Re: [RFC PATCH v2 2/2] selinux: fix overlayfs mmap() and mprotect() access checks

[Paul Moore <paul@paul-moore.com>]

On Mon, Mar 23, 2026 at 12:25 AM Paul Moore <paul@paul-moore.com> wrote:
>
> The existing SELinux security model for overlayfs is to allow access if
> the current task is able to access the top level file (the "user" file)
> and the mounter's credentials are sufficient to access the lower
> level file (the "backing" file).  Unfortunately, the current code does
> not properly enforce these access controls for both mmap() and mprotect()
> operations on overlayfs filesystems.
>
> This patch makes use of the newly created security_mmap_backing_file()
> LSM hook to provide the missing backing file enforcement for mmap()
> operations, and leverages the backing file API and new LSM blob to
> provide the necessary information to properly enforce the mprotect()
> access controls.
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Paul Moore <paul@paul-moore.com>
> ---
>  security/selinux/hooks.c          | 252 ++++++++++++++++++++++--------
>  security/selinux/include/objsec.h |  17 ++
>  2 files changed, 200 insertions(+), 69 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index d8224ea113d1..2a3d524dce24 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -1745,6 +1745,60 @@ static inline int file_path_has_perm(const struct cred *cred,
>  static int bpf_fd_pass(const struct file *file, u32 sid);
>  #endif
>
> +static int __file_has_perm(bool bf_user_file, const struct cred *cred,
> +                          const struct file *file, u32 av)
> +
> +{
> +       struct common_audit_data ad;
> +       struct inode *inode;
> +       u32 ssid = cred_sid(cred);
> +       u32 tsid_fd;
> +       int rc;
> +
> +       if (bf_user_file) {
> +               struct backing_file_security_struct *bfsec;
> +               const struct path *path;
> +
> +               if (WARN_ON(!(file->f_mode & FMODE_BACKING)))
> +                       return -EPERM;

Based on other code paths, we should return -EIO here.  I've updated
the patch, but I'm holding off on posting another version for a day or
so in case anyone else is able to take a look.

> +               bfsec = selinux_backing_file(file);
> +               path = backing_file_user_path(file);
> +               tsid_fd = bfsec->uf_sid;
> +               inode = d_inode(path->dentry);
> +
> +               ad.type = LSM_AUDIT_DATA_PATH;
> +               ad.u.path = *path;
> +       } else {
> +               struct file_security_struct *fsec = selinux_file(file);
> +
> +               tsid_fd = fsec->sid;
> +               inode = file_inode(file);
> +
> +               ad.type = LSM_AUDIT_DATA_FILE;
> +               ad.u.file = file;
> +       }
> +
> +       if (ssid != tsid_fd) {
> +               rc = avc_has_perm(ssid, tsid_fd, SECCLASS_FD, FD__USE, &ad);
> +               if (rc)
> +                       return rc;
> +       }
> +
> +#ifdef CONFIG_BPF_SYSCALL
> +       /* regardless of backing vs user file, use the underlying file here */
> +       rc = bpf_fd_pass(file, ssid);
> +       if (rc)
> +               return rc;
> +#endif
> +
> +       /* av is zero if only checking access to the descriptor. */
> +       if (av)
> +               return inode_has_perm(cred, inode, av, &ad);
> +
> +       return 0;
> +}

-- 
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