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

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Hi Rich,

On 26/08/15 11:26, Greg Ungerer wrote:
On 21/08/15 05:11, Rich Felker wrote:
From: Rich Felker <dalias@xxxxxxxx>

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@xxxxxxxx>

It would make sense to run this by David Howells <dhowells@xxxxxxxxxx>,
I think he wrote this code (added to CC list).

I have no problem with it, so:

Acked-by: Greg Ungerer <gerg@xxxxxxxxxxx>

Has anybody picked this up to push to Linus?
If not I can take it via the m68knommu tree.



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 Anyway, just some historical reference, not really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.


--- 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

-	/* 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;

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