2022-01-27 02:56:13

by Paul Moore

[permalink] [raw]
Subject: Re: [RFC PATCH] selinux: split no transition execve check

On Tue, Jan 25, 2022 at 9:59 AM Christian Göttsche
<[email protected]> wrote:
>
> In case a setuid or setgid binary is mislabeled with a generic context,
> either via a policy mistake or a move by the distribution package,
> executing it will be checked by the file permission execute_no_trans on
> the generic file context (e.g. bin_t). The setuid(2)/setgid(2) syscall
> within will then be checked against the unchanged caller process
> context, which might have been granted the capability permission setuid/
> setgid to initially drop privileges. To avoid that scenario split the
> execute_no_trans permission in case of a setuid/setgid binary into a new
> permission execute_sxid_no_trans.
>
> For backward compatibility this behavior is contained in a new policy
> capability.
>
> Signed-off-by: Christian Göttsche <[email protected]>
> ---
> security/selinux/hooks.c | 9 ++++++++-
> security/selinux/include/classmap.h | 2 +-
> security/selinux/include/policycap.h | 1 +
> security/selinux/include/policycap_names.h | 3 ++-
> security/selinux/include/security.h | 8 ++++++++
> 5 files changed, 20 insertions(+), 3 deletions(-)

Adding the refpolicy list to this thread as their opinion seems
particularly relevant to this discussion.

FWIW, this looks reasonable to me but I would like to hear what others
have to say.

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 5b6895e4fc29..b825fee39a70 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2348,9 +2348,16 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
> ad.u.file = bprm->file;
>
> if (new_tsec->sid == old_tsec->sid) {
> + u32 perm;
> +
> + if (selinux_policycap_execute_sxid_no_trans() && is_sxid(inode->i_mode))
> + perm = FILE__EXECUTE_SXID_NO_TRANS;
> + else
> + perm = FILE__EXECUTE_NO_TRANS;
> +
> rc = avc_has_perm(&selinux_state,
> old_tsec->sid, isec->sid,
> - SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
> + SECCLASS_FILE, perm, &ad);
> if (rc)
> return rc;
> } else {
> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> index 35aac62a662e..53a1eeeb86fb 100644
> --- a/security/selinux/include/classmap.h
> +++ b/security/selinux/include/classmap.h
> @@ -65,7 +65,7 @@ struct security_class_mapping secclass_map[] = {
> "quotaget", "watch", NULL } },
> { "file",
> { COMMON_FILE_PERMS,
> - "execute_no_trans", "entrypoint", NULL } },
> + "execute_no_trans", "entrypoint", "execute_sxid_no_trans", NULL } },
> { "dir",
> { COMMON_FILE_PERMS, "add_name", "remove_name",
> "reparent", "search", "rmdir", NULL } },
> diff --git a/security/selinux/include/policycap.h b/security/selinux/include/policycap.h
> index 2ec038efbb03..23929dc3e1db 100644
> --- a/security/selinux/include/policycap.h
> +++ b/security/selinux/include/policycap.h
> @@ -11,6 +11,7 @@ enum {
> POLICYDB_CAPABILITY_CGROUPSECLABEL,
> POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
> POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS,
> + POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS,
> __POLICYDB_CAPABILITY_MAX
> };
> #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
> diff --git a/security/selinux/include/policycap_names.h b/security/selinux/include/policycap_names.h
> index b89289f092c9..4c014c2cf352 100644
> --- a/security/selinux/include/policycap_names.h
> +++ b/security/selinux/include/policycap_names.h
> @@ -12,7 +12,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
> "always_check_network",
> "cgroup_seclabel",
> "nnp_nosuid_transition",
> - "genfs_seclabel_symlinks"
> + "genfs_seclabel_symlinks",
> + "execute_sxid_no_trans",
> };
>
> #endif /* _SELINUX_POLICYCAP_NAMES_H_ */
> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> index ac0ece01305a..ab95241b6b7b 100644
> --- a/security/selinux/include/security.h
> +++ b/security/selinux/include/security.h
> @@ -219,6 +219,14 @@ static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
> return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS]);
> }
>
> +static inline bool selinux_policycap_execute_sxid_no_trans(void)
> +{
> + struct selinux_state *state = &selinux_state;
> +
> + return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS]);
> +}
> +
> +
> struct selinux_policy_convert_data;
>
> struct selinux_load_state {
> --
> 2.34.1
>


--
paul-moore.com


2022-01-28 02:56:14

by Chris PeBenito

[permalink] [raw]
Subject: Re: [RFC PATCH] selinux: split no transition execve check

