[PATCH 8/8] mm: enable maple tree RCU mode by default.

Liam R. Howlett
2023-03-27T18:56:16Z

From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>

Use the maple tree in RCU mode for VMA tracking.

The maple tree tracks the stack and is able to update the pivot
(lower/upper boundary) in-place to allow the page fault handler to write
to the tree while holding just the mmap read lock. This is safe as the
writes to the stack have a guard VMA which ensures there will always be
a NULL in the direction of the growth and thus will only update a pivot.

It is possible, but not recommended, to have VMAs that grow up/down
without guard VMAs. syzbot has constructed a testcase which sets up a
VMA to grow and consume the empty space. Overwriting the entire NULL
entry causes the tree to be altered in a way that is not safe for
concurrent readers; the readers may see a node being rewritten or one
that does not match the maple state they are using.

Enabling RCU mode allows the concurrent readers to see a stable node and
will return the expected result.

Link: https://lkml.kernel.org/r/20230227173632.3292573-9-surenb@google.com
Link: https://lore.kernel.org/linux-mm/000000000000b0a65805f663ace6@google.com/
Cc: stable@vger.kernel.org
Fixes: d4af56c5c7c6 ("mm: start tracking VMAs with maple tree")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: syzbot+8d95422d3537159ca390@syzkaller.appspotmail.com
---
include/linux/mm_types.h | 3 ++-
kernel/fork.c | 3 +++
mm/mmap.c | 3 ++-
3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0722859c3647..a57e6ae78e65 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -774,7 +774,8 @@ struct mm_struct {
unsigned long cpu_bitmap[];
};

-#define MM_MT_FLAGS (MT_FLAGS_ALLOC_RANGE | MT_FLAGS_LOCK_EXTERN)
+#define MM_MT_FLAGS (MT_FLAGS_ALLOC_RANGE | MT_FLAGS_LOCK_EXTERN | \
+ MT_FLAGS_USE_RCU)
extern struct mm_struct init_mm;

/* Pointer magic because the dynamic array size confuses some compilers. */
diff --git a/kernel/fork.c b/kernel/fork.c
index d8cda4c6de6c..1bf31ba07e85 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -617,6 +617,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
if (retval)
goto out;

+ mt_clear_in_rcu(vmi.mas.tree);
for_each_vma(old_vmi, mpnt) {
struct file *file;

@@ -700,6 +701,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
retval = arch_dup_mmap(oldmm, mm);
loop_out:
vma_iter_free(&vmi);
+ if (!retval)
+ mt_set_in_rcu(vmi.mas.tree);
out:
mmap_write_unlock(mm);
flush_tlb_mm(oldmm);
diff --git a/mm/mmap.c b/mm/mmap.c
index 740b54be3ed4..16cbb83b3ec6 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2277,7 +2277,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
int count = 0;
int error = -ENOMEM;
MA_STATE(mas_detach, &mt_detach, 0, 0);
- mt_init_flags(&mt_detach, MT_FLAGS_LOCK_EXTERN);
+ mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
mt_set_external_lock(&mt_detach, &mm->mmap_lock);

/*
@@ -3042,6 +3042,7 @@ void exit_mmap(struct mm_struct *mm)
*/
set_bit(MMF_OOM_SKIP, &mm->flags);
mmap_write_lock(mm);
+ mt_clear_in_rcu(&mm->mm_mt);
free_pgtables(&tlb, &mm->mm_mt, vma, FIRST_USER_ADDRESS,
USER_PGTABLES_CEILING);
tlb_finish_mmu(&tlb);
--
2.39.2