[PATCH] selinux: allow kernfs symlinks to inherit parent directory context

[PATCH] selinux: allow kernfs symlinks to inherit parent directory context

[Christian Göttsche]

Currently symlinks on kernel filesystems, like sysfs, are labeled on
creation with the parent fs root sid.

Allow symlinks to inherit the parent directory context, so fine-grained
kernfs labeling can be applied to symlinks too and checking contexts
doesn't complain about them.

For backward-compatibility this behavior is contained in a new policy
capability: kernfs_sovereign_symlinks

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
 security/selinux/hooks.c            | 5 ++++-
 security/selinux/include/security.h | 8 ++++++++
 security/selinux/ss/services.c      | 3 ++-
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d9e8b2131..1303bc8c4 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1475,7 +1475,10 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 		/* Default to the fs superblock SID. */
 		sid = sbsec->sid;
 
-		if ((sbsec->flags & SE_SBGENFS) && !S_ISLNK(inode->i_mode)) {
+		if (((sbsec->flags & SE_SBGENFS) &&
+		     (!S_ISLNK(inode->i_mode))) ||
+		    (selinux_policycap_kernfs_sovereign_symlinks() &&
+		     (sbsec->flags & SE_SBGENFS_XATTR))) {
 			/* We must have a dentry to determine the label on
 			 * procfs inodes */
 			if (opt_dentry) {
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index a39f9565d..cc8217848 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -79,6 +79,7 @@ enum {
 	POLICYDB_CAPABILITY_ALWAYSNETWORK,
 	POLICYDB_CAPABILITY_CGROUPSECLABEL,
 	POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
+	POLICYDB_CAPABILITY_KERNFS_SOVEREIGN_SYMLINKS,
 	__POLICYDB_CAPABILITY_MAX
 };
 #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
@@ -209,6 +210,13 @@ static inline bool selinux_policycap_nnp_nosuid_transition(void)
 	return state->policycap[POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION];
 }
 
+static inline bool selinux_policycap_kernfs_sovereign_symlinks(void)
+{
+	struct selinux_state *state = &selinux_state;
+
+	return state->policycap[POLICYDB_CAPABILITY_KERNFS_SOVEREIGN_SYMLINKS];
+}
+
 int security_mls_enabled(struct selinux_state *state);
 int security_load_policy(struct selinux_state *state,
 			 void *data, size_t len);
diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 216ce602a..b70380947 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -73,7 +73,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
 	"extended_socket_class",
 	"always_check_network",
 	"cgroup_seclabel",
-	"nnp_nosuid_transition"
+	"nnp_nosuid_transition",
+	"kernfs_sovereign_symlinks"
 };
 
 static struct selinux_ss selinux_ss;
-- 
2.25.0

Re: [PATCH] selinux: allow kernfs symlinks to inherit parent directory context

[Stephen Smalley]

On 1/24/20 1:42 PM, Christian Göttsche wrote:
> Currently symlinks on kernel filesystems, like sysfs, are labeled on
> creation with the parent fs root sid.
> 
> Allow symlinks to inherit the parent directory context, so fine-grained
> kernfs labeling can be applied to symlinks too and checking contexts
> doesn't complain about them.
> 
> For backward-compatibility this behavior is contained in a new policy
> capability: kernfs_sovereign_symlinks
> 
> Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> ---
>   security/selinux/hooks.c            | 5 ++++-
>   security/selinux/include/security.h | 8 ++++++++
>   security/selinux/ss/services.c      | 3 ++-
>   3 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index d9e8b2131..1303bc8c4 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -1475,7 +1475,10 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
>   		/* Default to the fs superblock SID. */
>   		sid = sbsec->sid;
>   
> -		if ((sbsec->flags & SE_SBGENFS) && !S_ISLNK(inode->i_mode)) {
> +		if (((sbsec->flags & SE_SBGENFS) &&
> +		     (!S_ISLNK(inode->i_mode))) ||
> +		    (selinux_policycap_kernfs_sovereign_symlinks() &&

Not fond of the name.  1) kernfs is a kernel implementation detail, 
shouldn't be exposed to policy; genfs is the policy construct 2) 
sovereign doesn't seem to fit the meaning of this capability; seclabel 
would be more appropriate.

> +		     (sbsec->flags & SE_SBGENFS_XATTR))) {

Why limit this to SE_SBGENFS_XATTR filesystems?  Why not just make the test:
	if ((sbsec->flags & SE_SBGENFS) && (!S_ISLNK(inode->i_mode) || 
selinux_policycap_genfs_symlinkseclabel()))
or similar.

