From 561b5e0709e4a248c67d024d4d94b6e31e3edf2f Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Mon, 10 Jul 2017 15:49:51 -0700 Subject: [PATCH] mm/mmap.c: do not blow on PROT_NONE MAP_FIXED holes in the stack Commit 1be7107fbe18 ("mm: larger stack guard gap, between vmas") 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: 1be7107fbe18 ("mm: larger stack guard gap, between vmas") Link: http://lkml.kernel.org/r/20170705182849.GA18027@dhcp22.suse.cz Signed-off-by: Michal Hocko Debugged-by: Vlastimil Babka Cc: Ben Hutchings Cc: Willy Tarreau Cc: Oleg Nesterov Cc: Rik van Riel Cc: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/mmap.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/mm/mmap.c b/mm/mmap.c index 7f8cfe9d9b4d..d1902872414f 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? */ @@ -2328,7 +2329,8 @@ int expand_downwards(struct vm_area_struct *vma, if (gap_addr > address) return -ENOMEM; prev = vma->vm_prev; - if (prev && prev->vm_end > gap_addr) { + if (prev && prev->vm_end > gap_addr && + (prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) { if (!(prev->vm_flags & VM_GROWSDOWN)) return -ENOMEM; /* Check that both stack segments have the same anon_vma? */ -- 2.20.1