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[PATCH 4.4 01/26] fs/exec.c: account for argv/envp pointers

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4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kees Cook <keescook@chromium.org>

commit 98da7d08850fb8bdeb395d6368ed15753304aa0c upstream.

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: b6a2fea39318 ("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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/exec.c |   28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -206,8 +206,26 @@ 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);
 
 		/*
@@ -225,13 +243,15 @@ 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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[PATCH 4.4 00/26] 4.4.75-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 01/26] fs/exec.c: account for argv/envp pointers Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 10/26] time: Fix clock->read(clock) race around clocksource changes Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 13/26] drm/radeon: add a PX quirk for another K53TK variant Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 07/26] signal: Only reschedule timers on signals timers have sent Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 14/26] drm/radeon: add a quirk for Toshiba Satellite L20-183 Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 04/26] KVM: PPC: Book3S HV: Preserve userspace HTM state properly Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 17/26] USB: usbip: fix nonconforming hub descriptor Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 11/26] target: Fix kref->refcount underflow in transport_cmd_finish_abort Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 05/26] CIFS: Improve readdir verbosity Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 03/26] lib/cmdline.c: fix get_options() overflow while parsing ranges Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
    Re: [PATCH 4.4 03/26] lib/cmdline.c: fix get_options() overflow  while parsing ranges Ben Hutchings <ben.hutchings@codethink.co.uk> - 2017-06-29 20:30 +0200
  [PATCH 4.4 09/26] Input: i8042 - add Fujitsu Lifebook AH544 to notimeout list Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 26/26] nvme: apply DELAY_BEFORE_CHK_RDY quirk at probe time too Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  [PATCH 4.4 19/26] of: Add check to of_scan_flat_dt() before accessing initial_boot_params Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-06-27 15:00 +0200
  Re: [PATCH 4.4 00/26] 4.4.75-stable review Guenter Roeck <linux@roeck-us.net> - 2017-06-27 21:10 +0200
  Re: [PATCH 4.4 00/26] 4.4.75-stable review Shuah Khan <shuahkh@osg.samsung.com> - 2017-06-28 16:00 +0200

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