On Fri, Aug 07, 2020 at 08:45:00PM +0200, Jann Horn wrote:
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. 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") Acked-by: Todd Kjos tkjos@google.com Signed-off-by: Jann Horn jannh@google.com Reviewed-by: Martijn Coenen maco@android.com Link: https://lore.kernel.org/r/20200727120424.1627555-1-jannh@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org [manual backport: remove fine-grained locking and error reporting that don't exist in <=4.9] Signed-off-by: Jann Horn jannh@google.com
Thanks for the backport, now queued up.
greg k-h