Re: [PATCH] fs: clear file set[ug]id when writing via mmap

[Kees Cook]

On Mon, Nov 23, 2015 at 4:26 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Thu 19-11-15 16:10:43, Kees Cook wrote:
>> Normally, when a user can modify a file that has setuid or setgid bits,
>> those bits are cleared when they are not the file owner or a member of the
>> group. This is enforced when using write() directly but not when writing
>> to a shared mmap on the file. This could allow the file writer to gain
>> privileges by changing the binary without losing the setuid/setgid bits.
>>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>
> So I had another look at this and now I understand why we didn't do it from
> the start:
>
> To call file_remove_privs() safely, we need to hold inode->i_mutex since
> that operations is going to modify file mode / extended attributes and
> i_mutex protects those. However we cannot get i_mutex in the page fault
> path as that ranks above mmap_sem which we hold during the whole page
> fault.

Ah, I see the notation in __generic_file_write_iter about i_mutex.
Should file_remove_privs() get some debug annotation to catch callers
that don't hold that mutex? (That would have alerted me much earlier.)

> So calling file_remove_privs() when opening the file is probably as good as
> it can get. It doesn't catch the case when suid bits / IMA attrs are set
> while the file is already open but I don't see easy way around this.

I agree with Eric: mmap time seems like the right place.

> BTW: This is another example where page fault locking is constraining us
> and life would be simpler for filesystems we they get called without
> mmap_sem held...
>
>                                                                 Honza
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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