Patch "exec: Limit arg stack to at most 75% of _STK_LIM" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

exec: Limit arg stack to at most 75% of _STK_LIM

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     exec-limit-arg-stack-to-at-most-75-of-_stk_lim.patch

and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Kees Cook <[email protected]>
Date: Fri, 7 Jul 2017 11:57:29 -0700
Subject: exec: Limit arg stack to at most 75% of _STK_LIM

From: Kees Cook <[email protected]>

commit da029c11e6b12f321f36dac8771e833b65cec962 upstream.

To avoid pathological stack usage or the need to special-case setuid execs, just limit all arg stack usage to at most 75% of _STK_LIM (6MB).

---
 fs/exec.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

@@ -215,8 +215,7 @@ static struct page *get_arg_page(struct
 
 	if (write) {
 		unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
-		unsigned long ptr_size;
-		struct rlimit *rlim;
+		unsigned long ptr_size, limit;
 
 		/*
 		 * Since the stack will hold pointers to the strings, we

@@ -245,14 +244,16 @@ static struct page *get_arg_page(struct
 			return page;
 
 		/*
-		 * Limit to 1/4-th the stack size for the argv+env strings.
+		 * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM
+		 * (whichever is smaller) for the argv+env strings.
 		 * This ensures that:
 		 *  - the remaining binfmt code will not run out of stack space,
 		 *  - the program will have a reasonable amount of stack left
 		 *    to work from.
 		 */
-		rlim = current->signal->rlim;
-		if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
+		limit = _STK_LIM / 4 * 3;
+		limit = min(limit, rlimit(RLIMIT_STACK) / 4);
+		if (size > limit)
 			goto fail;
 	}
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/binfmt_elf-use-elf_et_dyn_base-only-for-pie.patch queue-4.9/exec-limit-arg-stack-to-at-most-75-of-_stk_lim.patch queue-4.9/s390-reduce-elf_et_dyn_base.patch queue-4.9/arm-move-elf_et_dyn_base-to-4mb.patch queue-4.9/arm64-move-elf_et_dyn_base-to-4gb-4mb.patch queue-4.9/selftests-capabilities-fix-the-test_execve-test.patch queue-4.9/powerpc-move-elf_et_dyn_base-to-4gb-4mb.patch