Re: [PATCH v2 4/6] cifs: Fix parsing native symlinks directory/file type

[Steve French]

Would this break any pure Linux client example, mounted to Windows,
where previously the Linux client created all symlinks as file
symlinks?  e.g. If there were two Linux clients writing to the share,
one that included this fix and one that did not.

On Sat, Oct 5, 2024 at 9:03 AM Pali Rohár <pali@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> ---
>  fs/smb/client/inode.c     |  5 ++++
>  fs/smb/client/smb2file.c  | 55 +++++++++++++++++++++++++++++++++++++++
>  fs/smb/client/smb2inode.c |  4 +++
>  fs/smb/client/smb2proto.h |  1 +
>  4 files changed, 65 insertions(+)
>
> diff --git a/fs/smb/client/inode.c b/fs/smb/client/inode.c
> index 0fe54b2d2561..aa38a3935f8f 100644
> --- a/fs/smb/client/inode.c
> +++ b/fs/smb/client/inode.c
> @@ -1110,6 +1110,11 @@ 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 dc52995f5591..149449d9c1c0 100644
> --- a/fs/smb/client/smb2file.c
> +++ b/fs/smb/client/smb2file.c
> @@ -63,6 +63,56 @@ 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 = kzalloc(len+2, GFP_KERNEL);
> +               if (!buf)
> +                       return -ENOMEM;
> +               memcpy(buf, *target, len);
> +               buf[len] = '/';
> +               kfree(*target);
> +               *target = buf;
> +       }
> +
> +       /*
> +        * If this is a symlink which points to file name with trailing slash,
> +        * or to file named "." or file named ".." then this symlink cannot be
> +        * resolved on Linux because Linux does not allow files with such names.
> +        * So return an error to prevent resolving this file type symlink to
> +        * directory, as it do not point to directory at all.
> +        */
> +       if (!directory) {
> +               const char *basename = kbasename(*target);
> +               int basename_len = strlen(basename);
> +               if (basename_len == 0 || /* symname ends with slash */
> +                   (basename_len == 1 && basename[0] == '.') || /* last component is "." */
> +                   (basename_len == 2 && basename[0] == '.' && basename[1] == '.')) /* or ".." */
> +                       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 +183,11 @@ 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 9a28a30ec1a3..06bb6f7fbf0f 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 aa01ae234732..1828b825c7d3 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
>
>


-- 
Thanks,

Steve