Re: [PATCH] ceph: fix allocation under spinlock in mount option parsing

[Ilya Dryomov <idryomov@gmail.com>]

On Fri, Feb 7, 2020 at 10:49 PM Jeff Layton <jlayton@kernel.org> wrote:
>
> Al and syzbot reported that 4fbc0c711b24 (ceph: remove the extra slashes
> in the server path) had caused a regression where an allocation could be
> done under spinlock.
>
> Fix this by keeping a canonicalized version of the path in the mount
> options. Then we can simply compare those without making copies at all
> during the comparison.
>
> Fixes: 4fbc0c711b24 ("ceph: remove the extra slashes in the server path")
> Reported-by: syzbot+98704a51af8e3d9425a9@syzkaller.appspotmail.com
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/ceph/super.c | 170 ++++++++++++++++++++++--------------------------
>  fs/ceph/super.h |   1 +
>  2 files changed, 79 insertions(+), 92 deletions(-)
>
> diff --git a/fs/ceph/super.c b/fs/ceph/super.c
> index 5fa28e98d2b8..196d547c7054 100644
> --- a/fs/ceph/super.c
> +++ b/fs/ceph/super.c
> @@ -208,6 +208,69 @@ struct ceph_parse_opts_ctx {
>         struct ceph_mount_options       *opts;
>  };
>
> +/**
> + * canonicalize_path - Remove the extra slashes in the server path
> + * @server_path: the server path and could be NULL

Hi Jeff,

It doesn't look like server_path can be NULL, and the code doesn't
handle that either.

> + *
> + * Return NULL if the path is NULL or only consists of "/", or a string
> + * without any extra slashes including the leading slash(es) and the

It can return an error, so this should say "string, NULL or error", but
see below.

> + * slash(es) at the end of the server path, such as:
> + * "//dir1////dir2///" --> "dir1/dir2"
> + */
> +static char *canonicalize_path(const char *server_path)
> +{
> +       const char *path = server_path;
> +       const char *cur, *end;
> +       char *buf, *p;
> +       int len;
> +
> +       /* remove all the leading slashes */
> +       while (*path == '/')
> +               path++;
> +
> +       /* if the server path only consists of slashes */
> +       if (*path == '\0')
> +               return NULL;
> +
> +       len = strlen(path);
> +
> +       buf = kmalloc(len + 1, GFP_KERNEL);
> +       if (!buf)
> +               return ERR_PTR(-ENOMEM);
> +
> +       end = path + len;
> +       p = buf;
> +       do {
> +               cur = strchr(path, '/');
> +               if (!cur)
> +                       cur = end;
> +
> +               len = cur - path;
> +
> +               /* including one '/' */
> +               if (cur != end)
> +                       len += 1;
> +
> +               memcpy(p, path, len);
> +               p += len;
> +
> +               while (cur <= end && *cur == '/')
> +                       cur++;
> +               path = cur;
> +       } while (path < end);
> +
> +       *p = '\0';
> +
> +       /*
> +        * remove the last slash if there has and just to make sure that
> +        * we will get something like "dir1/dir2"
> +        */
> +       if (*(--p) == '/')
> +               *p = '\0';
> +
> +       return buf;
> +}

I realize that you just adapted the existing function, but it really
looks like a mouthful -- both the signature (string, NULL or error)
and the code.  It could be a lot more concise...

> +
>  /*
>   * Parse the source parameter.  Distinguish the server list from the path.
>   *
> @@ -230,15 +293,23 @@ static int ceph_parse_source(struct fs_parameter *param, struct fs_context *fc)
>
>         dev_name_end = strchr(dev_name, '/');
>         if (dev_name_end) {
> -               kfree(fsopt->server_path);
>

Blank line.

