[PATCH v2] security/selinux: fix potential memleak in error branch

[PATCH v2] security/selinux: fix potential memleak in error branch

[Bernard Zhao]

This patch try to fix potential memleak in error branch.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
---
 security/selinux/hooks.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 62d30c0a30c2..8dc140399a23 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -983,18 +983,22 @@ static int selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
 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;
 
 	if (token == Opt_seclabel)	/* eaten and completely ignored */
 		return 0;
 
+	if (!s)
+		return -ENOMEM;
+
 	if (!opts) {
 		opts = kzalloc(sizeof(struct selinux_mnt_opts), GFP_KERNEL);
 		if (!opts)
 			return -ENOMEM;
 		*mnt_opts = opts;
+		is_alloc_opts = true;
 	}
-	if (!s)
-		return -ENOMEM;
+
 	switch (token) {
 	case Opt_context:
 		if (opts->context || opts->defcontext)
@@ -1019,6 +1023,8 @@ static int selinux_add_opt(int token, const char *s, void **mnt_opts)
 	}
 	return 0;
 Einval:
+	if (is_alloc_opts)
+		kfree(opts);
 	pr_warn(SEL_MOUNT_FAIL_MSG);
 	return -EINVAL;
 }
-- 
2.33.1

Re: [PATCH v2] security/selinux: fix potential memleak in error branch

[Ondrej Mosnacek]

On Mon, Dec 6, 2021 at 8:19 AM Bernard Zhao <bernard@vivo.com> wrote:
> This patch try to fix potential memleak in error branch.
>
> Signed-off-by: Bernard Zhao <bernard@vivo.com>
> ---
>  security/selinux/hooks.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 62d30c0a30c2..8dc140399a23 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -983,18 +983,22 @@ static int selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
>  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;
>
>         if (token == Opt_seclabel)      /* eaten and completely ignored */
>                 return 0;
>
> +       if (!s)
> +               return -ENOMEM;
> +
>         if (!opts) {
>                 opts = kzalloc(sizeof(struct selinux_mnt_opts), GFP_KERNEL);
>                 if (!opts)
>                         return -ENOMEM;
>                 *mnt_opts = opts;
> +               is_alloc_opts = true;
>         }
> -       if (!s)
> -               return -ENOMEM;
> +
>         switch (token) {
>         case Opt_context:
>                 if (opts->context || opts->defcontext)
> @@ -1019,6 +1023,8 @@ static int selinux_add_opt(int token, const char *s, void **mnt_opts)
>         }
>         return 0;
>  Einval:
> +       if (is_alloc_opts)
> +               kfree(opts);
>         pr_warn(SEL_MOUNT_FAIL_MSG);
>         return -EINVAL;
>  }
> --
> 2.33.1

The problem is a bit more tricky... As is, this patch will lead to
double frees in some cases. Look at security_sb_eat_lsm_opts()
callers, for example - some of them don't do anything when error is
returned, some call security_free_mnt_opts() on the opts regardless,
some will let it store the opts in fc->security, where
put_fs_context() will eventually call security_free_mnt_opts() on
them.

You need to at least *mnt_opts = NULL after kfree(opts), but it would
be also nice to make the LSM hook callers more consistent in what they
do in the error path and document the fact that *mnt_opts will be NULL
on error in lsm_hooks.h (in case of the sb_eat_lsm_opts hook).

-- 
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.

答复: [PATCH v2] security/selinux: fix potential memleak in error branch

[赵军奎]

-----邮件原件-----
发件人: bernard@vivo.com <bernard@vivo.com> 代表 Ondrej Mosnacek
发送时间: 2021年12月6日 17:11
收件人: 赵军奎 <bernard@vivo.com>
抄送: Paul Moore <paul@paul-moore.com>; Stephen Smalley <stephen.smalley.work@gmail.com>; Eric Paris <eparis@parisplace.org>; SElinux list <selinux@vger.kernel.org>; Linux kernel mailing list <linux-kernel@vger.kernel.org>
主题: Re: [PATCH v2] security/selinux: fix potential memleak in error branch

On Mon, Dec 6, 2021 at 8:19 AM Bernard Zhao <bernard@vivo.com> wrote:
> This patch try to fix potential memleak in error branch.
>
> Signed-off-by: Bernard Zhao <bernard@vivo.com>
> ---
>  security/selinux/hooks.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index 
> 62d30c0a30c2..8dc140399a23 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -983,18 +983,22 @@ static int selinux_sb_clone_mnt_opts(const 
> struct super_block *oldsb,  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;
>
>         if (token == Opt_seclabel)      /* eaten and completely ignored */
>                 return 0;
>
> +       if (!s)
> +               return -ENOMEM;
> +
>         if (!opts) {
>                 opts = kzalloc(sizeof(struct selinux_mnt_opts), GFP_KERNEL);
>                 if (!opts)
>                         return -ENOMEM;
>                 *mnt_opts = opts;
> +               is_alloc_opts = true;
>         }
> -       if (!s)
> -               return -ENOMEM;
> +
>         switch (token) {
>         case Opt_context:
>                 if (opts->context || opts->defcontext) @@ -1019,6 
> +1023,8 @@ static int selinux_add_opt(int token, const char *s, void **mnt_opts)
>         }
>         return 0;
>  Einval:
> +       if (is_alloc_opts)
> +               kfree(opts);
>         pr_warn(SEL_MOUNT_FAIL_MSG);
>         return -EINVAL;
>  }
> --
> 2.33.1

>The problem is a bit more tricky... As is, this patch will lead to double frees in some cases. Look at security_sb_eat_lsm_opts() callers, for example - some of them don't do anything when error is returned, some call 
>security_free_mnt_opts() on the opts regardless, some will let it store the opts in fc->security, where
>put_fs_context() will eventually call security_free_mnt_opts() on them.

