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

[Kees Cook]

On Thu, Nov 19, 2015 at 5:00 PM, Willy Tarreau <w@xxxxxx> wrote:
> Hi Kees,
>
> On Thu, Nov 19, 2015 at 04:10:43PM -0800, 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
>> ---
>>  mm/memory.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index deb679c31f2a..4c970a4e0057 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2036,6 +2036,7 @@ static inline int wp_page_reuse(struct mm_struct *mm,
>>
>>               if (!page_mkwrite)
>>                       file_update_time(vma->vm_file);
>> +             file_remove_privs(vma->vm_file);
>
> I thought you said in one of the early mails of this thread that it
> didn't work. Or maybe I misunderstood.

I had a think-o in my earlier attempts. I understood the meaning of
page_mkwrite incorrectly.

> Also, don't you think we should move that into the if (!page_mkwrite)
> just like for the time update ?

Nope, page_mkwrite indicates if there was a vmops call to
page_mkwrite. In this case, it means "I will update the file time if
the filesystem driver didn't take care of it like it should". For
file_remove_privs, we want to always do it, since we should not depend
on filesystems to do it.

-Kees

-- 
Kees Cook
Chrome OS Security
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