[PATCH v2 2/2] kernel/fork: set VMA's mm/prev/next right after vm_area_dup in dup_mmap

Konstantin Khlebnikov
2020-01-07T10:20:04Z

Function vm_area_dup() makes exact copy of structure. It's better to stop
referencing parent structures because various helpers use these pointers.

For example if VM_WIPEONFORK is set then anon_vma_prepare() will try to
merge anon_vma with previous and next VMA. Poking parent VMAs and sharing
their anon_vma is safe for now but is not expected behavior.

Note: this will break commit 4e4a9eb92133 ("mm/rmap.c: reuse mergeable
anon_vma as parent when fork") without related fix because it contains
several flaws hidden by current initialization sequence in dup_mmap().

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
kernel/fork.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index c33626993831..784c9ae56aa9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -544,10 +544,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
tmp = vm_area_dup(mpnt);
if (!tmp)
goto fail_nomem;
+ tmp->vm_mm = mm;
+ tmp->vm_prev = prev;
+ tmp->vm_next = NULL;
retval = vma_dup_policy(mpnt, tmp);
if (retval)
goto fail_nomem_policy;
- tmp->vm_mm = mm;
retval = dup_userfaultfd(tmp, &uf);
if (retval)
goto fail_nomem_anon_vma_fork;
@@ -559,7 +561,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
} else if (anon_vma_fork(tmp, mpnt, prev))
goto fail_nomem_anon_vma_fork;
tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
- tmp->vm_next = tmp->vm_prev = NULL;
file = tmp->vm_file;
if (file) {
struct inode *inode = file_inode(file);
@@ -592,7 +593,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
*/
*pprev = tmp;
pprev = &tmp->vm_next;
- tmp->vm_prev = prev;
prev = tmp;

__vma_link_rb(mm, tmp, rb_link, rb_parent);