2019-06-12 20:30:21

by Todd Kjos

Subject: [PATCH] binder: fix possible UAF when freeing buffer

There is a race between the binder driver cleaning
up a completed transaction via binder_free_transaction()
and a user calling binder_ioctl(BC_FREE_BUFFER) to
release a buffer. It doesn't matter which is first but
they need to be protected against running concurrently
which can result in a UAF.

Signed-off-by: Todd Kjos <[email protected]>
---
drivers/android/binder.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 748ac489ef7eb..bc26b5511f0a9 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -1941,8 +1941,18 @@ static void binder_free_txn_fixups(struct binder_transaction *t)

static void binder_free_transaction(struct binder_transaction *t)
{
- if (t->buffer)
- t->buffer->transaction = NULL;
+ struct binder_proc *target_proc = t->to_proc;
+
+ if (target_proc) {
+ binder_inner_proc_lock(target_proc);
+ if (t->buffer)
+ t->buffer->transaction = NULL;
+ binder_inner_proc_unlock(target_proc);
+ }
+ /*
+ * If the transaction has no target_proc, then
+ * t->buffer->transaction has already been cleared.
+ */
binder_free_txn_fixups(t);
kfree(t);
binder_stats_deleted(BINDER_STAT_TRANSACTION);
@@ -3551,10 +3561,12 @@ static void binder_transaction(struct binder_proc *proc,
static void
binder_free_buf(struct binder_proc *proc, struct binder_buffer *buffer)
{
+ binder_inner_proc_lock(proc);
if (buffer->transaction) {
buffer->transaction->buffer = NULL;
buffer->transaction = NULL;
}
+ binder_inner_proc_unlock(proc);
if (buffer->async_transaction && buffer->target_node) {
struct binder_node *buf_node;
struct binder_work *w;
--
2.22.0.rc2.383.gf4fbbf30c2-goog

2019-06-13 15:45:08

by Todd Kjos

Subject: Re: [PATCH] binder: fix possible UAF when freeing buffer

On Wed, Jun 12, 2019 at 10:41 PM Greg KH <[email protected]> wrote:
>
> On Wed, Jun 12, 2019 at 01:29:27PM -0700, Todd Kjos wrote:
> > There is a race between the binder driver cleaning
> > up a completed transaction via binder_free_transaction()
> > and a user calling binder_ioctl(BC_FREE_BUFFER) to
> > release a buffer. It doesn't matter which is first but
> > they need to be protected against running concurrently
> > which can result in a UAF.
> >
> > Signed-off-by: Todd Kjos <[email protected]>
> > ---
> > drivers/android/binder.c | 16 ++++++++++++++--
> > 1 file changed, 14 insertions(+), 2 deletions(-)
>
> Does this also need to go to the stable kernels?

This patch won't apply cleanly to stable kernels so I'll submit a new
patch directly to stable@ (4.14, 4.19) once this one lands.

>
> thanks,
>
> greg k-h

2019-06-13 16:46:20

by Greg Kroah-Hartman

Subject: Re: [PATCH] binder: fix possible UAF when freeing buffer

On Wed, Jun 12, 2019 at 01:29:27PM -0700, Todd Kjos wrote:
> There is a race between the binder driver cleaning
> up a completed transaction via binder_free_transaction()
> and a user calling binder_ioctl(BC_FREE_BUFFER) to
> release a buffer. It doesn't matter which is first but
> they need to be protected against running concurrently
> which can result in a UAF.
>
> Signed-off-by: Todd Kjos <[email protected]>
> ---
> drivers/android/binder.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)

Does this also need to go to the stable kernels?

thanks,

greg k-h