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

[Lev R. Oshvang .]

On Thu, May 14, 2020 at 6:48 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Thu, May 14, 2020 at 11:14:04AM +0300, Lev R. Oshvang . wrote:
> > New sysctl is indeed required to allow userspace that places scripts
> > or libs under noexec mounts.
>
> But since this is a not-uncommon environment, we must have the sysctl
> otherwise this change would break those systems.
>
 But I proposed sysctl on a line below.

> > fs.mnt_noexec_strict =1 (allow, e) , 1 (deny any file with --x
> > permission), 2 (deny when O_MAYEXEC absent), for any file with ---x
> > permissions)
>
> I don't think we want another mount option -- this is already fully
> expressed with noexec and the system-wide sysctl.
>
> --

The intended use of proposed sysctl is to ebable sysadmin to decide
whar is desired semantics  mount with NO_EXEC option.

fs.mnt_noexec_scope =0 |1|2|3
0  - means old behaviour i.e do nor run executables and scripts (default)
1 - deny any file with --x permissions, i.e executables , script and libs
2 - deny any file when O_MAYEXEC is present.

I think this is enough to handle all use cases and to not break
current sysadmin file mounts setting
I oppose the new O_MAY_EXECMOUNT flag, kernel already has MNT_NO_EXEC,
SB_NOEXEC and SB_I_NOEXEC and I frankly do not understand why so many
variants exist.
Lev