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mm/mmap.c: do not blow on PROT_NONE MAP_FIXED holes in the stack

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Commit 1be7107fbe ("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: 1be7107fbe ("mm: larger stack guard gap, between vmas")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Debugged-by: Vlastimil Babka <[email protected]>
Cc: Ben Hutchings <[email protected]>
Cc: Willy Tarreau <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Michal Hocko 2017-07-10 15:49:51 -07:00 • committed by Linus Torvalds
commit 561b5e0709
1 changed files with 4 additions and 2 deletions
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6
mm/mmap.c
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@ -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? */