As SMB protocol distinguish between symlink to directory and symlink to
file, add some mechanism to disallow resolving incompatible types.

When SMB symlink is of the directory type, ensure that its target path ends
with slash. This forces Linux to not allow resolving such symlink to file.

And when SMB symlink is of the file type and its target path ends with
slash then returns an error as such symlink is unresolvable. Such symlink
always points to invalid location as file cannot end with slash.

As POSIX server does not distinguish between symlinks to file and symlink
directory, do not apply this change for symlinks from POSIX SMB server. For
POSIX SMB servers, this change does nothing.

This mimics Windows behavior of native SMB symlinks.

Signed-off-by: Pali Rohár <pali@kernel.org>
---
Changes in v3:
* Relax non-directory case condition in smb2_fix_symlink_target_type()
  for compatibility with older Linux clients.
* Use krealloc() instead of kzalloc()+memcpy()
---
 fs/smb/client/inode.c     |  5 ++++
 fs/smb/client/smb2file.c  | 48 +++++++++++++++++++++++++++++++++++++++
 fs/smb/client/smb2inode.c |  4 ++++
 fs/smb/client/smb2proto.h |  1 +
 4 files changed, 58 insertions(+)

diff --git a/fs/smb/client/inode.c b/fs/smb/client/inode.c
index 2ac9cc8d327d..3fd625b356bd 100644
--- a/fs/smb/client/inode.c
+++ b/fs/smb/client/inode.c
@@ -1140,6 +1140,11 @@ static int reparse_info_to_fattr(struct cifs_open_info_data *data,
 							      full_path,
 							      iov, data);
 		}
+
+		if (data->reparse.tag == IO_REPARSE_TAG_SYMLINK && !rc) {
+			bool directory = le32_to_cpu(data->fi.Attributes) & ATTR_DIRECTORY;
+			rc = smb2_fix_symlink_target_type(&data->symlink_target, directory, cifs_sb);
+		}
 		break;
 	}
 
diff --git a/fs/smb/client/smb2file.c b/fs/smb/client/smb2file.c
index e836bc2193dd..4b07274e824a 100644
--- a/fs/smb/client/smb2file.c
+++ b/fs/smb/client/smb2file.c
@@ -63,6 +63,49 @@ static struct smb2_symlink_err_rsp *symlink_data(const struct kvec *iov)
 	return sym;
 }
 
+int smb2_fix_symlink_target_type(char **target, bool directory, struct cifs_sb_info *cifs_sb)
+{
+	char *buf;
+	int len;
+
+	/*
+	 * POSIX server does not distinguish between symlinks to file and
+	 * symlink directory. So nothing is needed to fix on the client side.
+	 */
+	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_POSIX_PATHS)
+		return 0;
+
+	len = strlen(*target);
+	if (!len)
+		return -EIO;
+
+	/*
+	 * If this is directory symlink and it does not have trailing slash then
+	 * append it. Trailing slash simulates Windows/SMB behavior which do not
+	 * allow resolving directory symlink to file.
+	 */
+	if (directory && (*target)[len-1] != '/') {
+		buf = krealloc(*target, len+2, GFP_KERNEL);
+		if (!buf)
+			return -ENOMEM;
+		buf[len] = '/';
+		buf[len+1] = '\0';
+		*target = buf;
+		len++;
+	}
+
+	/*
+	 * If this is a file (non-directory) symlink and it points to path name
+	 * with trailing slash then this is an invalid symlink because file name
+	 * cannot contain slash character. File name with slash is invalid on
+	 * both Windows and Linux systems. So return an error for such symlink.
+	 */
+	if (!directory && (*target)[len-1] == '/')
+		return -EIO;
+
+	return 0;
+}
+
 int smb2_parse_symlink_response(struct cifs_sb_info *cifs_sb, const struct kvec *iov,
 				const char *full_path, char **path)
 {
@@ -133,6 +176,11 @@ int smb2_open_file(const unsigned int xid, struct cifs_open_parms *oparms, __u32
 					       NULL, NULL, NULL);
 				oparms->create_options &= ~OPEN_REPARSE_POINT;
 			}
+			if (!rc) {
+				bool directory = le32_to_cpu(data->fi.Attributes) & ATTR_DIRECTORY;
+				rc = smb2_fix_symlink_target_type(&data->symlink_target,
+								  directory, oparms->cifs_sb);
+			}
 		}
 	}
 
diff --git a/fs/smb/client/smb2inode.c b/fs/smb/client/smb2inode.c
index a188908914fe..b8ccc8fd88f2 100644
--- a/fs/smb/client/smb2inode.c
+++ b/fs/smb/client/smb2inode.c
@@ -960,6 +960,10 @@ int smb2_query_path_info(const unsigned int xid,
 		rc = smb2_compound_op(xid, tcon, cifs_sb, full_path,
 				      &oparms, in_iov, cmds, num_cmds,
 				      cfile, NULL, NULL, NULL);
+		if (data->reparse.tag == IO_REPARSE_TAG_SYMLINK && !rc) {
+			bool directory = le32_to_cpu(data->fi.Attributes) & ATTR_DIRECTORY;
+			rc = smb2_fix_symlink_target_type(&data->symlink_target, directory, cifs_sb);
+		}
 		break;
 	case -EREMOTE:
 		break;
diff --git a/fs/smb/client/smb2proto.h b/fs/smb/client/smb2proto.h
index db93447f0f5a..5390d5a61039 100644
--- a/fs/smb/client/smb2proto.h
+++ b/fs/smb/client/smb2proto.h
@@ -113,6 +113,7 @@ extern int smb3_query_mf_symlink(unsigned int xid, struct cifs_tcon *tcon,
 			  struct cifs_sb_info *cifs_sb,
 			  const unsigned char *path, char *pbuf,
 			  unsigned int *pbytes_read);
+int smb2_fix_symlink_target_type(char **target, bool directory, struct cifs_sb_info *cifs_sb);
 int smb2_parse_native_symlink(char **target, const char *buf, unsigned int len,
 			      bool unicode, bool relative,
 			      const char *full_path,
-- 
2.20.1