linux-kernel - Re: [PATCH] fs/binfmt_elf_fdpic.c: fix brk area overlap with stack on NOMMU

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Date:	Mon, 14 Sep 2015 11:17:17 -0400
From:	Rich Felker <dalias@...c.org>
To:	Greg Ungerer <gregungerer@...tnet.com.au>
Cc:	linux-embedded@...r.kernel.org,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Matt Mackall <mpm@...enic.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] fs/binfmt_elf_fdpic.c: fix brk area overlap with stack
 on NOMMU

On Mon, Sep 14, 2015 at 10:13:03PM +1000, Greg Ungerer wrote:
> Hi Rich,
> 
> 
> On 26/08/15 11:26, Greg Ungerer wrote:
> >On 21/08/15 05:11, Rich Felker wrote:
> >>From: Rich Felker <dalias@...c.org>
> >>
> >>On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
> >>overlap with all but the last PAGE_SIZE bytes of the stack. This leads
> >>to catastrophic memory reuse/corruption if brk is used. Fix by setting
> >>the brk area to zero size to disable its use.
> >>
> >>Signed-off-by: Rich Felker <dalias@...c.org>
> >
> >It would make sense to run this by David Howells <dhowells@...hat.com>,
> >I think he wrote this code (added to CC list).
> >
> >I have no problem with it, so:
> >
> >Acked-by: Greg Ungerer <gerg@...inux.org>
> 
> Has anybody picked this up to push to Linus?
> If not I can take it via the m68knommu tree.

As far as I know, no. If you can do it that would be great.

Rich


> >>---
> >>
> >>There is no reason for the kernel to be providing a brk area at all on
> >>NOMMU; the bFLT loader does not provide one, uClibc never uses brk on
> >>NOMMU targets, and musl libc goes out of its way to avoid using brk
> >>that might run into the stack.
> >
> >I recall a long time back someone was playing with the idea of setting
> >the brk to the unused parts of the last data area page. (Somewhat like
> >this code seems to be trying). That scheme still allocated the full
> >requested stack size (IIRC) though. And that would have been on bFLT
> >executables. Anyway, just some historical reference, not really
> >relevant now.
> >
> >Regards
> >Greg
> >
> >
> >
> >>--- fs/binfmt_elf_fdpic.c.orig	2015-08-20 18:05:19.089888654 +0000
> >>+++ fs/binfmt_elf_fdpic.c	2015-08-20 18:10:01.519871432 +0000
> >>@@ -374,10 +388,7 @@ static int load_elf_fdpic_binary(struct
> >>  		PAGE_ALIGN(current->mm->start_brk);
> >>
> >>  #else
> >>-	/* create a stack and brk area big enough for everyone
> >>-	 * - the brk heap starts at the bottom and works up
> >>-	 * - the stack starts at the top and works down
> >>-	 */
> >>+	/* create a stack area and zero-size brk area */
> >>  	stack_size = (stack_size + PAGE_SIZE - 1) & PAGE_MASK;
> >>  	if (stack_size < PAGE_SIZE * 2)
> >>  		stack_size = PAGE_SIZE * 2;
> >>@@ -400,8 +411,6 @@ static int load_elf_fdpic_binary(struct
> >>
> >>  	current->mm->brk = current->mm->start_brk;
> >>  	current->mm->context.end_brk = current->mm->start_brk;
> >>-	current->mm->context.end_brk +=
> >>-		(stack_size > PAGE_SIZE) ? (stack_size - PAGE_SIZE) : 0;
> >>  	current->mm->start_stack = current->mm->start_brk + stack_size;
> >>  #endif
> >>
> >>
> >
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