>                 /*
>                  * The server_path will include the whole chars from userland
>                  * including the leading '/'.
>                  */
> +               kfree(fsopt->server_path);
>                 fsopt->server_path = kstrdup(dev_name_end, GFP_KERNEL);
>                 if (!fsopt->server_path)
>                         return -ENOMEM;
> +
> +               kfree(fsopt->canon_path);
> +               fsopt->canon_path = canonicalize_path(fsopt->server_path);
> +               if (fsopt->canon_path && IS_ERR(fsopt->canon_path)) {
> +                       ret = PTR_ERR(fsopt->canon_path);
> +                       fsopt->canon_path = NULL;
> +                       return ret;
> +               }
>         } else {
>                 dev_name_end = dev_name + strlen(dev_name);
>         }
> @@ -447,6 +518,7 @@ static void destroy_mount_options(struct ceph_mount_options *args)
>         kfree(args->snapdir_name);
>         kfree(args->mds_namespace);
>         kfree(args->server_path);
> +       kfree(args->canon_path);
>         kfree(args->fscache_uniq);
>         kfree(args);
>  }
> @@ -462,73 +534,6 @@ static int strcmp_null(const char *s1, const char *s2)
>         return strcmp(s1, s2);
>  }
>
> -/**
> - * path_remove_extra_slash - Remove the extra slashes in the server path
> - * @server_path: the server path and could be NULL
> - *
> - * Return NULL if the path is NULL or only consists of "/", or a string
> - * without any extra slashes including the leading slash(es) and the
> - * slash(es) at the end of the server path, such as:
> - * "//dir1////dir2///" --> "dir1/dir2"
> - */
> -static char *path_remove_extra_slash(const char *server_path)
> -{
> -       const char *path = server_path;
> -       const char *cur, *end;
> -       char *buf, *p;
> -       int len;
> -
> -       /* if the server path is omitted */
> -       if (!path)
> -               return NULL;
> -
> -       /* remove all the leading slashes */
> -       while (*path == '/')
> -               path++;
> -
> -       /* if the server path only consists of slashes */
> -       if (*path == '\0')
> -               return NULL;
> -
> -       len = strlen(path);
> -
> -       buf = kmalloc(len + 1, GFP_KERNEL);
> -       if (!buf)
> -               return ERR_PTR(-ENOMEM);
> -
> -       end = path + len;
> -       p = buf;
> -       do {
> -               cur = strchr(path, '/');
> -               if (!cur)
> -                       cur = end;
> -
> -               len = cur - path;
> -
> -               /* including one '/' */
> -               if (cur != end)
> -                       len += 1;
> -
> -               memcpy(p, path, len);
> -               p += len;
> -
> -               while (cur <= end && *cur == '/')
> -                       cur++;
> -               path = cur;
> -       } while (path < end);
> -
> -       *p = '\0';
> -
> -       /*
> -        * remove the last slash if there has and just to make sure that
> -        * we will get something like "dir1/dir2"
> -        */
> -       if (*(--p) == '/')
> -               *p = '\0';
> -
> -       return buf;
> -}
> -
>  static int compare_mount_options(struct ceph_mount_options *new_fsopt,
>                                  struct ceph_options *new_opt,
>                                  struct ceph_fs_client *fsc)
> @@ -536,7 +541,6 @@ static int compare_mount_options(struct ceph_mount_options *new_fsopt,
>         struct ceph_mount_options *fsopt1 = new_fsopt;
>         struct ceph_mount_options *fsopt2 = fsc->mount_options;
>         int ofs = offsetof(struct ceph_mount_options, snapdir_name);
> -       char *p1, *p2;
>         int ret;
>
>         ret = memcmp(fsopt1, fsopt2, ofs);
> @@ -546,21 +550,12 @@ static int compare_mount_options(struct ceph_mount_options *new_fsopt,
>         ret = strcmp_null(fsopt1->snapdir_name, fsopt2->snapdir_name);
>         if (ret)
>                 return ret;
> +
>         ret = strcmp_null(fsopt1->mds_namespace, fsopt2->mds_namespace);
>         if (ret)
>                 return ret;
>
> -       p1 = path_remove_extra_slash(fsopt1->server_path);
> -       if (IS_ERR(p1))
> -               return PTR_ERR(p1);
> -       p2 = path_remove_extra_slash(fsopt2->server_path);
> -       if (IS_ERR(p2)) {
> -               kfree(p1);
> -               return PTR_ERR(p2);
> -       }
> -       ret = strcmp_null(p1, p2);
> -       kfree(p1);
> -       kfree(p2);
> +       ret = strcmp_null(fsopt1->canon_path, fsopt2->canon_path);
>         if (ret)
>                 return ret;
>
> @@ -963,7 +958,9 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc,
>         mutex_lock(&fsc->client->mount_mutex);
>
>         if (!fsc->sb->s_root) {
> -               const char *path, *p;
> +               const char *path = fsc->mount_options->canon_path ?
> +                                       fsc->mount_options->canon_path : "";
> +
>                 err = __ceph_open_session(fsc->client, started);
>                 if (err < 0)
>                         goto out;
> @@ -975,22 +972,11 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc,
>                                 goto out;
>                 }
>
> -               p = path_remove_extra_slash(fsc->mount_options->server_path);
> -               if (IS_ERR(p)) {
> -                       err = PTR_ERR(p);
> -                       goto out;
> -               }
> -               /* if the server path is omitted or just consists of '/' */
> -               if (!p)
> -                       path = "";
> -               else
> -                       path = p;
>                 dout("mount opening path '%s'\n", path);
>
>                 ceph_fs_debugfs_init(fsc);
>
>                 root = open_root_dentry(fsc, path, started);
> -               kfree(p);
>                 if (IS_ERR(root)) {
>                         err = PTR_ERR(root);
>                         goto out;
> diff --git a/fs/ceph/super.h b/fs/ceph/super.h
> index e8f891770f9d..70aa32cfb64d 100644
> --- a/fs/ceph/super.h
> +++ b/fs/ceph/super.h
> @@ -94,6 +94,7 @@ struct ceph_mount_options {
>         char *snapdir_name;   /* default ".snap" */
>         char *mds_namespace;  /* default NULL */
>         char *server_path;    /* default  "/" */
> +       char *canon_path;     /* default "/" */

Why keep both the original and canon_path around?  It looks like
the only remaining use of server_path is in create_session_open_msg().
Since that's just metadata, I think we can safely switch it over.

Also, the comment is misleading.  The default is NULL (which _means_
"/" for metadata purposes but "/" is never stored here).

I'll post what I have in mind as a patch shortly.

Thanks,

                Ilya