Re: [RFC PATCH 2/2] selinux: Propagate RCU walk status from 'security_inode_follow_link()'

[Stephen Smalley <sds@tycho.nsa.gov>]

On 11/19/19 1:40 PM, Will Deacon wrote:
> 'selinux_inode_follow_link()' can be called as part of an RCU path walk,
> and is passed a 'bool rcu' parameter to indicate whether or not it is
> being called from within an RCU read-side critical section.
> 
> Unfortunately, this knowledge is not propagated further and, instead,
> 'avc_has_perm()' unconditionally passes a flags argument of '0' to both
> 'avc_has_perm_noaudit()' and 'avc_audit()' which may block.
> 
> Introduce 'avc_has_perm_flags()' which can be used safely from within an
> RCU read-side critical section.

Please see e46e01eebbbcf2ff6d28ee7cae9f117e9d1572c8 ("selinux: stop 
passing MAY_NOT_BLOCK to the AVC upon follow_link").

> 
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>   security/selinux/avc.c         | 12 +++++++-----
>   security/selinux/hooks.c       |  5 +++--
>   security/selinux/include/avc.h | 12 ++++++++----
>   3 files changed, 18 insertions(+), 11 deletions(-)
> 
> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> index 9c183c899e92..7d99dadd24d0 100644
> --- a/security/selinux/avc.c
> +++ b/security/selinux/avc.c
> @@ -1177,11 +1177,12 @@ inline int avc_has_perm_noaudit(struct selinux_state *state,
>   }
>   
>   /**
> - * avc_has_perm - Check permissions and perform any appropriate auditing.
> + * avc_has_perm_flags - Check permissions and perform any appropriate auditing.
>    * @ssid: source security identifier
>    * @tsid: target security identifier
>    * @tclass: target security class
>    * @requested: requested permissions, interpreted based on @tclass
> + * @flags: AVC_STRICT, AVC_NONBLOCKING, or 0
>    * @auditdata: auxiliary audit data
>    *
>    * Check the AVC to determine whether the @requested permissions are granted
> @@ -1192,17 +1193,18 @@ inline int avc_has_perm_noaudit(struct selinux_state *state,
>    * permissions are granted, -%EACCES if any permissions are denied, or
>    * another -errno upon other errors.
>    */
> -int avc_has_perm(struct selinux_state *state, u32 ssid, u32 tsid, u16 tclass,
> -		 u32 requested, struct common_audit_data *auditdata)
> +int avc_has_perm_flags(struct selinux_state *state, u32 ssid, u32 tsid,
> +		       u16 tclass, u32 requested, unsigned int flags,
> +		       struct common_audit_data *auditdata)
>   {
>   	struct av_decision avd;
>   	int rc, rc2;
>   
> -	rc = avc_has_perm_noaudit(state, ssid, tsid, tclass, requested, 0,
> +	rc = avc_has_perm_noaudit(state, ssid, tsid, tclass, requested, flags,
>   				  &avd);
>   
>   	rc2 = avc_audit(state, ssid, tsid, tclass, requested, &avd, rc,
> -			auditdata, 0);
> +			auditdata, flags);
>   	if (rc2)
>   		return rc2;
>   	return rc;
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 9625b99e677f..0c09f59a2740 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3008,8 +3008,9 @@ static int selinux_inode_follow_link(struct dentry *dentry, struct inode *inode,
>   	if (IS_ERR(isec))
>   		return PTR_ERR(isec);
>   
> -	return avc_has_perm(&selinux_state,
> -			    sid, isec->sid, isec->sclass, FILE__READ, &ad);
> +	return avc_has_perm_flags(&selinux_state, sid, isec->sid, isec->sclass,
> +				  rcu ? AVC_NONBLOCKING : 0,
> +				  FILE__READ, &ad);
>   }
>   
>   static noinline int audit_inode_permission(struct inode *inode,
> diff --git a/security/selinux/include/avc.h b/security/selinux/include/avc.h
> index 7be0e1e90e8b..0450e1b88182 100644
> --- a/security/selinux/include/avc.h
> +++ b/security/selinux/include/avc.h
> @@ -149,10 +149,14 @@ int avc_has_perm_noaudit(struct selinux_state *state,
>   			 unsigned flags,
>   			 struct av_decision *avd);
>   
> -int avc_has_perm(struct selinux_state *state,
> -		 u32 ssid, u32 tsid,
> -		 u16 tclass, u32 requested,
> -		 struct common_audit_data *auditdata);
> +int avc_has_perm_flags(struct selinux_state *state,
> +		       u32 ssid, u32 tsid,
> +		       u16 tclass, u32 requested,
> +		       unsigned flags,
> +		       struct common_audit_data *auditdata);
> +
> +#define avc_has_perm(state, ssid, tsid, tclass, requested, auditdata) \
> +	avc_has_perm_flags(state, ssid, tsid, tclass, requested, 0, auditdata)
>   
>   int avc_has_extended_perms(struct selinux_state *state,
>   			   u32 ssid, u32 tsid, u16 tclass, u32 requested,
>