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

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Jan 31, 2017 at 3:43 AM, Jeff Layton <jlayton@redhat.com> wrote:
> 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@vger.kernel.org
>> Signed-off-by: Andy Lutomirski <luto@kernel.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) {
>
> 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.

I'll keep that in mind if I do further cleanups here.

>
>> +             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'm having trouble thinking of any legitimate use.  Sure, some package
manager or untar-like tool could create a setgid file like this, but
as soon as it tries to write to the file, unless it exploits a
different bug, the setgid bit would be cleared.

--Andy

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