On 1/26/22 17:51, Paul Moore wrote:
> On Tue, Jan 25, 2022 at 9:59 AM Christian Göttsche
> <[email protected]> wrote:
>>
>> In case a setuid or setgid binary is mislabeled with a generic context,
>> either via a policy mistake or a move by the distribution package,
>> executing it will be checked by the file permission execute_no_trans on
>> the generic file context (e.g. bin_t). The setuid(2)/setgid(2) syscall
>> within will then be checked against the unchanged caller process
>> context, which might have been granted the capability permission setuid/
>> setgid to initially drop privileges. To avoid that scenario split the
>> execute_no_trans permission in case of a setuid/setgid binary into a new
>> permission execute_sxid_no_trans.
>>
>> For backward compatibility this behavior is contained in a new policy
>> capability.
>>
>> Signed-off-by: Christian Göttsche <[email protected]>
>> ---
>> security/selinux/hooks.c | 9 ++++++++-
>> security/selinux/include/classmap.h | 2 +-
>> security/selinux/include/policycap.h | 1 +
>> security/selinux/include/policycap_names.h | 3 ++-
>> security/selinux/include/security.h | 8 ++++++++
>> 5 files changed, 20 insertions(+), 3 deletions(-)
>
> Adding the refpolicy list to this thread as their opinion seems
> particularly relevant to this discussion.
>
> FWIW, this looks reasonable to me but I would like to hear what others
> have to say.

I think this a band-aid to cover up the real problem, which is the mislabeled files.




>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>> index 5b6895e4fc29..b825fee39a70 100644
>> --- a/security/selinux/hooks.c
>> +++ b/security/selinux/hooks.c
>> @@ -2348,9 +2348,16 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
>> ad.u.file = bprm->file;
>>
>> if (new_tsec->sid == old_tsec->sid) {
>> + u32 perm;
>> +
>> + if (selinux_policycap_execute_sxid_no_trans() && is_sxid(inode->i_mode))
>> + perm = FILE__EXECUTE_SXID_NO_TRANS;
>> + else
>> + perm = FILE__EXECUTE_NO_TRANS;
>> +
>> rc = avc_has_perm(&selinux_state,
>> old_tsec->sid, isec->sid,
>> - SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
>> + SECCLASS_FILE, perm, &ad);
>> if (rc)
>> return rc;
>> } else {
>> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
>> index 35aac62a662e..53a1eeeb86fb 100644
>> --- a/security/selinux/include/classmap.h
>> +++ b/security/selinux/include/classmap.h
>> @@ -65,7 +65,7 @@ struct security_class_mapping secclass_map[] = {
>> "quotaget", "watch", NULL } },
>> { "file",
>> { COMMON_FILE_PERMS,
>> - "execute_no_trans", "entrypoint", NULL } },
>> + "execute_no_trans", "entrypoint", "execute_sxid_no_trans", NULL } },
>> { "dir",
>> { COMMON_FILE_PERMS, "add_name", "remove_name",
>> "reparent", "search", "rmdir", NULL } },
>> diff --git a/security/selinux/include/policycap.h b/security/selinux/include/policycap.h
>> index 2ec038efbb03..23929dc3e1db 100644
>> --- a/security/selinux/include/policycap.h
>> +++ b/security/selinux/include/policycap.h
>> @@ -11,6 +11,7 @@ enum {
>> POLICYDB_CAPABILITY_CGROUPSECLABEL,
>> POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
>> POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS,
>> + POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS,
>> __POLICYDB_CAPABILITY_MAX
>> };
>> #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
>> diff --git a/security/selinux/include/policycap_names.h b/security/selinux/include/policycap_names.h
>> index b89289f092c9..4c014c2cf352 100644
>> --- a/security/selinux/include/policycap_names.h
>> +++ b/security/selinux/include/policycap_names.h
>> @@ -12,7 +12,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
>> "always_check_network",
>> "cgroup_seclabel",
>> "nnp_nosuid_transition",
>> - "genfs_seclabel_symlinks"
>> + "genfs_seclabel_symlinks",
>> + "execute_sxid_no_trans",
>> };
>>
>> #endif /* _SELINUX_POLICYCAP_NAMES_H_ */
>> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
>> index ac0ece01305a..ab95241b6b7b 100644
>> --- a/security/selinux/include/security.h
>> +++ b/security/selinux/include/security.h
>> @@ -219,6 +219,14 @@ static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
>> return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS]);
>> }
>>
>> +static inline bool selinux_policycap_execute_sxid_no_trans(void)
>> +{
>> + struct selinux_state *state = &selinux_state;
>> +
>> + return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS]);
>> +}
>> +
>> +
>> struct selinux_policy_convert_data;
>>
>> struct selinux_load_state {
>> --
>> 2.34.1
>>
>
>


--
Chris PeBenito

2022-01-29 08:11:38

by Paul Moore

[permalink] [raw]
Subject: Re: [RFC PATCH] selinux: split no transition execve check

