From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751859AbdGEQ4R (ORCPT ); Wed, 5 Jul 2017 12:56:17 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:34501 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751653AbdGEQ4Q (ORCPT ); Wed, 5 Jul 2017 12:56:16 -0400 From: Michal Hocko To: Linus Torvalds , Andrew Morton Cc: Vlastimil Babka , Ben Hutchings , Willy Tarreau , Oleg Nesterov , Rik van Riel , LKML , , Michal Hocko Subject: [PATCH] mm: mm, mmap: do not blow on PROT_NONE MAP_FIXED holes in the stack Date: Wed, 5 Jul 2017 18:56:02 +0200 Message-Id: <20170705165602.15005-1-mhocko@kernel.org> X-Mailer: git-send-email 2.11.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Michal Hocko "mm: enlarge stack guard gap" has introduced a regression in some rust and Java environments which are trying to implement their own stack guard page. They are punching a new MAP_FIXED mapping inside the existing stack Vma. This will confuse expand_{downwards,upwards} into thinking that the stack expansion would in fact get us too close to an existing non-stack vma which is a correct behavior wrt. safety. It is a real regression on the other hand. Let's work around the problem by considering PROT_NONE mapping as a part of the stack. This is a gros hack but overflowing to such a mapping would trap anyway an we only can hope that usespace knows what it is doing and handle it propely. Fixes: d4d2d35e6ef9 ("mm: larger stack guard gap, between vmas") Debugged-by: Vlastimil Babka Cc: stable Signed-off-by: Michal Hocko --- Hi, the original thread [1] has grown quite large and also a bit confusing. At least the rust part should be fixed by this patch. 32b java will probably need something more on top of this. Btw. JNI environments rely on MAP_FIXED PROT_NONE as well they were just lucky to not hit the issue yet I guess. [1] http://lkml.kernel.org/r/1499126133.2707.20.camel@decadent.org.uk mm/mmap.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/mm/mmap.c b/mm/mmap.c index f60a8bc2869c..2e996cbf4ff3 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2244,7 +2244,8 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address) gap_addr = TASK_SIZE; next = vma->vm_next; - if (next && next->vm_start < gap_addr) { + if (next && next->vm_start < gap_addr && + (next->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) { if (!(next->vm_flags & VM_GROWSUP)) return -ENOMEM; /* Check that both stack segments have the same anon_vma? */ @@ -2325,7 +2326,8 @@ int expand_downwards(struct vm_area_struct *vma, /* Enforce stack_guard_gap */ prev = vma->vm_prev; /* Check that both stack segments have the same anon_vma? */ - if (prev && !(prev->vm_flags & VM_GROWSDOWN)) { + if (prev && !(prev->vm_flags & VM_GROWSDOWN) && + (prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) { if (address - prev->vm_end < stack_guard_gap) return -ENOMEM; } -- 2.11.0