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

[Ben Hutchings <ben@decadent.org.uk>]

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On Wed, 2017-01-25 at 13:06 -0800, Andy Lutomirski 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.
> 
> > Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  fs/inode.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index f7029c40cfbd..d7e4b80470dd 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2007,11 +2007,28 @@ void inode_init_owner(struct inode *inode, const struct inode *dir,
>  {
>  	inode->i_uid = current_fsuid();
>  	if (dir && dir->i_mode & S_ISGID) {
> +		bool changing_gid = !gid_eq(inode->i_gid, dir->i_gid);
[...]

inode->i_gid hasn't been initialised yet.  This should compare with
current_fsgid(), shouldn't it?

Ben.

-- 
Ben Hutchings
It is easier to write an incorrect program than to understand a correct
one.


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