On Thu, Jan 27, 2022 at 8:42 AM Chris PeBenito <[email protected]> wrote:
> On 1/26/22 17:51, Paul Moore wrote:
> > On Tue, Jan 25, 2022 at 9:59 AM Christian Göttsche
> > <[email protected]> wrote:
> >>
> >> In case a setuid or setgid binary is mislabeled with a generic context,
> >> either via a policy mistake or a move by the distribution package,
> >> executing it will be checked by the file permission execute_no_trans on
> >> the generic file context (e.g. bin_t). The setuid(2)/setgid(2) syscall
> >> within will then be checked against the unchanged caller process
> >> context, which might have been granted the capability permission setuid/
> >> setgid to initially drop privileges. To avoid that scenario split the
> >> execute_no_trans permission in case of a setuid/setgid binary into a new
> >> permission execute_sxid_no_trans.
> >>
> >> For backward compatibility this behavior is contained in a new policy
> >> capability.
> >>
> >> Signed-off-by: Christian Göttsche <[email protected]>
> >> ---
> >> security/selinux/hooks.c | 9 ++++++++-
> >> security/selinux/include/classmap.h | 2 +-
> >> security/selinux/include/policycap.h | 1 +
> >> security/selinux/include/policycap_names.h | 3 ++-
> >> security/selinux/include/security.h | 8 ++++++++
> >> 5 files changed, 20 insertions(+), 3 deletions(-)
> >
> > Adding the refpolicy list to this thread as their opinion seems
> > particularly relevant to this discussion.
> >
> > FWIW, this looks reasonable to me but I would like to hear what others
> > have to say.
>
> I think this a band-aid to cover up the real problem, which is the mislabeled files.

It's hard to disagree with that, and the kernel is probably the wrong
place to apply a band-aid unless it is the only option left.

> >> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> >> index 5b6895e4fc29..b825fee39a70 100644
> >> --- a/security/selinux/hooks.c
> >> +++ b/security/selinux/hooks.c
> >> @@ -2348,9 +2348,16 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
> >> ad.u.file = bprm->file;
> >>
> >> if (new_tsec->sid == old_tsec->sid) {
> >> + u32 perm;
> >> +
> >> + if (selinux_policycap_execute_sxid_no_trans() && is_sxid(inode->i_mode))
> >> + perm = FILE__EXECUTE_SXID_NO_TRANS;
> >> + else
> >> + perm = FILE__EXECUTE_NO_TRANS;
> >> +
> >> rc = avc_has_perm(&selinux_state,
> >> old_tsec->sid, isec->sid,
> >> - SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
> >> + SECCLASS_FILE, perm, &ad);
> >> if (rc)
> >> return rc;
> >> } else {
> >> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> >> index 35aac62a662e..53a1eeeb86fb 100644
> >> --- a/security/selinux/include/classmap.h
> >> +++ b/security/selinux/include/classmap.h
> >> @@ -65,7 +65,7 @@ struct security_class_mapping secclass_map[] = {
> >> "quotaget", "watch", NULL } },
> >> { "file",
> >> { COMMON_FILE_PERMS,
> >> - "execute_no_trans", "entrypoint", NULL } },
> >> + "execute_no_trans", "entrypoint", "execute_sxid_no_trans", NULL } },
> >> { "dir",
> >> { COMMON_FILE_PERMS, "add_name", "remove_name",
> >> "reparent", "search", "rmdir", NULL } },
> >> diff --git a/security/selinux/include/policycap.h b/security/selinux/include/policycap.h
> >> index 2ec038efbb03..23929dc3e1db 100644
> >> --- a/security/selinux/include/policycap.h
> >> +++ b/security/selinux/include/policycap.h
> >> @@ -11,6 +11,7 @@ enum {
> >> POLICYDB_CAPABILITY_CGROUPSECLABEL,
> >> POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
> >> POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS,
> >> + POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS,
> >> __POLICYDB_CAPABILITY_MAX
> >> };
> >> #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
> >> diff --git a/security/selinux/include/policycap_names.h b/security/selinux/include/policycap_names.h
> >> index b89289f092c9..4c014c2cf352 100644
> >> --- a/security/selinux/include/policycap_names.h
> >> +++ b/security/selinux/include/policycap_names.h
> >> @@ -12,7 +12,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
> >> "always_check_network",
> >> "cgroup_seclabel",
> >> "nnp_nosuid_transition",
> >> - "genfs_seclabel_symlinks"
> >> + "genfs_seclabel_symlinks",
> >> + "execute_sxid_no_trans",
> >> };
> >>
> >> #endif /* _SELINUX_POLICYCAP_NAMES_H_ */
> >> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> >> index ac0ece01305a..ab95241b6b7b 100644
> >> --- a/security/selinux/include/security.h
> >> +++ b/security/selinux/include/security.h
> >> @@ -219,6 +219,14 @@ static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
> >> return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS]);
> >> }
> >>
> >> +static inline bool selinux_policycap_execute_sxid_no_trans(void)
> >> +{
> >> + struct selinux_state *state = &selinux_state;
> >> +
> >> + return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS]);
> >> +}
> >> +
> >> +
> >> struct selinux_policy_convert_data;
> >>
> >> struct selinux_load_state {
> >> --
> >> 2.34.1
> >>
> >
> >
>
>
> --
> Chris PeBenito



--
paul-moore.com