CIFS: New mount option for cifs.upcall namespace resolution

commit db363b0a1d9e6b9dc556296f1b1007aeb496a8cf upstream.

In the current implementation, the SMB filesystem on a mount point can
trigger upcalls from the kernel to the userspace to enable certain
functionalities like spnego, dns_resolution, amongst others. These upcalls
usually either happen in the context of the mount or in the context of an
application/user. The upcall handler for cifs, cifs.upcall already has
existing code which switches the namespaces to the caller's namespace
before handling the upcall. This behaviour is expected for scenarios like
multiuser mounts, but might not cover all single user scenario with
services such as Kubernetes, where the mount can happen from different
locations such as on the host, from an app container, or a driver pod
which does the mount on behalf of a different pod.

This patch introduces a new mount option called upcall_target, to
customise the upcall behaviour. upcall_target can take 'mount' and 'app'
as possible values. This aids use cases like Kubernetes where the mount
happens on behalf of the application in another container altogether.
Having this new mount option allows the mount command to specify where the
upcall should happen: 'mount' for resolving the upcall to the host
namespace, and 'app' for resolving the upcall to the ns of the calling
thread. This will enable both the scenarios where the Kerberos credentials
can be found on the application namespace or the host namespace to which
just the mount operation is "delegated".

Reviewed-by: Shyam Prasad <shyam.prasad@microsoft.com>
Reviewed-by: Bharath S M <bharathsm@microsoft.com>
Reviewed-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Signed-off-by: Ritvik Budhiraja <rbudhiraja@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Cc: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 20 insertions, 0 deletions

diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
index d5549e06a533..112057c7ca11 100644
--- a/fs/smb/client/connect.c
+++ b/fs/smb/client/connect.c
@@ -2381,6 +2381,26 @@ retry_old_session:
 
 	ses->sectype = ctx->sectype;
 	ses->sign = ctx->sign;
+
+	/*
+	 *Explicitly marking upcall_target mount option for easier handling
+	 * by cifs_spnego.c and eventually cifs.upcall.c
+	 */
+
+	switch (ctx->upcall_target) {
+	case UPTARGET_UNSPECIFIED: /* default to app */
+	case UPTARGET_APP:
+		ses->upcall_target = UPTARGET_APP;
+		break;
+	case UPTARGET_MOUNT:
+		ses->upcall_target = UPTARGET_MOUNT;
+		break;
+	default:
+		// should never happen
+		ses->upcall_target = UPTARGET_APP;
+		break;
+	}
+
 	ses->local_nls = load_nls(ctx->local_nls->charset);
 
 	/* add server as first channel */