[libvirt] [PATCH] RFC: security: Make sure to decrease ref count label value

[Michal Privoznik]

On 7/26/19 8:38 PM, Stefan Berger wrote:
> I noticed that if a domain fails to restore, the ref count in the xattr
> 'trusted.libvirt.security.ref_selinux' keeps on increasing indefinitely
> and the VM will never restore even if the root cause for the restore
> failure has been removed. The reason seems to be that the code to decrease
> the ref count never gets called because the block above it fails due
> to virSecuritySELinuxTransactionAppend() failing. The simple solution
> seems to be to revert the order in which things are done.
> 
> Signed-off-by: Stefan Berger <stefanb at linux.ibm.com>
> ---
>   src/security/security_selinux.c | 19 +++++++++++--------
>   1 file changed, 11 insertions(+), 8 deletions(-)

So what was the root cause, did you ever find out?

> 
> diff --git a/src/security/security_selinux.c b/src/security/security_selinux.c
> index ea20373a90..9fd29e9bca 100644
> --- a/src/security/security_selinux.c
> +++ b/src/security/security_selinux.c
> @@ -1499,14 +1499,9 @@ virSecuritySELinuxRestoreFileLabel(virSecurityManagerPtr mgr,
>           goto cleanup;
>       }
>   
> -    if ((rc = virSecuritySELinuxTransactionAppend(path, NULL,
> -                                                  false, recall, true)) < 0) {
> -        goto cleanup;
> -    } else if (rc > 0) {
> -        ret = 0;
> -        goto cleanup;
> -    }
> -
> +    /* Recall the label so the ref count label decreases its counter
> +     * even if transaction append below fails.
> +     */
>       if (recall) {
>           rc = virSecuritySELinuxRecallLabel(newpath, &fcon);
>           if (rc == -2) {
> @@ -1519,6 +1514,14 @@ virSecuritySELinuxRestoreFileLabel(virSecurityManagerPtr mgr,
>           }
>       }
>   
> +    if ((rc = virSecuritySELinuxTransactionAppend(path, NULL,
> +                                                  false, recall, true)) < 0) {
> +        goto cleanup;
> +    } else if (rc > 0) {
> +        ret = 0;
> +        goto cleanup;
> +    }
> +
>       if (!recall || rc == -2) {
>           if (stat(newpath, &buf) != 0) {
>               VIR_WARN("cannot stat %s: %s", newpath,
> 

IIUC, this gets then called twice, doesn't it? I mean, transactions are 
hackish a bit. We wanted chown() and setfilecon_raw() to run inside the 
mount namespace of a domain. So instead of rewriting our secdrivers and 
making them enter the namespace on every chown()/setfilecon_raw() we put 
small code snippets just before these calls that would record arguments 
for these functions in a thread local variable and then 
transactionCommit() would enter the namespace and replay all 
chown()/setfilecon_raw().

That's why there can't be any code before transactionAppend(), because 
it would be called twice - once from regular call and from 
transactionCommit(). For instance:

virSecuritySELinuxDomainSetPathLabel()
  -> virSecuritySELinuxSetFilecon()
    -> virSecuritySELinuxSetFileconHelper()
      -> virSecuritySELinuxTransactionAppend()

virSecuritySELinuxTransactionCommit()
   -> virSecuritySELinuxTransactionRun()
     -> virSecuritySELinuxSetFileconHelper()
       -> virSecuritySELinuxSetFileconImpl()
         -> setfilecon_raw()

We need to find the root cause. Maybe our set is not on par with 
restore, i.e. we are calling more set than restore and thus the 
refcounter just keeps growing?

Michal