[PATCH] Making shares unaccessible at root level mountable (aka solving bsc#8950 ...again)

["Aurélien Aptel" <aaptel-IBi9RG/b67k@public.gmane.org>]

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Hi all,

I've come up with a new solution for the problem (attached). Between
the long, old and irrelevant comments on the bug report and the various
mailing-list threads it was getting hard to understand. So I've written
a summary which explains the problem in details and the 2 proposed
solutions.

http://diobla.info/stuff/bugs/bsc799133/

The new solution is basically switching to the old prefixpath system
when the normal method of querying intermediary path fails. It's much
more simpler.

*Any* feedback would be nice. I haven't noticed any problems so far but
I haven't run the xfs test suite (I still have to figure out how
to make it play nice with cifs...). I'm sending to samba-tech too in
hopes of reviews/comments.

-- 
Aurélien Aptel / SUSE Labs Samba Team
GPG: 1839 CB5F 9F5B FB9B AA97  8C99 03C8 A49B 521B D5D3
SUSE Linux GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG
Nürnberg)

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From 3e9af75bea055bf88a4700fced50a7ba38b39b6f Mon Sep 17 00:00:00 2001
From: Aurelien Aptel <aaptel-IBi9RG/b67k@public.gmane.org>
Date: Wed, 25 May 2016 19:59:09 +0200
Subject: [PATCH] fs/cifs: make share unaccessible at root level mountable

if, when mounting //HOST/share/sub/dir/foo we can query /sub/dir/foo but
not any of the path components above:

- store the /sub/dir/foo prefix in the cifs super_block info
- in the superblock, set root dentry to the subpath dentry (instead of
  the share root)
- set a flag in the superblock to remember it
- use prefixpath when building path from a dentry

fixes bso#8950

Signed-off-by: Aurelien Aptel <aaptel-IBi9RG/b67k@public.gmane.org>
---
 fs/cifs/cifs_fs_sb.h |  2 ++
 fs/cifs/cifsfs.c     | 19 +++++++++++++++++++
 fs/cifs/connect.c    |  3 +++
 fs/cifs/dir.c        | 19 +++++++++++++++++--
 4 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/fs/cifs/cifs_fs_sb.h b/fs/cifs/cifs_fs_sb.h
index 3182273..02b9ac3 100644
--- a/fs/cifs/cifs_fs_sb.h
+++ b/fs/cifs/cifs_fs_sb.h
@@ -46,6 +46,7 @@
 #define CIFS_MOUNT_CIFS_BACKUPUID 0x200000 /* backup intent bit for a user */
 #define CIFS_MOUNT_CIFS_BACKUPGID 0x400000 /* backup intent bit for a group */
 #define CIFS_MOUNT_MAP_SFM_CHR	0x800000 /* SFM/MAC mapping for illegal chars */
+#define CIFS_MOUNT_USE_PREFIX_PATH 0x1000000 /* make subpath with unaccessible root mountable */
 
 struct cifs_sb_info {
 	struct rb_root tlink_tree;
@@ -67,5 +68,6 @@ struct cifs_sb_info {
 	struct backing_dev_info bdi;
 	struct delayed_work prune_tlinks;
 	struct rcu_head rcu;
+	char *prepath;
 };
 #endif				/* _CIFS_FS_SB_H */
diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 5d8b7ed..d1fc593 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -649,6 +649,17 @@ cifs_get_root(struct smb_vol *vol, struct super_block *sb)
 		dentry = child;
 	} while (!IS_ERR(dentry));
 	kfree(full_path);
+
+	if (IS_ERR(dentry) /* && path accessible */) {
+		/* we know the path is accessible (it is tested
+		 * earlier in cifs_do_mount()) so there must be a perm
+		 * problem */
+		cifs_dbg(VFS, "cannot query directories between root and final path, "
+			 "enabling CIFS_MOUNT_USE_PREFIX_PATH\n");
+		cifs_sb->mnt_cifs_flags |= CIFS_MOUNT_USE_PREFIX_PATH;
+		dentry = dget(sb->s_root);
+	}
+
 	return dentry;
 }
 
@@ -688,6 +699,14 @@ cifs_do_mount(struct file_system_type *fs_type,
 		goto out_cifs_sb;
 	}
 
+	if (volume_info->prepath) {
+		cifs_sb->prepath = kstrdup(volume_info->prepath, GFP_KERNEL);
+		if (cifs_sb->prepath == NULL) {
+			root = ERR_PTR(-ENOMEM);
+			goto out_cifs_sb;
+		}
+	}
+
 	cifs_setup_cifs_sb(volume_info, cifs_sb);
 
 	rc = cifs_mount(cifs_sb, volume_info);
diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index 66736f5..90e57ee 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -3887,6 +3887,9 @@ cifs_umount(struct cifs_sb_info *cifs_sb)
 
 	bdi_destroy(&cifs_sb->bdi);
 	kfree(cifs_sb->mountdata);
+	if (cifs_sb->prepath) {
+		kfree(cifs_sb->prepath);
+	}
 	call_rcu(&cifs_sb->rcu, delayed_free);
 }
 
diff --git a/fs/cifs/dir.c b/fs/cifs/dir.c
index c3eb998..5374253 100644
--- a/fs/cifs/dir.c
+++ b/fs/cifs/dir.c
@@ -84,6 +84,7 @@ build_path_from_dentry(struct dentry *direntry)
 	struct dentry *temp;
 	int namelen;
 	int dfsplen;
+	int pplen = 0;
 	char *full_path;
 	char dirsep;
 	struct cifs_sb_info *cifs_sb = CIFS_SB(direntry->d_sb);
@@ -95,8 +96,12 @@ build_path_from_dentry(struct dentry *direntry)
 		dfsplen = strnlen(tcon->treeName, MAX_TREE_SIZE + 1);
 	else
 		dfsplen = 0;
+
+	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_USE_PREFIX_PATH)
+		pplen = cifs_sb->prepath ? strlen(cifs_sb->prepath) + 1 : 0;
+
 cifs_bp_rename_retry:
-	namelen = dfsplen;
+	namelen = dfsplen + pplen;
 	seq = read_seqbegin(&rename_lock);
 	rcu_read_lock();
 	for (temp = direntry; !IS_ROOT(temp);) {
@@ -137,7 +142,7 @@ cifs_bp_rename_retry:
 		}
 	}
 	rcu_read_unlock();
-	if (namelen != dfsplen || read_seqretry(&rename_lock, seq)) {
+	if (namelen != dfsplen + pplen || read_seqretry(&rename_lock, seq)) {
 		cifs_dbg(FYI, "did not end path lookup where expected. namelen=%ddfsplen=%d\n",
 			 namelen, dfsplen);
 		/* presumably this is only possible if racing with a rename
@@ -153,6 +158,16 @@ cifs_bp_rename_retry:
 	   those safely to '/' if any are found in the middle of the prepath */
 	/* BB test paths to Windows with '/' in the midst of prepath */
 
+	if (pplen) {
+		int i;
+		cifs_dbg(FYI, "using cifs_sb prepath <%s>\n", cifs_sb->prepath);
+		memcpy(full_path+dfsplen+1, cifs_sb->prepath, pplen-1);
+		full_path[dfsplen] = '\\';
+		for (i = 0; i < pplen-1; i++)
+			if (full_path[dfsplen+1+i] == '/')
+				full_path[dfsplen+1+i] = CIFS_DIR_SEP(cifs_sb);
+	}
+
 	if (dfsplen) {
 		strncpy(full_path, tcon->treeName, dfsplen);
 		if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_POSIX_PATHS) {
-- 
2.1.4


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