Re: [Qemu-devel] [PATCH 23/29] 9pfs: local: chmod: don't follow symlinks

[Greg Kurz <groug@kaod.org>]

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On Thu, 23 Feb 2017 15:10:42 +0000
Stefan Hajnoczi <stefanha@gmail.com> wrote:

> On Mon, Feb 20, 2017 at 03:42:19PM +0100, Greg Kurz wrote:
> > The local_chmod() callback is vulnerable to symlink attacks because it
> > calls:
> > 
> > (1) chmod() which follows symbolic links for all path elements
> > (2) local_set_xattr()->setxattr() which follows symbolic links for all
> >     path elements
> > (3) local_set_mapped_file_attr() which calls in turn local_fopen() and
> >     mkdir(), both functions following symbolic links for all path
> >     elements but the rightmost one
> > 
> > This patch converts local_chmod() to rely on open_nofollow() and
> > fchmod() to fix (1).
> > 
> > It introduces a local_set_xattrat() replacement to local_set_xattr()
> > based on fsetxattrat() to fix (2), and a local_set_mapped_file_attrat()
> > replacement to local_set_mapped_file_attr() based on local_fopenat()
> > and mkdirat() to fix (3). No effort is made to factor out code because
> > both local_set_xattr() and local_set_mapped_file_attr() will be dropped
> > when all users have been converted to use the "at" versions.
> > 
> > This partly fixes CVE-2016-9602.
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >  hw/9pfs/9p-local.c |  163 ++++++++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 150 insertions(+), 13 deletions(-)
> > 
> > diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
> > index fb536fdb3082..56d7731f7ce1 100644
> > --- a/hw/9pfs/9p-local.c
> > +++ b/hw/9pfs/9p-local.c
> > @@ -327,6 +327,87 @@ err_out:
> >      return ret;
> >  }
> >  
> > +static int local_set_mapped_file_attrat(int dirfd, const char *name,
> > +                                        FsCred *credp)
> > +{
> > +    FILE *fp;
> > +    int ret;
> > +    char buf[ATTR_MAX];
> > +    int uid = -1, gid = -1, mode = -1, rdev = -1;
> > +    int map_dirfd;
> > +
> > +    ret = mkdirat(dirfd, VIRTFS_META_DIR, 0700);
> > +    if (ret < 0 && errno != EEXIST) {
> > +        return -1;
> > +    }
> > +
> > +    map_dirfd = openat(dirfd, VIRTFS_META_DIR,
> > +                       O_RDONLY | O_DIRECTORY | O_NOFOLLOW);
> > +    if (map_dirfd == -1) {
> > +        return -1;
> > +    }
> > +
> > +    fp = local_fopenat(map_dirfd, name, "r");
> > +    if (!fp) {
> > +        if (errno == ENOENT) {
> > +            goto update_map_file;
> > +        } else {
> > +            close_preserve_errno(map_dirfd);
> > +            return -1;
> > +        }
> > +    }
> > +    memset(buf, 0, ATTR_MAX);
> > +    while (fgets(buf, ATTR_MAX, fp)) {
> > +        if (!strncmp(buf, "virtfs.uid", 10)) {
> > +            uid = atoi(buf + 11);
> > +        } else if (!strncmp(buf, "virtfs.gid", 10)) {
> > +            gid = atoi(buf + 11);
> > +        } else if (!strncmp(buf, "virtfs.mode", 11)) {
> > +            mode = atoi(buf + 12);
> > +        } else if (!strncmp(buf, "virtfs.rdev", 11)) {
> > +            rdev = atoi(buf + 12);
> > +        }
> > +        memset(buf, 0, ATTR_MAX);
> > +    }
> > +    fclose(fp);
> > +
> > +update_map_file:
> > +    fp = local_fopenat(map_dirfd, name, "w");
> > +    close_preserve_errno(map_dirfd);
> > +    if (!fp) {
> > +        return -1;
> > +    }
> > +
> > +    if (credp->fc_uid != -1) {
> > +        uid = credp->fc_uid;
> > +    }
> > +    if (credp->fc_gid != -1) {
> > +        gid = credp->fc_gid;
> > +    }
> > +    if (credp->fc_mode != -1) {
> > +        mode = credp->fc_mode;
> > +    }
> > +    if (credp->fc_rdev != -1) {
> > +        rdev = credp->fc_rdev;
> > +    }
> > +
> > +    if (uid != -1) {
> > +        fprintf(fp, "virtfs.uid=%d\n", uid);
> > +    }
> > +    if (gid != -1) {
> > +        fprintf(fp, "virtfs.gid=%d\n", gid);
> > +    }
> > +    if (mode != -1) {
> > +        fprintf(fp, "virtfs.mode=%d\n", mode);
> > +    }
> > +    if (rdev != -1) {
> > +        fprintf(fp, "virtfs.rdev=%d\n", rdev);
> > +    }
> > +    fclose(fp);
> > +
> > +    return 0;
> > +}
> > +
> >  static int local_set_xattr(const char *path, FsCred *credp)
