Re: [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property

["Lev R. Oshvang ." <levonshe@gmail.com>]

On Wed, May 13, 2020 at 12:09 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Tue, May 05, 2020 at 05:31:52PM +0200, Mickaël Salaün wrote:
> > This new MAY_EXECMOUNT flag enables to check if the underlying mount
> > point of an inode is marked as executable.  This is useful to implement
> > a security policy taking advantage of the noexec mount option.
> >
Security policy is expressed by sysadmin by mount -noexec very clear,
I don't think there is a need
in sysctl, wish is system-wide

> > This flag is set according to path_noexec(), which checks if a mount
> > point is mounted with MNT_NOEXEC or if the underlying superblock is
> > SB_I_NOEXEC.
> >
> > Signed-off-by: Mickaël Salaün <mic@digikod.net>
> > Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
> > Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
> > Cc: Aleksa Sarai <cyphar@cyphar.com>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Kees Cook <keescook@chromium.org>
> > ---
> >  fs/namei.c         | 2 ++
> >  include/linux/fs.h | 2 ++
> >  2 files changed, 4 insertions(+)
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index a320371899cf..33b6d372e74a 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -2849,6 +2849,8 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> >               break;
> >       }
> >
> > +     /* Pass the mount point executability. */
> > +     acc_mode |= path_noexec(path) ? 0 : MAY_EXECMOUNT;
> >       error = inode_permission(inode, MAY_OPEN | acc_mode);
> >       if (error)
> >               return error;
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 313c934de9ee..79435fca6c3e 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -103,6 +103,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
> >  #define MAY_NOT_BLOCK                0x00000080
> >  /* the inode is opened with O_MAYEXEC */
> >  #define MAY_OPENEXEC         0x00000100
> > +/* the mount point is marked as executable */
> > +#define MAY_EXECMOUNT                0x00000200
> >
> >  /*
> >   * flags in file.f_mode.  Note that FMODE_READ and FMODE_WRITE must correspond
>
> I find this name unintuitive, but I cannot think of anything better,
> since I think my problem is that "MAY" doesn't map to the language I
> want to use to describe what this flag is indicating.
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> --
> Kees Cook


I think that the original patch was perfect, I quite it again
@@ -3167,6 +3167,14 @@ static int may_open(struct path *path, int
acc_mode, int flag)

+
+ if ((acc_mode & MAY_OPENEXEC)
+ && (S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode))
+ && (path->mnt && (path->mnt->mnt_flags & MNT_NOEXEC)))
+            return -EACCES;
+
+
+
error = inode_permission(inode, MAY_OPEN | acc_mode);

As I said in the inline comment above, sysadmin had already express
security policy in a very clear way,
mount -noexec !
I would only check inside inode_permission() whether the file mode is
any  ---x  permission and deny such
open when file is opened with O_MAYEXEC under MNT_NOEXEC mount point

New sysctl is indeed required to allow userspace that places scripts
or libs under noexec mounts.
fs.mnt_noexec_strict =0 (allow, e) , 1 (deny any file with --x
permission), 2 (deny when O_MAYEXEC absent), for any file with ---x
permissions)