cifs: during remount, make sure passwords are in sync

This fixes scenarios where remount can overwrite the only currently
working password, breaking reconnect.

We recently introduced a password2 field in both ses and ctx structs.
This was done so as to allow the client to rotate passwords for a mount
without any downtime. However, when the client transparently handles
password rotation, it can swap the values of the two password fields
in the ses struct, but not in smb3_fs_context struct that hangs off
cifs_sb. This can lead to a situation where a remount unintentionally
overwrites a working password in the ses struct.

In order to fix this, we first get the passwords in ctx struct
in-sync with ses struct, before replacing them with what the passwords
that could be passed as a part of remount.

Also, in order to avoid race condition between smb2_reconnect and
smb3_reconfigure, we make sure to lock session_mutex before changing
password and password2 fields of the ses structure.

Fixes: 35f8342 ("smb3: fix broken reconnect when password changing on the server by allowing password rotation")
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Meetakshi Setiya <msetiya@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: sprasad-microsoft

fs/smb/client/fs_context.c

@@ -920,12 +920,37 @@ do {									\
 	cifs_sb->ctx->field = NULL;					\
 } while (0)
 
+int smb3_sync_session_ctx_passwords(struct cifs_sb_info *cifs_sb, struct cifs_ses *ses)
+{
+	if (ses->password &&
+	    cifs_sb->ctx->password &&
+	    strcmp(ses->password, cifs_sb->ctx->password)) {
+		kfree_sensitive(cifs_sb->ctx->password);
+		cifs_sb->ctx->password = kstrdup(ses->password, GFP_KERNEL);
+		if (!cifs_sb->ctx->password)
+			return -ENOMEM;
+	}
+	if (ses->password2 &&
+	    cifs_sb->ctx->password2 &&
+	    strcmp(ses->password2, cifs_sb->ctx->password2)) {
+		kfree_sensitive(cifs_sb->ctx->password2);
+		cifs_sb->ctx->password2 = kstrdup(ses->password2, GFP_KERNEL);
+		if (!cifs_sb->ctx->password2) {
+			kfree_sensitive(cifs_sb->ctx->password);
+			cifs_sb->ctx->password = NULL;
+			return -ENOMEM;
+		}
+	}
+	return 0;
+}
+
 static int smb3_reconfigure(struct fs_context *fc)
 {
 	struct smb3_fs_context *ctx = smb3_fc2context(fc);
 	struct dentry *root = fc->root;
 	struct cifs_sb_info *cifs_sb = CIFS_SB(root->d_sb);
 	struct cifs_ses *ses = cifs_sb_master_tcon(cifs_sb)->ses;
+	char *new_password = NULL, *new_password2 = NULL;
 	bool need_recon = false;
 	int rc;
 
@@ -945,21 +970,61 @@ static int smb3_reconfigure(struct fs_context *fc)
 	STEAL_STRING(cifs_sb, ctx, UNC);
 	STEAL_STRING(cifs_sb, ctx, source);
 	STEAL_STRING(cifs_sb, ctx, username);
+
 	if (need_recon == false)
 		STEAL_STRING_SENSITIVE(cifs_sb, ctx, password);
 	else  {
-		kfree_sensitive(ses->password);
-		ses->password = kstrdup(ctx->password, GFP_KERNEL);
-		if (!ses->password)
-			return -ENOMEM;
-		kfree_sensitive(ses->password2);
-		ses->password2 = kstrdup(ctx->password2, GFP_KERNEL);
-		if (!ses->password2) {
-			kfree_sensitive(ses->password);
-			ses->password = NULL;
+		if (ctx->password) {
+			new_password = kstrdup(ctx->password, GFP_KERNEL);
+			if (!new_password)
+				return -ENOMEM;
+		} else
+			STEAL_STRING_SENSITIVE(cifs_sb, ctx, password);
+	}
+
+	/*
+	 * if a new password2 has been specified, then reset it's value
+	 * inside the ses struct
+	 */
+	if (ctx->password2) {
+		new_password2 = kstrdup(ctx->password2, GFP_KERNEL);
+		if (!new_password2) {
+			kfree_sensitive(new_password);
 			return -ENOMEM;
 		}
+	} else
+		STEAL_STRING_SENSITIVE(cifs_sb, ctx, password2);
+
+	/*
+	 * we may update the passwords in the ses struct below. Make sure we do
+	 * not race with smb2_reconnect
+	 */
+	mutex_lock(&ses->session_mutex);
+
+	/*
+	 * smb2_reconnect may swap password and password2 in case session setup
+	 * failed. First get ctx passwords in sync with ses passwords. It should
+	 * be okay to do this even if this function were to return an error at a
+	 * later stage
+	 */
+	rc = smb3_sync_session_ctx_passwords(cifs_sb, ses);
+	if (rc)
+		return rc;
+
+	/*
+	 * now that allocations for passwords are done, commit them
+	 */
+	if (new_password) {
+		kfree_sensitive(ses->password);
+		ses->password = new_password;
 	}
+	if (new_password2) {
+		kfree_sensitive(ses->password2);
+		ses->password2 = new_password2;
+	}
+
+	mutex_unlock(&ses->session_mutex);
+
 	STEAL_STRING(cifs_sb, ctx, domainname);
 	STEAL_STRING(cifs_sb, ctx, nodename);
 	STEAL_STRING(cifs_sb, ctx, iocharset);

fs/smb/client/fs_context.h

@@ -309,6 +309,7 @@ static inline struct smb3_fs_context *smb3_fc2context(const struct fs_context *f
 }
 
 extern int smb3_fs_context_dup(struct smb3_fs_context *new_ctx, struct smb3_fs_context *ctx);
+extern int smb3_sync_session_ctx_passwords(struct cifs_sb_info *cifs_sb, struct cifs_ses *ses);
 extern void smb3_update_mnt_flags(struct cifs_sb_info *cifs_sb);
 
 /*