On Thu, Jul 9, 2020 at 3:40 PM Jann Horn <jannh@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx > Fixes: 457b9a6f09f0 ("Staging: android: add binder driver") > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> Nice catch. Acked-by: Todd Kjos <tkjos@xxxxxxxxxx> > --- > 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 > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
