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| author | Karl Mehltretter <kmehltretter@gmail.com> | 2026-08-29 23:32:55 +0200 |
|---|---|---|
| committer | Paul Moore <paul@paul-moore.com> | 2026-09-15 23:30:07 -0400 |
| commit | 8c0c602202b9a4909b00bc3354e3c0355bc69e65 (patch) | |
| tree | 54c9a5766a8979e5723f56fa3ee518aa27ea4cae /security | |
| parent | 8861db305103107199b1426f25fde1fb6d465583 (diff) | |
| download | lwn-8c0c602202b9a4909b00bc3354e3c0355bc69e65.tar.gz lwn-8c0c602202b9a4909b00bc3354e3c0355bc69e65.zip | |
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.
Cc: stable@vger.kernel.org
Fixes: 82544d36b172 ("selinux: fix overlayfs mmap() and mprotect() access checks")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Diffstat (limited to 'security')
| -rw-r--r-- | security/selinux/hooks.c | 9 | ||||
| -rw-r--r-- | security/selinux/include/objsec.h | 2 |
2 files changed, 9 insertions, 2 deletions
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index e5e17f100aae..b0bb5f8c90bf 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -3843,13 +3843,20 @@ static int selinux_file_alloc_security(struct file *file) return 0; } +static inline u32 selinux_file_user_sid(const struct file *file) +{ + if (unlikely(file->f_mode & FMODE_BACKING)) + return selinux_backing_file(file)->uf_sid; + return selinux_file(file)->sid; +} + static int selinux_backing_file_alloc(struct file *backing_file, const struct file *user_file) { struct backing_file_security_struct *bfsec; bfsec = selinux_backing_file(backing_file); - bfsec->uf_sid = selinux_file(user_file)->sid; + bfsec->uf_sid = selinux_file_user_sid(user_file); return 0; } diff --git a/security/selinux/include/objsec.h b/security/selinux/include/objsec.h index 3c0a16ec978b..853f7266ed18 100644 --- a/security/selinux/include/objsec.h +++ b/security/selinux/include/objsec.h @@ -87,7 +87,7 @@ struct file_security_struct { }; struct backing_file_security_struct { - u32 uf_sid; /* associated user file fsec->sid */ + u32 uf_sid; /* top-level user file fsec->sid */ }; struct superblock_security_struct { |