Re: [PATCH] exec: Limit arg stack to at most _STK_LIM / 4 * 3

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Fri, Jul 7, 2017 at 11:57 AM, Kees Cook <keescook@chromium.org> wrote:
> To avoid pathological stack usage or the need to special-case setuid
> execs, just limit all arg stack usage to at most _STK_LIM / 4 * 3 (6MB).

Ok, this I think I should just apply, but would prefer to avoid
multi-line complex conditionals around things like this.

So how about the attached slightly edited version instead?

I didn't test it (and I'm not really committing it until I get an ack
or two), but it seemed all ObviouslyCorrect(tm). FamousLastWords(tm).

Comments?

                Linus

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From 4f677953627bc64393bcc310f4f779282a089ae3 Mon Sep 17 00:00:00 2001
From: Kees Cook <keescook@chromium.org>
Date: Fri, 7 Jul 2017 11:57:29 -0700
Subject: [PATCH] exec: Limit arg stack to at most 75% of _STK_LIM

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).

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 fs/exec.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 904199086490..62175cbcc801 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -220,8 +220,7 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
 
 	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
@@ -250,14 +249,16 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
 			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;
 	}
 
-- 
2.13.1.518.g0d864c4df