs mounts and SELinux enforcing,
mprotect(PROT_READ) returns EACCES with an fd { use } denial against the
mounter SID. With this change, mprotect() succeeds.
Tested on arm64 QEMU with a small BusyBox initramfs and a purpose-built
SELinux policy. The original test was also repeated with Fedora Cloud
Base 44 userspace and gave the same result.
Metrics
Affected Vendors & Products
Advisories
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Fixes
Solution
No solution given by the vendor.
Workaround
No workaround given by the vendor.
References
History
Tue, 06 Oct 2026 09:00:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | In the Linux kernel, the following vulnerability has been resolved: selinux: preserve user SID across nested backing files SELinux saves the user file SID in a backing-file security blob so it remains available after mmap() replaces vma->vm_file with a backing file. For nested backing files (overlayfs over overlayfs, or FUSE passthrough backed by overlayfs), user_file may itself be a backing file. Its fsec->sid is the SID of the mounter that opened it, rather than the user that opened the top-level file. mprotect() then checks fd { use } against the mounter SID. This can incorrectly deny access without a domain transition, or check the wrong target SID after one. Copy the saved user SID when user_file is a backing file. Keep using the regular file SID for the first backing layer. With two nested overlayfs mounts and SELinux enforcing, mprotect(PROT_READ) returns EACCES with an fd { use } denial against the mounter SID. With this change, mprotect() succeeds. Tested on arm64 QEMU with a small BusyBox initramfs and a purpose-built SELinux policy. The original test was also repeated with Fedora Cloud Base 44 userspace and gave the same result. | |
| Title | selinux: preserve user SID across nested backing files | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
|
Projects
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MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-10-06T08:44:51.412Z
Reserved: 2026-09-25T10:25:14.326Z
Link: CVE-2026-98215
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-10-06T09:18:08.107
Modified: 2026-10-06T09:18:08.107
Link: CVE-2026-98215
Redhat
No data.
OpenCVE Enrichment
No data.
Weaknesses
No weakness.