Commit c84daf60 authored by Jann Horn's avatar Jann Horn Committed by Kelsey Skunberg

binder: Prevent context manager from incrementing ref 0

BugLink: https://bugs.launchpad.net/bugs/1892822

commit 4b836a14 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: 457b9a6f ("Staging: android: add binder driver")
Acked-by: default avatarTodd Kjos <tkjos@google.com>
Signed-off-by: default avatarJann Horn <jannh@google.com>
Reviewed-by: default avatarMartijn Coenen <maco@android.com>
Link: https://lore.kernel.org/r/20200727120424.1627555-1-jannh@google.com
[manual backport: remove fine-grained locking and error reporting that
                  don't exist in <=4.9]
Signed-off-by: default avatarJann Horn <jannh@google.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarKhalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: default avatarIan May <ian.may@canonical.com>
Signed-off-by: default avatarKelsey Skunberg <kelsey.skunberg@canonical.com>
parent 95ad1793
......@@ -1415,6 +1415,10 @@ static void binder_transaction(struct binder_proc *proc,
return_error = BR_DEAD_REPLY;
goto err_dead_binder;
}
if (WARN_ON(proc == target_proc)) {
return_error = BR_FAILED_REPLY;
goto err_invalid_target_handle;
}
if (security_binder_transaction(proc->tsk,
target_proc->tsk) < 0) {
return_error = BR_FAILED_REPLY;
......@@ -1812,6 +1816,11 @@ static int binder_thread_write(struct binder_proc *proc,
ptr += sizeof(uint32_t);
if (target == 0 && binder_context_mgr_node &&
(cmd == BC_INCREFS || cmd == BC_ACQUIRE)) {
if (binder_context_mgr_node->proc == proc) {
binder_user_error("%d:%d context manager tried to acquire desc 0\n",
proc->pid, thread->pid);
return -EINVAL;
}
ref = binder_get_ref_for_node(proc,
binder_context_mgr_node);
if (ref->desc != target) {
......
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