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

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Jan 25, 2017 at 1:31 PM, Ben Hutchings <ben@decadent.org.uk> wrote:
> 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?

Whoops.  In v2, I'll fix it by inode->i_gid first -- that'll simplify
the control flow.

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



-- 
Andy Lutomirski
AMA Capital Management, LLC