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

[Kees Cook]

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 <keesc...@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