linux-kernel - [PATCH] binder: validate alloc->mm in ->mmap() handler

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Message-ID: <20221104231235.348958-1-cmllamas@google.com>
Date:   Fri,  4 Nov 2022 23:12:35 +0000
From:   Carlos Llamas <cmllamas@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Arve Hjønnevåg" <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <brauner@...nel.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Jann Horn <jannh@...gle.com>, stable@...r.kernel.org,
        Todd Kjos <tkjos@...gle.com>
Subject: [PATCH] binder: validate alloc->mm in ->mmap() handler

Since commit 1da52815d5f1 ("binder: fix alloc->vma_vm_mm null-ptr
dereference") binder caches a pointer to the current->mm during open().
This fixes a null-ptr dereference reported by syzkaller. Unfortunately,
it also opens the door for a process to update its mm after the open(),
(e.g. via execve) making the cached alloc->mm pointer invalid.

Things get worse when the process continues to mmap() a vma. From this
point forward, binder will attempt to find this vma using an obsolete
alloc->mm reference. Such as in binder_update_page_range(), where the
wrong vma is obtained via vma_lookup(), yet binder proceeds to happily
insert new pages into it.

To avoid this issue fail the ->mmap() callback if we detect a mismatch
between the vma->vm_mm and the original alloc->mm pointer. This prevents
alloc->vm_addr from getting set, so that any subsequent vma_lookup()
calls fail as expected.

Fixes: 1da52815d5f1 ("binder: fix alloc->vma_vm_mm null-ptr dereference")
Reported-by: Jann Horn <jannh@...gle.com>
Cc: <stable@...r.kernel.org> # 5.15+
Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
---
 drivers/android/binder_alloc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 1c39cfce32fa..4ad42b0f75cd 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -739,6 +739,12 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
 	const char *failure_string;
 	struct binder_buffer *buffer;

+	if (unlikely(vma->vm_mm != alloc->mm)) {
+		ret = -EINVAL;
+		failure_string = "invalid vma->vm_mm";
+		goto err_invalid_mm;
+	}
+
 	mutex_lock(&binder_alloc_mmap_lock);
 	if (alloc->buffer_size) {
 		ret = -EBUSY;
@@ -785,6 +791,7 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
 	alloc->buffer_size = 0;
 err_already_mapped:
 	mutex_unlock(&binder_alloc_mmap_lock);
+err_invalid_mm:
 	binder_alloc_debug(BINDER_DEBUG_USER_ERROR,
 			   "%s: %d %lx-%lx %s failed %d\n", __func__,
 			   alloc->pid, vma->vm_start, vma->vm_end,
-- 
2.38.1.431.g37b22c650d-goog