[PATCH 5.8 19/38] binder: Prevent context manager from incrementing ref 0

From: Greg Kroah-Hartman
Date: Mon Aug 10 2020 - 11:20:48 EST

From: Jann Horn <jannh@xxxxxxxxxx>

commit 4b836a1426cb0f1ef2a6e211d7e553221594f8fc upstream.

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.

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

- 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
- 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

Additionally, print a warning if someone manages to find another way to
trigger a transaction-to-self bug in the future.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Acked-by: Todd Kjos <tkjos@xxxxxxxxxx>
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Reviewed-by: Martijn Coenen <maco@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200727120424.1627555-1-jannh@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

drivers/android/binder.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)

--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2982,6 +2982,12 @@ static void binder_transaction(struct bi
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 +3641,17 @@ static int binder_thread_write(struct bi
struct binder_node *ctx_mgr_node;
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",
+ proc->pid, thread->pid);
+ mutex_unlock(&context->context_mgr_node_lock);
+ return -EINVAL;
+ }
ret = binder_inc_ref_for_node(
proc, ctx_mgr_node,
strong, NULL, &rdata);
+ }
if (ret)