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SubjectRe: [PATCH resend] binder: Prevent context manager from incrementing ref 0
On Thu, Jul 9, 2020 at 3:40 PM Jann Horn <jannh@google.com> wrote:
>
> Binder is designed such that a binder_proc never has references to
> itself. If this rule is violated, memory corruption can occur when a
> process sends a transaction to itself; see e.g.
> <https://syzkaller.appspot.com/bug?extid=09e05aba06723a94d43d>.
>
> There is a remaining edgecase through which such a transaction-to-self
> can still occur from the context of a task with BINDER_SET_CONTEXT_MGR
> access:
>
> - task A opens /dev/binder twice, creating binder_proc instances P1
> and P2
> - P1 becomes context manager
> - P2 calls ACQUIRE on the magic handle 0, allocating index 0 in its
> handle table
> - P1 dies (by closing the /dev/binder fd and waiting a bit)
> - P2 becomes context manager
> - P2 calls ACQUIRE on the magic handle 0, allocating index 1 in its
> handle table
> [this triggers a warning: "binder: 1974:1974 tried to acquire
> reference to desc 0, got 1 instead"]
> - task B opens /dev/binder once, creating binder_proc instance P3
> - P3 calls P2 (via magic handle 0) with (void*)1 as argument (two-way
> transaction)
> - P2 receives the handle and uses it to call P3 (two-way transaction)
> - P3 calls P2 (via magic handle 0) (two-way transaction)
> - P2 calls P2 (via handle 1) (two-way transaction)
>
> And then, if P2 does *NOT* accept the incoming transaction work, but
> instead closes the binder fd, we get a crash.
>
> Solve it by preventing the context manager from using ACQUIRE on ref 0.
> There shouldn't be any legitimate reason for the context manager to do
> that.
>
> Additionally, print a warning if someone manages to find another way to
> trigger a transaction-to-self bug in the future.
>
> Cc: stable@vger.kernel.org
> Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> Signed-off-by: Jann Horn <jannh@google.com>

Nice catch.

Acked-by: Todd Kjos <tkjos@google.com>

> ---
> sending again because I forgot to CC LKML the first time... sorry about
> the spam.
>
> drivers/android/binder.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index f50c5f182bb5..cac65ff3a257 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -2982,6 +2982,12 @@ static void binder_transaction(struct binder_proc *proc,
> goto err_dead_binder;
> }
> e->to_node = target_node->debug_id;
> + if (WARN_ON(proc == target_proc)) {
> + return_error = BR_FAILED_REPLY;
> + return_error_param = -EINVAL;
> + return_error_line = __LINE__;
> + goto err_invalid_target_handle;
> + }
> if (security_binder_transaction(proc->tsk,
> target_proc->tsk) < 0) {
> return_error = BR_FAILED_REPLY;
> @@ -3635,10 +3641,16 @@ static int binder_thread_write(struct binder_proc *proc,
> struct binder_node *ctx_mgr_node;
> mutex_lock(&context->context_mgr_node_lock);
> ctx_mgr_node = context->binder_context_mgr_node;
> - if (ctx_mgr_node)
> + if (ctx_mgr_node) {
> + if (ctx_mgr_node->proc == proc) {
> + binder_user_error("%d:%d context manager tried to acquire desc 0\n");
> + mutex_unlock(&context->context_mgr_node_lock);
> + return -EINVAL;
> + }
> ret = binder_inc_ref_for_node(
> proc, ctx_mgr_node,
> strong, NULL, &rdata);
> + }
> mutex_unlock(&context->context_mgr_node_lock);
> }
> if (ret)
>
> base-commit: 2a89b99f580371b86ae9bafd6cbeccd3bfab524a
> --
> 2.27.0.389.gc38d7665816-goog
>

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