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

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Message-ID: <CAGXu5jJR1KqLRUmD5_WM51k=v74gRWNA+CjsrL_oO6D494FMog@mail.gmail.com>
Date:	Thu, 19 Nov 2015 17:03:15 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Willy Tarreau <w@....eu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Andy Lutomirski <luto@...capital.net>, Jan Kara <jack@...e.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Boaz Harrosh <boaz@...xistor.com>,
	Michal Hocko <mhocko@...e.cz>,
	Haggai Eran <haggaie@...lanox.com>,
	Theodore Tso <tytso@...gle.com>,
	Dirk Steinmetz <public@...tdrjgfuzkfg.com>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Seth Forshee <seth.forshee@...onical.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] fs: clear file set[ug]id when writing via mmap

On Thu, Nov 19, 2015 at 5:00 PM, Willy Tarreau <w@....eu> 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@...omium.org>
>> Cc: stable@...r.kernel.org
>> ---
>>  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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