linux-kernel - [PATCH] binder: Address corner cases in deferred copy and fixup

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Date:   Wed, 13 Apr 2022 10:54:27 +0200
From:   Alessandro Astone <ales.astone@...il.com>
To:     tkjos@...roid.com, gregkh@...uxfoundation.org, brauner@...nel.org,
        arve@...roid.com, linux-kernel@...r.kernel.org, maco@...roid.com
Cc:     Alessandro Astone <ales.astone@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: [PATCH] binder: Address corner cases in deferred copy and fixup

When handling BINDER_TYPE_FDA object we are pushing a parent fixup
with a certain skip_size but no scatter-gather copy object, since
the copy is handled standalone.
If BINDER_TYPE_FDA is the last children the scatter-gather copy
loop will never stop to skip it, thus we are left with an item in
the parent fixup list. This will trigger the BUG_ON().

Furthermore, it is possible to receive BINDER_TYPE_FDA object
with num_fds=0 which will confuse the scatter-gather code.

In the android userspace I could only find these usecases in the
libstagefright OMX implementation, so it might be that they're
doing something very weird, but nonetheless the kernel should not
panic about it.

Fixes: 09184ae9b575 ("binder: defer copies of pre-patched txn data")
Signed-off-by: Alessandro Astone <ales.astone@...il.com>
---
 drivers/android/binder.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 8351c5638880..18ad6825ba30 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2295,7 +2295,7 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
 {
 	int ret = 0;
 	struct binder_sg_copy *sgc, *tmpsgc;
-	struct binder_ptr_fixup *pf =
+	struct binder_ptr_fixup *tmppf, *pf =
 		list_first_entry_or_null(pf_head, struct binder_ptr_fixup,
 					 node);
 
@@ -2349,7 +2349,11 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
 		list_del(&sgc->node);
 		kfree(sgc);
 	}
-	BUG_ON(!list_empty(pf_head));
+	list_for_each_entry_safe(pf, tmppf, pf_head, node) {
+		BUG_ON(pf->skip_size == 0);
+		list_del(&pf->node);
+		kfree(pf);
+	}
 	BUG_ON(!list_empty(sgc_head));
 
 	return ret > 0 ? -EINVAL : ret;
@@ -2486,6 +2490,9 @@ static int binder_translate_fd_array(struct list_head *pf_head,
 	struct binder_proc *proc = thread->proc;
 	int ret;
 
+	if (fda->num_fds == 0)
+		return 0;
+
 	fd_buf_size = sizeof(u32) * fda->num_fds;
 	if (fda->num_fds >= SIZE_MAX / sizeof(u32)) {
 		binder_user_error("%d:%d got transaction with invalid number of fds (%lld)\n",
-- 
2.35.1