Re: [PATCH RFC v2 5/5] SELinux: Support SELinux determination of side-channel vulnerability

From: Stephen Smalley
Date: Mon Aug 20 2018 - 13:51:38 EST

On 08/20/2018 12:59 PM, Schaufler, Casey wrote:
-----Original Message-----
From: Stephen Smalley [mailto:sds@xxxxxxxxxxxxx]
Sent: Monday, August 20, 2018 9:03 AM
To: Schaufler, Casey <casey.schaufler@xxxxxxxxx>; kernel-
hardening@xxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-security-
module@xxxxxxxxxxxxxxx; selinux@xxxxxxxxxxxxx; Hansen, Dave
<dave.hansen@xxxxxxxxx>; Dock, Deneen T <deneen.t.dock@xxxxxxxxx>;
kristen@xxxxxxxxxxxxxxx; arjan@xxxxxxxxxxxxxxx
Subject: Re: [PATCH RFC v2 5/5] SELinux: Support SELinux determination of
side-channel vulnerability

On 08/17/2018 06:16 PM, Casey Schaufler wrote:
SELinux considers tasks to be side-channel safe if they
have PROCESS_SHARE access.

Now the description and the code no longer match.

You're right.

Signed-off-by: Casey Schaufler <casey.schaufler@xxxxxxxxx>
security/selinux/hooks.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index a8bf324130f5..7fbd7d7ac1cb 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -4219,6 +4219,14 @@ static void selinux_task_to_inode(struct
task_struct *p,

+static int selinux_task_safe_sidechannel(struct task_struct *p)
+ struct av_decision avd;
+ return avc_has_perm_noaudit(&selinux_state, current_sid(),

And my question from before still stands: why do we need a new hook and
new security module instead of just using ptrace_may_access()?

Locking. The SELinux check, for example, will lock up solid while trying
to generate an audit record. There is no good reason aside from coding
convenience to assume that the same restrictions will apply for side-channel
as apply to ptrace. I'm actually a touch surprised you're not suggesting a
separate SECCLASS or access mode for the SELinux hook.

The PTRACE_MODE_NOAUDIT flag to ptrace_may_access() would address the locking concern. Duplicating the ptrace access checking logic seems prone to errors and inconsistencies. I can't imagine policy writers understanding what "safe sidechannel" means, much less deciding when to allow it.

/* Returns error only if unable to parse addresses */
static int selinux_parse_skb_ipv4(struct sk_buff *skb,
struct common_audit_data *ad, u8 *proto)
@@ -7002,6 +7010,7 @@ static struct security_hook_list selinux_hooks[]
__lsm_ro_after_init = {
LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
LSM_HOOK_INIT(task_kill, selinux_task_kill),
LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode),
+ LSM_HOOK_INIT(task_safe_sidechannel,

LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission),
LSM_HOOK_INIT(ipc_getsecid, selinux_ipc_getsecid),