> >  {
> >      int err;
> > @@ -362,6 +443,45 @@ static int local_set_xattr(const char *path, FsCred *credp)
> >      return 0;
> >  }
> >  
> > +static int local_set_xattrat(int dirfd, const char *path, FsCred *credp)
> > +{
> > +    int err;
> > +
> > +    if (credp->fc_uid != -1) {
> > +        uint32_t tmp_uid = cpu_to_le32(credp->fc_uid);
> > +        err = fsetxattrat_nofollow(dirfd, path, "user.virtfs.uid", &tmp_uid,
> > +                                   sizeof(uid_t), 0);
> > +        if (err) {
> > +            return err;
> > +        }
> > +    }
> > +    if (credp->fc_gid != -1) {
> > +        uint32_t tmp_gid = cpu_to_le32(credp->fc_gid);
> > +        err = fsetxattrat_nofollow(dirfd, path, "user.virtfs.gid", &tmp_gid,
> > +                                   sizeof(gid_t), 0);
> > +        if (err) {
> > +            return err;
> > +        }
> > +    }
> > +    if (credp->fc_mode != -1) {
> > +        uint32_t tmp_mode = cpu_to_le32(credp->fc_mode);
> > +        err = fsetxattrat_nofollow(dirfd, path, "user.virtfs.mode", &tmp_mode,
> > +                                   sizeof(mode_t), 0);
> > +        if (err) {
> > +            return err;
> > +        }
> > +    }
> > +    if (credp->fc_rdev != -1) {
> > +        uint64_t tmp_rdev = cpu_to_le64(credp->fc_rdev);
> > +        err = fsetxattrat_nofollow(dirfd, path, "user.virtfs.rdev", &tmp_rdev,
> > +                                   sizeof(dev_t), 0);
> > +        if (err) {
> > +            return err;
> > +        }
> > +    }
> > +    return 0;
> > +}
> > +
> >  static int local_post_create_passthrough(FsContext *fs_ctx, const char *path,
> >                                           FsCred *credp)
> >  {
> > @@ -553,21 +673,38 @@ static ssize_t local_pwritev(FsContext *ctx, V9fsFidOpenState *fs,
> >  
> >  static int local_chmod(FsContext *fs_ctx, V9fsPath *fs_path, FsCred *credp)
> >  {
> > -    char *buffer;
> >      int ret = -1;
> > -    char *path = fs_path->data;
> > +    int dirfd;
> >  
> > -    if (fs_ctx->export_flags & V9FS_SM_MAPPED) {
> > -        buffer = rpath(fs_ctx, path);
> > -        ret = local_set_xattr(buffer, credp);
> > -        g_free(buffer);
> > -    } else if (fs_ctx->export_flags & V9FS_SM_MAPPED_FILE) {
> > -        return local_set_mapped_file_attr(fs_ctx, path, credp);
> > -    } else if ((fs_ctx->export_flags & V9FS_SM_PASSTHROUGH) ||
> > -               (fs_ctx->export_flags & V9FS_SM_NONE)) {
> > -        buffer = rpath(fs_ctx, path);
> > -        ret = chmod(buffer, credp->fc_mode);
> > -        g_free(buffer);
> > +    if (fs_ctx->export_flags & V9FS_SM_MAPPED ||
> > +        fs_ctx->export_flags & V9FS_SM_MAPPED_FILE) {
> > +        char *dirpath, *name;
> > +
> > +        dirpath = g_path_get_dirname(fs_path->data);
> > +        dirfd = local_opendir_nofollow(fs_ctx, dirpath);
> > +        g_free(dirpath);
> > +        if (dirfd == -1) {
> > +            return -1;
> > +        }
> > +
> > +        name = g_path_get_basename(fs_path->data);
> > +        if (fs_ctx->export_flags & V9FS_SM_MAPPED) {
> > +            ret = local_set_xattrat(dirfd, name, credp);
> > +        } else {
> > +            ret = local_set_mapped_file_attrat(dirfd, name, credp);
> > +        }
> > +        g_free(name);
> > +        close_preserve_errno(dirfd);
> > +    } else if (fs_ctx->export_flags & V9FS_SM_PASSTHROUGH ||
> > +               fs_ctx->export_flags & V9FS_SM_NONE) {
> > +        int fd;
> > +
> > +        fd = local_open_nofollow(fs_ctx, fs_path->data, O_RDONLY, 0);
> > +        if (fd == -1) {
> > +            return -1;
> > +        }
> > +        ret = fchmod(fd, credp->fc_mode);
> > +        close_preserve_errno(fd);  
> 
> As mentioned on IRC, not sure this is workable since chmod(2) must not
> require read permission on the file.  This patch introduces failures
> when the file isn't readable.
> 

Yeah :-\ This one is the more painful issue to address. I need a
chmod() variant that would fail on symlinks instead of following
them... and I'm kinda suffering to implement this in a race-free
manner.

Cc'ing Eric for insights.

> Stefan


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