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

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Message-ID: <20151120001043.GA28204@www.outflux.net>
Date:	Thu, 19 Nov 2015 16:10:43 -0800
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	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>, Willy Tarreau <w@....eu>,
	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>,
	LKML <linux-kernel@...r.kernel.org>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>, linux-mm@...ck.org
Subject: [PATCH] fs: clear file set[ug]id when writing via mmap

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);
 	}
 
 	return VM_FAULT_WRITE;
-- 
1.9.1


-- 
Kees Cook
Chrome OS Security
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