From: Michal Hocko <mhocko@xxxxxxxx>
Subject: mm/mmap.c: do not blow on PROT_NONE MAP_FIXED holes in the stack
"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: 1be7107fbe18 ("mm: larger stack guard gap, between vmas")
Link: http://lkml.kernel.org/r/20170705182849.GA18027@xxxxxxxxxxxxxx
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
Debugged-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Cc: Willy Tarreau <w@xxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mmap.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff -puN mm/mmap.c~mm-mm-mmap-do-not-blow-on-prot_none-map_fixed-holes-in-the-stack mm/mmap.c
--- a/mm/mmap.c~mm-mm-mmap-do-not-blow-on-prot_none-map_fixed-holes-in-the-stack
+++ a/mm/mmap.c
@@ -2244,7 +2244,8 @@ int expand_upwards(struct vm_area_struct
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_stru
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? */
_
--
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