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: [email protected] Fixes: 457b9a6f09f0 ("Staging: android: add binder driver") Acked-by: Todd Kjos <[email protected]> Signed-off-by: Jann Horn <[email protected]> --- fixed that broken binder_user_error() from the first version... I sent v1 while I had a dirty tree containing the missing fix. whoops. drivers/android/binder.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/drivers/android/binder.c b/drivers/android/binder.c index f50c5f182bb5..5b310eea9e52 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 +3641,17 @@ 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", + 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); + } mutex_unlock(&context->context_mgr_node_lock); } if (ret) base-commit: 2a89b99f580371b86ae9bafd6cbeccd3bfab524a -- 2.28.0.rc0.142.g3c755180ce-goog
