linux-kernel - Re: [PATCH v2 2/2] fs: Harden against open(..., O

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Message-ID: <CAHO5Pa29Jnz8U9fp1zSy4RP2LdE0CwgB=ex1tog9SZKanAEwpQ@mail.gmail.com>
Date:   Tue, 31 Jan 2017 16:50:16 +1300
From:   Michael Kerrisk <mtk.manpages@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     security@...nel.org, Konstantin Khlebnikov <koct9i@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>, Willy Tarreau <w@....eu>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        yalin wang <yalin.wang2010@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Frank Filz <ffilzlnx@...dspring.com>, stable@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] fs: Harden against open(..., O_CREAT, 02777) in a
 setgid directory

[CC += linux-api@]

On Sat, Jan 28, 2017 at 3:49 PM, Andy Lutomirski <luto@...nel.org> 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@...r.kernel.org
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  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/