Re: [PATCH RFC v2 1/2] selinux: Fix selinux_sb_mnt_opts_compat()

[Paul Moore]

On Thu, Jan 20, 2022 at 4:50 PM Scott Mayhew <smayhew@xxxxxxxxxx> wrote:
>
> selinux_sb_mnt_opts_compat() is called under the sb_lock spinlock and
> shouldn't be performing any memory allocations.  Fix this by parsing the
> sids at the same time we're chopping up the security mount options
> string and then using the pre-parsed sids when doing the comparison.
>
> Fixes: cc274ae7763d ("selinux: fix sleeping function called from invalid context")
> Fixes: 69c4a42d72eb ("lsm,selinux: add new hook to compare new mount to an existing mount")
> Signed-off-by: Scott Mayhew <smayhew@xxxxxxxxxx>
> ---
>  security/selinux/hooks.c | 112 ++++++++++++++++++++++++++-------------
>  1 file changed, 76 insertions(+), 36 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 5b6895e4fc29..f27ca9e870c0 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -342,6 +342,11 @@ static void inode_free_security(struct inode *inode)
>
>  struct selinux_mnt_opts {
>         const char *fscontext, *context, *rootcontext, *defcontext;
> +       u32 fscontext_sid;
> +       u32 context_sid;
> +       u32 rootcontext_sid;
> +       u32 defcontext_sid;
> +       unsigned short preparsed;
>  };

Is the preparsed field strictly necessary?  Can't we just write the
code to assume that if a given SID field is not SECSID_NULL then it is
valid/preparsed?

> @@ -598,12 +603,11 @@ static int bad_option(struct superblock_security_struct *sbsec, char flag,
>         return 0;
>  }
>
> -static int parse_sid(struct super_block *sb, const char *s, u32 *sid,
> -                    gfp_t gfp)
> +static int parse_sid(struct super_block *sb, const char *s, u32 *sid)
>  {
>         int rc = security_context_str_to_sid(&selinux_state, s,
> -                                            sid, gfp);
> -       if (rc)
> +                                            sid, GFP_KERNEL);
> +       if (rc && sb != NULL)
>                 pr_warn("SELinux: security_context_str_to_sid"
>                        "(%s) failed for (dev %s, type %s) errno=%d\n",
>                        s, sb->s_id, sb->s_type->name, rc);

It seems like it would still be useful to see the warning even when sb
is NULL, wouldn't you say?  How about something like this:

  if (rc)
    pr_warn("SELinux: blah blah blah (dev %s, type %s) blah blah\n",
            (sb ? sb->s_id : "?"),
            (sb ? sb->s_type->name : "?"));

> @@ -976,6 +976,9 @@ static int selinux_add_opt(int token, const char *s, void **mnt_opts)
>  {
>         struct selinux_mnt_opts *opts = *mnt_opts;
>         bool is_alloc_opts = false;
> +       bool preparse_sid = false;
> +       u32 sid;
> +       int rc;
>
>         if (token == Opt_seclabel)
>                 /* eaten and completely ignored */
> @@ -991,26 +994,57 @@ static int selinux_add_opt(int token, const char *s, void **mnt_opts)
>                 is_alloc_opts = true;
>         }
>
> +       if (selinux_initialized(&selinux_state))
> +               preparse_sid = true;

Since there is no looping in selinux_add_opt, and you can only specify
one token/option for a given call to this function, it seems like we
can do away with preparse_sid and just do the selinux_initialized(...)
check directly in the code below, yes?

>         switch (token) {
>         case Opt_context:
>                 if (opts->context || opts->defcontext)
>                         goto err;
>                 opts->context = s;
> +               if (preparse_sid) {
> +                       rc = parse_sid(NULL, s, &sid);
> +                       if (rc == 0) {
> +                               opts->context_sid = sid;
> +                               opts->preparsed |= CONTEXT_MNT;
> +                       }
> +               }

Is there a reason why we need a dedicated sid variable as opposed to
passing opt->context_sid as the parameter?  For example:

  rc = parse_sid(NULL, s, &opts->context_sid);

-- 
paul moore
paul-moore.com