On Fri, 2017-01-27 at 18:49 -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.
>
> 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) {
I'm surprised the compiler doesn't complain about ambiguous order of ops
in the above if statement. Might be nice to add some parenthesis there
since you're in here, just for clarity.
> + 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);
It's hard to picture any applications that would rely on the legacy
behavior, but if they come out of the woodwork, we could always add a
"make my kernel unsafe" command-line or compile time switch to bring it
back.
I think this is reasonable thing to do, but Michael K. is correct that
we should document the behavior changes in the relevant manpages.
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
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