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>

diff --git a/fs/exec.c b/fs/exec.c
index 9041990..62175cb 100644 (file)
--- 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;
        }