>   			/* We must have a dentry to determine the label on
>   			 * procfs inodes */
>   			if (opt_dentry) {
> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> index a39f9565d..cc8217848 100644
> --- a/security/selinux/include/security.h
> +++ b/security/selinux/include/security.h
> @@ -79,6 +79,7 @@ enum {
>   	POLICYDB_CAPABILITY_ALWAYSNETWORK,
>   	POLICYDB_CAPABILITY_CGROUPSECLABEL,
>   	POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
> +	POLICYDB_CAPABILITY_KERNFS_SOVEREIGN_SYMLINKS,
>   	__POLICYDB_CAPABILITY_MAX
>   };
>   #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
> @@ -209,6 +210,13 @@ static inline bool selinux_policycap_nnp_nosuid_transition(void)
>   	return state->policycap[POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION];
>   }
>   
> +static inline bool selinux_policycap_kernfs_sovereign_symlinks(void)
> +{
> +	struct selinux_state *state = &selinux_state;
> +
> +	return state->policycap[POLICYDB_CAPABILITY_KERNFS_SOVEREIGN_SYMLINKS];
> +}
> +
>   int security_mls_enabled(struct selinux_state *state);
>   int security_load_policy(struct selinux_state *state,
>   			 void *data, size_t len);
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index 216ce602a..b70380947 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -73,7 +73,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
>   	"extended_socket_class",
>   	"always_check_network",
>   	"cgroup_seclabel",
> -	"nnp_nosuid_transition"
> +	"nnp_nosuid_transition",
> +	"kernfs_sovereign_symlinks"
>   };
>   
>   static struct selinux_ss selinux_ss;
> 

Re: [PATCH] selinux: allow kernfs symlinks to inherit parent directory context

[Christian Göttsche]

Am Fr., 24. Jan. 2020 um 19:53 Uhr schrieb Stephen Smalley <sds@tycho.nsa.gov>:
>
> On 1/24/20 1:42 PM, Christian Göttsche wrote:
> > Currently symlinks on kernel filesystems, like sysfs, are labeled on
> > creation with the parent fs root sid.
> >
> > Allow symlinks to inherit the parent directory context, so fine-grained
> > kernfs labeling can be applied to symlinks too and checking contexts
> > doesn't complain about them.
> >
> > For backward-compatibility this behavior is contained in a new policy
> > capability: kernfs_sovereign_symlinks
> >
> > Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> > ---
> >   security/selinux/hooks.c            | 5 ++++-
> >   security/selinux/include/security.h | 8 ++++++++
> >   security/selinux/ss/services.c      | 3 ++-
> >   3 files changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index d9e8b2131..1303bc8c4 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -1475,7 +1475,10 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
> >               /* Default to the fs superblock SID. */
> >               sid = sbsec->sid;
> >
> > -             if ((sbsec->flags & SE_SBGENFS) && !S_ISLNK(inode->i_mode)) {
> > +             if (((sbsec->flags & SE_SBGENFS) &&
> > +                  (!S_ISLNK(inode->i_mode))) ||
> > +                 (selinux_policycap_kernfs_sovereign_symlinks() &&
>
> Not fond of the name.  1) kernfs is a kernel implementation detail,
> shouldn't be exposed to policy; genfs is the policy construct 2)
> sovereign doesn't seem to fit the meaning of this capability; seclabel
> would be more appropriate.

Something like genfs_seclabel_symlinks?

> > +                  (sbsec->flags & SE_SBGENFS_XATTR))) {
>
> Why limit this to SE_SBGENFS_XATTR filesystems?  Why not just make the test:
>         if ((sbsec->flags & SE_SBGENFS) && (!S_ISLNK(inode->i_mode) ||
> selinux_policycap_genfs_symlinkseclabel()))
> or similar.

I somehow thought that this functionality is limited to filesystems
with SE_SBGENFS_XATTR;
so I can expand the check to SE_SBGENFS.

> >                       /* We must have a dentry to determine the label on
> >                        * procfs inodes */
> >                       if (opt_dentry) {
> > diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> > index a39f9565d..cc8217848 100644
> > --- a/security/selinux/include/security.h
> > +++ b/security/selinux/include/security.h
> > @@ -79,6 +79,7 @@ enum {
> >       POLICYDB_CAPABILITY_ALWAYSNETWORK,
> >       POLICYDB_CAPABILITY_CGROUPSECLABEL,
> >       POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
> > +     POLICYDB_CAPABILITY_KERNFS_SOVEREIGN_SYMLINKS,
> >       __POLICYDB_CAPABILITY_MAX
> >   };
> >   #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
> > @@ -209,6 +210,13 @@ static inline bool selinux_policycap_nnp_nosuid_transition(void)
> >       return state->policycap[POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION];
> >   }
> >
> > +static inline bool selinux_policycap_kernfs_sovereign_symlinks(void)
> > +{
> > +     struct selinux_state *state = &selinux_state;
> > +
> > +     return state->policycap[POLICYDB_CAPABILITY_KERNFS_SOVEREIGN_SYMLINKS];
> > +}
> > +
> >   int security_mls_enabled(struct selinux_state *state);
> >   int security_load_policy(struct selinux_state *state,
> >                        void *data, size_t len);
> > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> > index 216ce602a..b70380947 100644
> > --- a/security/selinux/ss/services.c
> > +++ b/security/selinux/ss/services.c
> > @@ -73,7 +73,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
> >       "extended_socket_class",
> >       "always_check_network",
> >       "cgroup_seclabel",
> > -     "nnp_nosuid_transition"
> > +     "nnp_nosuid_transition",
> > +     "kernfs_sovereign_symlinks"
> >   };
> >
> >   static struct selinux_ss selinux_ss;
> >
>

