[Devel] [PATCH RHEL8 COMMIT] ve/fs/binfmt: clean bm_data reference from ve on err path

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-305.3.1.vz8.7.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-305.3.1.vz8.7.11
------>
commit 8250ff41d1900f88dfba91bc138e2485c95faa11
Author: Konstantin Khorenko <khorenko at virtuozzo.com>
Date:   Fri Sep 10 19:35:27 2021 +0300

    ve/fs/binfmt: clean bm_data reference from ve on err path
    
    1. Make sure ve->binfmt_misc is NULL if error happens on binfmt_misc
    mount, otherwise on next attempt to mount binfmt_misc (probably
    successful) we won't even try to allocate/init structures for it.
    
    2. The current bm_fill_super() code makes us suppose we can get into
    the function with ve->binfmt_misc already initialized. If this is true
    and simple_fill_super() fails we will free preconfigured ve->binfmt_misc
    without proper deinitialization (ve_binfmt_fini()).
    
    Hopefully this is a wrong assumption, so rewrite the code not to confuse
    readers.
    
    Fixes: 90fb0e274 ("ve/fs/binfmt: virtualization")
    
    https://jira.sw.ru/browse/PSBM-131994
    
    Signed-off-by: Alexander Mikhalitsyn <alexander.mikhalitsyn at virtuozzo.com>
    Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 fs/binfmt_misc.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 2b586d61f4a9..0946e7e6caa5 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -853,18 +853,28 @@ static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
 	};
 
 	struct ve_struct *ve = get_exec_env();
-	struct binfmt_misc *bm_data = ve->binfmt_misc;
+	struct binfmt_misc *bm_data;
 
-	if (!bm_data) {
-		bm_data = kzalloc(sizeof(struct binfmt_misc), GFP_KERNEL);
-		if (!bm_data)
-			return -ENOMEM;
+	/*
+	 * bm_get_tree()
+	 *  get_tree_keyed(fc, bm_fill_super, get_ve(ve))
+	 *   fc->s_fs_info = current VE
+	 *   vfs_get_super(fc, vfs_get_keyed_super, bm_fill_super)
+	 *    sb = sget_fc(fc, test, set_anon_super_fc)
+	 *    if (!sb->s_root) {
+	 *		err = bm_fill_super(sb, fc);
+	 *
+	 * => we should never get here with initialized ve->binfmt_misc.
+	 */
+	if (WARN_ON_ONCE(ve->binfmt_misc))
+		return -EEXIST;
 
-		INIT_LIST_HEAD(&bm_data->entries);
-		rwlock_init(&bm_data->entries_lock);
+	bm_data = kzalloc(sizeof(struct binfmt_misc), GFP_KERNEL);
+	if (!bm_data)
+		return -ENOMEM;
 
-		ve->binfmt_misc = bm_data;
-	}
+	INIT_LIST_HEAD(&bm_data->entries);
+	rwlock_init(&bm_data->entries_lock);
 
 	err = simple_fill_super(sb, BINFMTFS_MAGIC, bm_files);
 	if (err) {
@@ -874,6 +884,7 @@ static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
 
 	sb->s_op = &s_ops;
 
+	ve->binfmt_misc = bm_data;
 	bm_data->enabled = 1;
 
 	return 0;