On Tue, Jan 31, 2017 at 3:43 AM, Jeff Layton <jlayton@xxxxxxxxxx> 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@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.
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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