Re: [PATCH] selinux: allow kernfs symlinks to inherit parent directory context

[Stephen Smalley]

On 1/24/20 2:08 PM, Christian Göttsche wrote:
> Am Fr., 24. Jan. 2020 um 19:53 Uhr schrieb Stephen Smalley <sds@tycho.nsa.gov>:
>>
>> On 1/24/20 1:42 PM, Christian Göttsche wrote:
>>> Currently symlinks on kernel filesystems, like sysfs, are labeled on
>>> creation with the parent fs root sid.
>>>
>>> Allow symlinks to inherit the parent directory context, so fine-grained
>>> kernfs labeling can be applied to symlinks too and checking contexts
>>> doesn't complain about them.
>>>
>>> For backward-compatibility this behavior is contained in a new policy
>>> capability: kernfs_sovereign_symlinks
>>>
>>> Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
>>> ---

>> Not fond of the name.  1) kernfs is a kernel implementation detail,
>> shouldn't be exposed to policy; genfs is the policy construct 2)
>> sovereign doesn't seem to fit the meaning of this capability; seclabel
>> would be more appropriate.
> 
> Something like genfs_seclabel_symlinks?

Works for me.

> 
>>> +                  (sbsec->flags & SE_SBGENFS_XATTR))) {
>>
>> Why limit this to SE_SBGENFS_XATTR filesystems?  Why not just make the test:
>>          if ((sbsec->flags & SE_SBGENFS) && (!S_ISLNK(inode->i_mode) ||
>> selinux_policycap_genfs_symlinkseclabel()))
>> or similar.
> 
> I somehow thought that this functionality is limited to filesystems
> with SE_SBGENFS_XATTR;
> so I can expand the check to SE_SBGENFS.

I could be wrong but I don't see why it would need to be limited in that 
way.

[PATCH v2] selinux: allow kernfs symlinks to inherit parent directory context

[Christian Göttsche]

Currently symlinks on kernel filesystems, like sysfs, are labeled on
creation with the parent filesystem root sid.

Allow symlinks to inherit the parent directory context, so fine-grained
kernfs labeling can be applied to symlinks too and checking contexts
doesn't complain about them.

For backward-compatibility this behavior is contained in a new policy
capability: genfs_seclabel_symlinks

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
v2:
incorporate feedback from Stephen Smalley
  - changed polcap name
  - extended affected filesystems from SE_SBGENFS_XATTR to SE_SBGENFS

 security/selinux/hooks.c            | 4 +++-
 security/selinux/include/security.h | 8 ++++++++
 security/selinux/ss/services.c      | 3 ++-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d9e8b2131a65..60a0b3553c70 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1475,7 +1475,9 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 		/* Default to the fs superblock SID. */
 		sid = sbsec->sid;
 
-		if ((sbsec->flags & SE_SBGENFS) && !S_ISLNK(inode->i_mode)) {
+		if ((sbsec->flags & SE_SBGENFS) &&
+		     (!S_ISLNK(inode->i_mode) ||
+		      selinux_policycap_genfs_seclabel_symlinks())) {
 			/* We must have a dentry to determine the label on
 			 * procfs inodes */
 			if (opt_dentry) {
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index a39f9565d80b..863ccf2bb629 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -79,6 +79,7 @@ enum {
 	POLICYDB_CAPABILITY_ALWAYSNETWORK,
 	POLICYDB_CAPABILITY_CGROUPSECLABEL,
 	POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
+	POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS,
 	__POLICYDB_CAPABILITY_MAX
 };
 #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
@@ -209,6 +210,13 @@ static inline bool selinux_policycap_nnp_nosuid_transition(void)
 	return state->policycap[POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION];
 }
 
+static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
+{
+	struct selinux_state *state = &selinux_state;
+
+	return state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS];
+}
+
 int security_mls_enabled(struct selinux_state *state);
 int security_load_policy(struct selinux_state *state,
 			 void *data, size_t len);
diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 216ce602a2b5..d9306f489060 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -73,7 +73,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
 	"extended_socket_class",
 	"always_check_network",
 	"cgroup_seclabel",
-	"nnp_nosuid_transition"
+	"nnp_nosuid_transition",
+	"genfs_seclabel_symlinks"
 };
 
 static struct selinux_ss selinux_ss;
-- 
2.25.0