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

[Kees Cook <keescook@chromium.org>]

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).

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/exec.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 904199086490..ddca2cf15f71 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -221,7 +221,6 @@ 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;
 
 		/*
 		 * Since the stack will hold pointers to the strings, we
@@ -250,14 +249,15 @@ 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)
+		if (size > min_t(unsigned long, rlimit(RLIMIT_STACK) / 4,
+						_STK_LIM / 4 * 3))
 			goto fail;
 	}
 
-- 
2.7.4


-- 
Kees Cook
Pixel Security