Commit 32e4e6d5 authored Jul 10, 2017 by Oleg Nesterov Committed by Linus Torvalds Jul 10, 2017

mm/mmap.c: expand_downwards: don't require the gap if !vm_prev

expand_stack(vma) fails if address < stack_guard_gap even if there is no
vma->vm_prev.  I don't think this makes sense, and we didn't do this
before the recent commit 1be7107f ("mm: larger stack guard gap,
between vmas").

We do not need a gap in this case, any address is fine as long as
security_mmap_addr() doesn't object.

This also simplifies the code, we know that address >= prev->vm_end and
thus underflow is not possible.

Link: http://lkml.kernel.org/r/20170628175258.GA24881@redhat.com


Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 561b5e07

mm/mmap.c

+3 −7

Original line number	Diff line number	Diff line
		@@ -2316,7 +2316,6 @@ int expand_downwards(struct vm_area_struct *vma,
		{
		struct mm_struct *mm = vma->vm_mm;
		struct vm_area_struct *prev;
		unsigned long gap_addr;
		int error;

		address &= PAGE_MASK;
		@@ -2325,15 +2324,12 @@ int expand_downwards(struct vm_area_struct *vma,
		return error;

		/* Enforce stack_guard_gap */
		gap_addr = address - stack_guard_gap;
		if (gap_addr > address)
		return -ENOMEM;
		prev = vma->vm_prev;
		if (prev && prev->vm_end > gap_addr &&
		/* Check that both stack segments have the same anon_vma? */
		if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
		(prev->vm_flags & (VM_WRITE\|VM_READ\|VM_EXEC))) {
		if (!(prev->vm_flags & VM_GROWSDOWN))
		if (address - prev->vm_end < stack_guard_gap)
		return -ENOMEM;
		/* Check that both stack segments have the same anon_vma? */
		}

		/* We must make sure the anon_vma is allocated. */