Re: [RFC][PATCH] selinux: avoid silent denials in permissive mode under RCU walk

[Stephen Smalley <sds@xxxxxxxxxxxxx>]

On 12/4/18 3:42 PM, Stephen Smalley wrote:
> commit 0dc1ba24f7fff6 ("SELINUX: Make selinux cache VFS RCU walks safe")
> results in no audit messages at all if in permissive mode because the
> cache is updated during the rcu walk and thus no denial occurs on
> the subsequent ref walk.  Fix this by not updating the cache when
> performing a non-blocking permission check.  This only affects search
> and symlink read checks during rcu walk.

Probably should drop "under RCU walk" from the subject line since the 
denials remain silent during the rcu walk itself but are then audited 
properly upon falling back to the ref walk when a denial occurs.

> Fixes: 0dc1ba24f7fff6 ("SELINUX: Make selinux cache VFS RCU walks safe")
> Reported-by: BMK <bmktuwien@xxxxxxxxx>
> Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>
> ---
>   security/selinux/avc.c         | 9 ++++++---
>   security/selinux/hooks.c       | 4 +++-
>   security/selinux/include/avc.h | 1 +
>   3 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> index 635e5c1e3e48..f0e7bc0dc442 100644
> --- a/security/selinux/avc.c
> +++ b/security/selinux/avc.c
> @@ -1021,8 +1021,10 @@ static noinline int avc_denied(struct selinux_state *state,
>   	    !(avd->flags & AVD_FLAGS_PERMISSIVE))
>   		return -EACCES;
>   
> -	avc_update_node(state->avc, AVC_CALLBACK_GRANT, requested, driver,
> -			xperm, ssid, tsid, tclass, avd->seqno, NULL, flags);
> +	if (!(flags & AVC_NONBLOCKING))
> +		avc_update_node(state->avc, AVC_CALLBACK_GRANT, requested,
> +				driver, xperm, ssid, tsid, tclass, avd->seqno,
> +				NULL, flags);
>   	return 0;
>   }
>   
> @@ -1199,7 +1201,8 @@ int avc_has_perm_flags(struct selinux_state *state,
>   	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 & MAY_NOT_BLOCK) ? AVC_NONBLOCKING : 0,
>   				  &avd);
>   
>   	rc2 = avc_audit(state, ssid, tsid, tclass, requested, &avd, rc,
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 7ce012d9ec51..9b05f84808d9 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3196,7 +3196,9 @@ static int selinux_inode_permission(struct inode *inode, int mask)
>   		return PTR_ERR(isec);
>   
>   	rc = avc_has_perm_noaudit(&selinux_state,
> -				  sid, isec->sid, isec->sclass, perms, 0, &avd);
> +				  sid, isec->sid, isec->sclass, perms,
> +				  (flags & MAY_NOT_BLOCK) ? AVC_NONBLOCKING : 0,
> +				  &avd);
>   	audited = avc_audit_required(perms, &avd, rc,
>   				     from_access ? FILE__AUDIT_ACCESS : 0,
>   				     &denied);
> diff --git a/security/selinux/include/avc.h b/security/selinux/include/avc.h
> index ef899bcfd2cb..74ea50977c20 100644
> --- a/security/selinux/include/avc.h
> +++ b/security/selinux/include/avc.h
> @@ -142,6 +142,7 @@ static inline int avc_audit(struct selinux_state *state,
>   
>   #define AVC_STRICT 1 /* Ignore permissive mode. */
>   #define AVC_EXTENDED_PERMS 2	/* update extended permissions */
> +#define AVC_NONBLOCKING    4	/* non blocking */
>   int avc_has_perm_noaudit(struct selinux_state *state,
>   			 u32 ssid, u32 tsid,
>   			 u16 tclass, u32 requested,