Re: [PATCH v2 2/2] fs: Harden against open(..., O_CREAT, 02777) in a setgid directory

[Michael Kerrisk]

[CC += linux-api@]

On Sat, Jan 28, 2017 at 3:49 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> Currently, if you open("foo", O_WRONLY | O_CREAT | ..., 02777) in a
> directory that is setgid and owned by a different gid than current's
> fsgid, you end up with an SGID executable that is owned by the
> directory's GID.  This is a Bad Thing (tm).  Exploiting this is
> nontrivial because most ways of creating a new file create an empty
> file and empty executables aren't particularly interesting, but this
> is nevertheless quite dangerous.
>
> Harden against this type of attack by detecting this particular
> corner case (unprivileged program creates SGID executable inode in
> SGID directory owned by a different GID) and clearing the new
> inode's SGID bit.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  fs/inode.c | 24 +++++++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index 0e1e141b094c..f6acb9232263 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2025,12 +2025,30 @@ void inode_init_owner(struct inode *inode, const struct inode *dir,
>                         umode_t mode)
>  {
>         inode->i_uid = current_fsuid();
> +       inode->i_gid = current_fsgid();
> +
>         if (dir && dir->i_mode & S_ISGID) {
> +               bool changing_gid = !gid_eq(inode->i_gid, dir->i_gid);
> +
>                 inode->i_gid = dir->i_gid;
> -               if (S_ISDIR(mode))
> +
> +               if (S_ISDIR(mode)) {
>                         mode |= S_ISGID;
> -       } else
> -               inode->i_gid = current_fsgid();
> +               } else if (((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
> +                          && S_ISREG(mode) && changing_gid
> +                          && !capable(CAP_FSETID)) {
> +                       /*
> +                        * Whoa there!  An unprivileged program just
> +                        * tried to create a new executable with SGID
> +                        * set in a directory with SGID set that belongs
> +                        * to a different group.  Don't let this program
> +                        * create a SGID executable that ends up owned
> +                        * by the wrong group.
> +                        */
> +                       mode &= ~S_ISGID;
> +               }
> +       }
> +
>         inode->i_mode = mode;
>  }
>  EXPORT_SYMBOL(inode_init_owner);
> --
> 2.9.3
>
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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/