2022-06-27 15:54:54

by Liam R. Howlett

Subject: [PATCH] android: binder: Fix lockdep check on clearing vma

When munmapping a vma, the mmap_lock can be degraded to a write before
calling close() on the file handle. The binder close() function calls
binder_alloc_set_vma() to clear the vma address, which now has a lock
dep check for writing on the mmap_lock. Change the lockdep check to
ensure the reading lock is held while clearing and keep the write check
while writing.

Reported-by: [email protected]
Fixes: 472a68df605b ("android: binder: stop saving a pointer to the VMA")
Signed-off-by: Liam R. Howlett <[email protected]>
---
drivers/android/binder_alloc.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index f555eebceef6..1014beb12802 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -315,12 +315,19 @@ static inline void binder_alloc_set_vma(struct binder_alloc *alloc,
{
unsigned long vm_start = 0;

+ /*
+ * Allow clearing the vma with holding just the read lock to allow
+ * munmapping downgrade of the write lock before freeing and closing the
+ * file using binder_alloc_vma_close().
+ */
if (vma) {
vm_start = vma->vm_start;
alloc->vma_vm_mm = vma->vm_mm;
+ mmap_assert_write_locked(alloc->vma_vm_mm);
+ } else {
+ mmap_assert_locked(alloc->vma_vm_mm);
}

- mmap_assert_write_locked(alloc->vma_vm_mm);
alloc->vma_addr = vm_start;
}

--
2.35.1

2022-06-27 16:06:53

by Todd Kjos

Subject: Re: [PATCH] android: binder: Fix lockdep check on clearing vma

On Mon, Jun 27, 2022 at 8:19 AM Liam Howlett <[email protected]> wrote:
>
> When munmapping a vma, the mmap_lock can be degraded to a write before
> calling close() on the file handle. The binder close() function calls
> binder_alloc_set_vma() to clear the vma address, which now has a lock
> dep check for writing on the mmap_lock. Change the lockdep check to
> ensure the reading lock is held while clearing and keep the write check
> while writing.
>
> Reported-by: [email protected]
> Fixes: 472a68df605b ("android: binder: stop saving a pointer to the VMA")
> Signed-off-by: Liam R. Howlett <[email protected]>

Acked-by: Todd Kjos <[email protected]>
> ---
> drivers/android/binder_alloc.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> index f555eebceef6..1014beb12802 100644
> --- a/drivers/android/binder_alloc.c
> +++ b/drivers/android/binder_alloc.c
> @@ -315,12 +315,19 @@ static inline void binder_alloc_set_vma(struct binder_alloc *alloc,
> {
> unsigned long vm_start = 0;
>
> + /*
> + * Allow clearing the vma with holding just the read lock to allow
> + * munmapping downgrade of the write lock before freeing and closing the
> + * file using binder_alloc_vma_close().
> + */
> if (vma) {
> vm_start = vma->vm_start;
> alloc->vma_vm_mm = vma->vm_mm;
> + mmap_assert_write_locked(alloc->vma_vm_mm);
> + } else {
> + mmap_assert_locked(alloc->vma_vm_mm);
> }
>
> - mmap_assert_write_locked(alloc->vma_vm_mm);
> alloc->vma_addr = vm_start;
> }
>
> --
> 2.35.1