From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932649AbXC3WOO (ORCPT ); Fri, 30 Mar 2007 18:14:14 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932562AbXC3WOO (ORCPT ); Fri, 30 Mar 2007 18:14:14 -0400 Received: from smtp.osdl.org ([65.172.181.24]:54782 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932649AbXC3WON (ORCPT ); Fri, 30 Mar 2007 18:14:13 -0400 Date: Fri, 30 Mar 2007 15:13:53 -0700 From: Andrew Morton To: Hugh Dickins , David Howells Cc: Brian Pomerantz , viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, Nick Piggin Subject: Re: [PATCH] fix page leak during core dump Message-Id: <20070330151353.95cd56ed.akpm@linux-foundation.org> In-Reply-To: References: <20070329203913.GA5190@skull.piratehaven.org> <20070330134359.ec6d95bc.akpm@linux-foundation.org> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.6; i686-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 30 Mar 2007 23:01:45 +0100 (BST) Hugh Dickins wrote: > On Fri, 30 Mar 2007, Andrew Morton wrote: > > On Thu, 29 Mar 2007 13:39:13 -0700 > > Brian Pomerantz wrote: > > > > > When the dump cannot occur most likely because of a full file system > > > and the page to be written is the zero page, the call to > > > page_cache_release() is missed. > > > > > > Signed-off-by: Brian Pomerantz > > > > > > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c > > > index a2fceba..9cc4f0a 100644 > > > --- a/fs/binfmt_elf.c > > > +++ b/fs/binfmt_elf.c > > > @@ -1704,7 +1704,10 @@ static int elf_core_dump(long signr, struct pt_regs *regs, struct file *file) > > > DUMP_SEEK(PAGE_SIZE); > > > } else { > > > if (page == ZERO_PAGE(addr)) { > > > - DUMP_SEEK(PAGE_SIZE); > > > + if (!dump_seek(file, PAGE_SIZE)) { > > > + page_cache_release(page); > > > + goto end_coredump; > > > + } > > > > Oh for gawds sake I wish we could be rid of those idiotic macros :( > > > > This patch looks OK to me, although a refcount leak on the ZERO_PAGE is > > special, because that page is PageReserved(). > > > > It used to be the case that we'd ignore attempts to change the refcount on > > reserved pages (or at least on the ZERO_PAGE), but we changed that, so we > > now actually refcount the ZERO_PAGE. (I think, from a quick read of the > > code. This contradicts my memory of how it works). > > > > So I expect the net effect here is that a sufficiently determined attacker > > can overflow the ZERO_PAGE's refcount, thus causing it to be "freed". The > > page allocator won't actually free the page due to PG_Reserved, but it'll > > all become very noisy. > > > > Nick, Hugh: agree? > > I think so - lots of "Bad page state" messages as the count bounces > around the 0 mark, but not actually freed. But when CONFIG_DEBUG_VM > you'll get BUG_ONs. And I can't swear bad things won't happen some- > where once the count wraps to negative. Easier to fix than work out > the consequences. > > (Of course, Nick is right now proposing a patch to take us back the > other way, back to not accounting the ZERO_PAGE: so the fix needs > to go in, then he'll need to reverse that again in his patch.) OK. > Doesn't fs/binfmt_elf_fdpic.c need the same fix? It looks slightly > different there, but I think when you look closer there's exactly > the same issue? Think so. David, does it look OK? From: Brian Pomerantz When the dump cannot occur most likely because of a full file system and the page to be written is the zero page, the call to page_cache_release() is missed. Signed-off-by: Brian Pomerantz Cc: Hugh Dickins Cc: Nick Piggin Cc: Signed-off-by: Andrew Morton --- fs/binfmt_elf.c | 5 ++++- fs/binfmt_elf_fdpic.c | 6 ++++-- 2 files changed, 8 insertions(+), 3 deletions(-) diff -puN fs/binfmt_elf.c~fix-page-leak-during-core-dump fs/binfmt_elf.c --- a/fs/binfmt_elf.c~fix-page-leak-during-core-dump +++ a/fs/binfmt_elf.c @@ -1704,7 +1704,10 @@ static int elf_core_dump(long signr, str DUMP_SEEK(PAGE_SIZE); } else { if (page == ZERO_PAGE(addr)) { - DUMP_SEEK(PAGE_SIZE); + if (!dump_seek(file, PAGE_SIZE)) { + page_cache_release(page); + goto end_coredump; + } } else { void *kaddr; flush_cache_page(vma, addr, diff -puN fs/binfmt_elf_fdpic.c~fix-page-leak-during-core-dump fs/binfmt_elf_fdpic.c --- a/fs/binfmt_elf_fdpic.c~fix-page-leak-during-core-dump +++ a/fs/binfmt_elf_fdpic.c @@ -1480,8 +1480,10 @@ static int elf_fdpic_dump_segments(struc DUMP_SEEK(file->f_pos + PAGE_SIZE); } else if (page == ZERO_PAGE(addr)) { - DUMP_SEEK(file->f_pos + PAGE_SIZE); - page_cache_release(page); + if (!dump_seek(file, file->f_pos + PAGE_SIZE)) { + page_cache_release(page); + return 0; + } } else { void *kaddr; _