From: Kees Cook <keescook@xxxxxxxxxxxx>
Subject: fs/exec.c: account for argv/envp pointers
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).
[akpm@xxxxxxxxxxxxxxxxxxxx: additional commenting from Kees]
Fixes: b6a2fea39318 ("mm: variable length argument support")
Link: http://lkml.kernel.org/r/20170622001720.GA32173@beast
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Qualys Security Advisory <qsa@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/exec.c | 28 ++++++++++++++++++++++++----
1 file changed, 24 insertions(+), 4 deletions(-)
diff -puN fs/exec.c~exec-account-for-argv-envp-pointers fs/exec.c
--- a/fs/exec.c~exec-account-for-argv-envp-pointers
+++ a/fs/exec.c
@@ -220,8 +220,26 @@ static struct page *get_arg_page(struct
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.
+ *
+ * The size calculation is the entire vma while each arg page is
+ * built, so each time we get here it's calculating how far it
+ * is currently (rather than each call being just the newly
+ * added size from the arg page). As a result, we need to
+ * always add the entire size of the pointers, so that on the
+ * last call to get_arg_page() we'll actually have the entire
+ * correct size.
+ */
+ 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 +257,15 @@ static struct page *get_arg_page(struct
* 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)
_
--
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