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

From: Rich Felker <[email protected]>

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 <[email protected]>
---

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.

--- 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