[PATCH] exec: Account for argv/envp pointers

[Kees Cook]

When limiting the argv/envp strings during exec to 1/4 of the stack limit,
the storage of the pointers to the strings was not included. This means
that an exec with huge numbers of tiny strings could eat 1/4 of the
stack limit in strings and then additional space would be later used
by the pointers to the strings. For example, on 32-bit with a 8MB stack
rlimit, an exec with 1677721 single-byte strings would consume less than
2MB of stack, the max (8MB / 4) amount allowed, but the pointers to the
strings would consume the remaining additional stack space (1677721 *
4 == 6710884). The result (1677721 + 6710884 == 8388605) would exhaust
stack space entirely. Controlling this stack exhaustion could result in
pathological behavior in setuid binaries (CVE-2017-1000365).

Fixes: b6a2fea39318 ("mm: variable length argument support")
Cc: sta...@vger.kernel.org
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
 fs/exec.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 72934df68471..8079ca70cfda 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -220,8 +220,18 @@ 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
+                * must account for them as well.
+                */
+               ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
+               if (ptr_size > ULONG_MAX - size)
+                       goto fail;
+               size += ptr_size;
+
                acct_arg_size(bprm, size / PAGE_SIZE);
 
                /*
@@ -239,13 +249,15 @@ static struct page *get_arg_page(struct linux_binprm 
*bprm, unsigned long pos,
                 *    to work from.
                 */
                rlim = current->signal->rlim;
-               if (size > ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4) {
-                       put_page(page);
-                       return NULL;
-               }
+               if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
+                       goto fail;
        }
 
        return page;
+
+fail:
+       put_page(page);
+       return NULL;
 }
 
 static void put_arg_page(struct page *page)
-- 
2.7.4


-- 
Kees Cook
Pixel Security