Commit 98da7d08 authored Jun 23, 2017 by Kees Cook Committed by Linus Torvalds Jun 23, 2017

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@linux-foundation.org: additional commenting from Kees]
Fixes: b6a2fea3 ("mm: variable length argument support")
Link: http://lkml.kernel.org/r/20170622001720.GA32173@beast


Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Qualys Security Advisory <qsa@qualys.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 8818efaa

fs/exec.c

+24 −4

Original line number	Diff line number	Diff line
		@@ -220,8 +220,26 @@ 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.
		*
		* 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 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)

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