[PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC

[Mickaël Salaün]

On 14/05/2020 01:27, Kees Cook wrote:
> On Wed, May 13, 2020 at 11:37:16AM -0400, Stephen Smalley wrote:
>> On Tue, May 5, 2020 at 11:33 AM Mickaël Salaün <mic at digikod.net> wrote:
>>>
>>> Enable to forbid access to files open with O_MAYEXEC.  Thanks to the
>>> noexec option from the underlying VFS mount, or to the file execute
>>> permission, userspace can enforce these execution policies.  This may
>>> allow script interpreters to check execution permission before reading
>>> commands from a file, or dynamic linkers to allow shared object loading.
>>>
>>> Add a new sysctl fs.open_mayexec_enforce to enable system administrators
>>> to enforce two complementary security policies according to the
>>> installed system: enforce the noexec mount option, and enforce
>>> executable file permission.  Indeed, because of compatibility with
>>> installed systems, only system administrators are able to check that
>>> this new enforcement is in line with the system mount points and file
>>> permissions.  A following patch adds documentation.
>>>
>>> For tailored Linux distributions, it is possible to enforce such
>>> restriction at build time thanks to the CONFIG_OMAYEXEC_STATIC option.
>>> The policy can then be configured with CONFIG_OMAYEXEC_ENFORCE_MOUNT and
>>> CONFIG_OMAYEXEC_ENFORCE_FILE.
>>>
>>> Being able to restrict execution also enables to protect the kernel by
>>> restricting arbitrary syscalls that an attacker could perform with a
>>> crafted binary or certain script languages.  It also improves multilevel
>>> isolation by reducing the ability of an attacker to use side channels
>>> with specific code.  These restrictions can natively be enforced for ELF
>>> binaries (with the noexec mount option) but require this kernel
>>> extension to properly handle scripts (e.g., Python, Perl).  To get a
>>> consistent execution policy, additional memory restrictions should also
>>> be enforced (e.g. thanks to SELinux).
>>>
>>> Signed-off-by: Mickaël Salaün <mic at digikod.net>
>>> Reviewed-by: Thibaut Sautereau <thibaut.sautereau at ssi.gouv.fr>
>>> Cc: Aleksa Sarai <cyphar at cyphar.com>
>>> Cc: Al Viro <viro at zeniv.linux.org.uk>
>>> Cc: Kees Cook <keescook at chromium.org>
>>> ---
>>
>>> diff --git a/fs/namei.c b/fs/namei.c
>>> index 33b6d372e74a..70f179f6bc6c 100644
>>> --- a/fs/namei.c
>>> +++ b/fs/namei.c
>>> @@ -411,10 +412,90 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
>> <snip>
>>> +#if defined(CONFIG_SYSCTL) && !defined(CONFIG_OMAYEXEC_STATIC)
>>> +int proc_omayexec(struct ctl_table *table, int write, void __user *buffer,
>>> +               size_t *lenp, loff_t *ppos)
>>> +{
>>> +       int error;
>>> +
>>> +       if (write) {
>>> +               struct ctl_table table_copy;
>>> +               int tmp_mayexec_enforce;
>>> +
>>> +               if (!capable(CAP_MAC_ADMIN))
>>> +                       return -EPERM;
>>
>> Not fond of using CAP_MAC_ADMIN here (or elsewhere outside of security
>> modules).  The ability to set this sysctl is not equivalent to being
>> able to load a MAC policy, set arbitrary MAC labels on
>> processes/files, etc.
> 
> That's fair. In that case, perhaps this could just use the existing
> _sysadmin helper? (Though I should note that these perm checks actually
> need to be in the open, not the read/write ... I thought there was a
> series to fix that, but I can't find it now. Regardless, that's
> orthogonal to this series.)

OK, I'll switch to CAP_SYS_ADMIN with proc_dointvec_minmax_sysadmin().