Re: [PATCH -next] selinux: Fix potential memory leak in selinux_add_opt

[Paul Moore]

On Mon, Jun 13, 2022 at 9:18 PM xiujianfeng <xiujianfeng@xxxxxxxxxx> wrote:
> 在 2022/6/14 4:22, Paul Moore 写道:
> > On Sat, Jun 11, 2022 at 5:07 AM Xiu Jianfeng <xiujianfeng@xxxxxxxxxx> wrote:
> >> In the entry of selinux_add_opt, *mnt_opts may be assigned to new
> >> allocated memory, and also may be freed and reset at the end of the
> >> function. however, if security_context_str_to_sid failed, it returns
> >> directly and skips the procedure for free and reset, even if it may be
> >> handled at the caller of this function, It is better to handle it
> >> inside.
> >>
> >> Fixes: 70f4169ab421 ("selinux: parse contexts for mount options early")
> >> Signed-off-by: Xiu Jianfeng <xiujianfeng@xxxxxxxxxx>
> >> ---
> >>   security/selinux/hooks.c | 12 +++++++-----
> >>   1 file changed, 7 insertions(+), 5 deletions(-)
> > Have you actually observed a memory leak from the selinux_mnt_opts
> > allocation in selinux_add_opt()?
> >
> > The selinux_add_opt() function has two callers:
> > selinux_sb_eat_lsm_opts() and selinux_fs_context_parse_param().  The
> > former cleans up the selinux_mnt_opts allocation it its error handler
> > while the latter will end up calling
> > security_free_mnt_opts()/selinux_free_mnt_opts() to free the
> > fs_context:security when the fs_context is destroyed.
> >
> > This patch shouldn't be necessary.
>
> I may not have made it clear, I said potential means may have a third
> caller in the future.

Let's not worry about it.  If you wanted to add a comment header to
the function (see selinux_skb_peerlbl_sid() for an example) to make it
clear that callers are responsible for cleaning up @mnt_opts on error
I think that would be okay ... although even that is going to be a
problem in the new mount API case where selinux_add_opt() is going to
be called multiple times.

> I think the error handler as following is not necessary:
>
> err:
>          if (is_alloc_opts) {
>                  kfree(opts);
>                  *mnt_opts = NULL;
>          }
>
> otherwise, some error paths goto err label while others don't, It's
> confusing.

That's a fair point.  Looking at the patch which added it, we should
probably also return EINVAL when @s is NULL instead of ENOMEM.  In
fact, in all the cases where we currently jump to @err, I think we are
guaranteed that @is_alloc_opts is false as it requires a previously
populated @opts.

If you want to submit another patch, I would suggest doing the
following in the patch:

1. Change the @s NULL check to return -EINVAL when @s is NULL.
2. Allocate @opts/@mnt_opts if NULL, but don't call kfree() on the
object in case of error.  The new mount API will cleanup when it is
done and selinux_sb_eat_lsm_opts() will cleanup on error.

If you don't have time to put together a patch for this, let me know and I will.

-- 
paul-moore.com