>You need to at least *mnt_opts = NULL after kfree(opts), but it would be also nice to make the LSM hook callers more consistent in what they do in the error path and document the fact that *mnt_opts will be NULL
>on error in lsm_hooks.h (in case of the sb_eat_lsm_opts hook).
Hi Ondrej Mosnacek:

Thanks for your comments!
I am not sure if there is some gap, for this part " it would be also nice to make the LSM hook callers more consistent in what they do in the error path and document the fact that *mnt_opts will be NULL
on error in lsm_hooks.h (in case of the sb_eat_lsm_opts hook)"
I am not sure if this is OK: 
116   * @sb_eat_lsm_opts:
117   * 	Eat (scan @orig options) and save them in @mnt_opts.
118   *     If error is returned, then the *mnt_opts will be NULL.
Please help to double check, thanks!

BR//Bernard
--
>Ondrej Mosnacek
>Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.

Re: [PATCH v2] security/selinux: fix potential memleak in error branch

[Ondrej Mosnacek]

On Tue, Dec 7, 2021 at 1:05 PM 赵军奎 <bernard@vivo.com> wrote:
> -----邮件原件-----
> 发件人: bernard@vivo.com <bernard@vivo.com> 代表 Ondrej Mosnacek
> 发送时间: 2021年12月6日 17:11
> 收件人: 赵军奎 <bernard@vivo.com>
> 抄送: Paul Moore <paul@paul-moore.com>; Stephen Smalley <stephen.smalley.work@gmail.com>; Eric Paris <eparis@parisplace.org>; SElinux list <selinux@vger.kernel.org>; Linux kernel mailing list <linux-kernel@vger.kernel.org>
> 主题: Re: [PATCH v2] security/selinux: fix potential memleak in error branch
>
> On Mon, Dec 6, 2021 at 8:19 AM Bernard Zhao <bernard@vivo.com> wrote:
> > This patch try to fix potential memleak in error branch.
> >
> > Signed-off-by: Bernard Zhao <bernard@vivo.com>
> > ---
> >  security/selinux/hooks.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index
> > 62d30c0a30c2..8dc140399a23 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -983,18 +983,22 @@ static int selinux_sb_clone_mnt_opts(const
> > struct super_block *oldsb,  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;
> >
> >         if (token == Opt_seclabel)      /* eaten and completely ignored */
> >                 return 0;
> >
> > +       if (!s)
> > +               return -ENOMEM;
> > +
> >         if (!opts) {
> >                 opts = kzalloc(sizeof(struct selinux_mnt_opts), GFP_KERNEL);
> >                 if (!opts)
> >                         return -ENOMEM;
> >                 *mnt_opts = opts;
> > +               is_alloc_opts = true;
> >         }
> > -       if (!s)
> > -               return -ENOMEM;
> > +
> >         switch (token) {
> >         case Opt_context:
> >                 if (opts->context || opts->defcontext) @@ -1019,6
> > +1023,8 @@ static int selinux_add_opt(int token, const char *s, void **mnt_opts)
> >         }
> >         return 0;
> >  Einval:
> > +       if (is_alloc_opts)
> > +               kfree(opts);
> >         pr_warn(SEL_MOUNT_FAIL_MSG);
> >         return -EINVAL;
> >  }
> > --
> > 2.33.1
>
> >The problem is a bit more tricky... As is, this patch will lead to double frees in some cases. Look at security_sb_eat_lsm_opts() callers, for example - some of them don't do anything when error is returned, some call
> >security_free_mnt_opts() on the opts regardless, some will let it store the opts in fc->security, where
> >put_fs_context() will eventually call security_free_mnt_opts() on them.
>
> >You need to at least *mnt_opts = NULL after kfree(opts), but it would be also nice to make the LSM hook callers more consistent in what they do in the error path and document the fact that *mnt_opts will be NULL
> >on error in lsm_hooks.h (in case of the sb_eat_lsm_opts hook).
> Hi Ondrej Mosnacek:
>
> Thanks for your comments!
> I am not sure if there is some gap, for this part " it would be also nice to make the LSM hook callers more consistent in what they do in the error path and document the fact that *mnt_opts will be NULL
> on error in lsm_hooks.h (in case of the sb_eat_lsm_opts hook)"
> I am not sure if this is OK:
> 116   * @sb_eat_lsm_opts:
> 117   *         Eat (scan @orig options) and save them in @mnt_opts.
> 118   *     If error is returned, then the *mnt_opts will be NULL.
> Please help to double check, thanks!

I'd prefer something like:

If the hook returns 0, the caller is responsible for destroying the
returned @mnt_opts using the @sb_free_mnt_opts hook. The LSMs must not
expect the callers to destroy @mnt_opts if the hook returns an error
and should always set it to NULL in such case.

(You may want to double-check that the other implementations of this
hook (i.e. security/smack/smack_lsm.c) follow that contract and fix
them if necessary.)

Thanks for your efforts to improve this!

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.