linux-kernel - [RFC PATCH 2/3] Revert "android: binder: stop saving a pointer to the VMA"

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Message-ID: <20230424205548.1935192-2-cmllamas@google.com>
Date:   Mon, 24 Apr 2023 20:55:47 +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>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Liam Howlett <liam.howlett@...cle.com>
Subject: [RFC PATCH 2/3] Revert "android: binder: stop saving a pointer to the VMA"

This reverts commit a43cfc87caaf46710c8027a8c23b8a55f1078f19.

This patch fixed an issue reported by syzkaller in [1]. However, this
turned out to be only a band-aid in binder. The root cause, as bisected
by syzkaller, was fixed by commit 5789151e48ac ("mm/mmap: undo ->mmap()
when mas_preallocate() fails"). We no longer need the patch for binder.

Reverting such patch allows us to have a lockless access to alloc->vma
in specific cases where the mmap_lock is not required. This approach
avoids the contention that caused a performance regression.

[1] https://lore.kernel.org/all/0000000000004a0dbe05e1d749e0@google.com

[cmllamas: resolved conflicts with rework of alloc->mm and removal of
 binder_alloc_set_vma() also fixed comment section]

Cc: Liam Howlett <liam.howlett@...cle.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>
Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
---
 drivers/android/binder_alloc.c          | 17 +++++++++--------
 drivers/android/binder_alloc.h          |  4 ++--
 drivers/android/binder_alloc_selftest.c |  2 +-
 3 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 92c814ec44fe..eb082b33115b 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -213,7 +213,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 
 	if (mm) {
 		mmap_read_lock(mm);
-		vma = vma_lookup(mm, alloc->vma_addr);
+		vma = alloc->vma;
 	}
 
 	if (!vma && need_mm) {
@@ -314,9 +314,11 @@ static inline struct vm_area_struct *binder_alloc_get_vma(
 {
 	struct vm_area_struct *vma = NULL;
 
-	if (alloc->vma_addr)
-		vma = vma_lookup(alloc->mm, alloc->vma_addr);
-
+	if (alloc->vma) {
+		/* Look at description in binder_alloc_set_vma */
+		smp_rmb();
+		vma = alloc->vma;
+	}
 	return vma;
 }
 
@@ -775,7 +777,7 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
 	buffer->free = 1;
 	binder_insert_free_buffer(alloc, buffer);
 	alloc->free_async_space = alloc->buffer_size / 2;
-	alloc->vma_addr = vma->vm_start;
+	alloc->vma = vma;
 
 	return 0;
 
@@ -805,8 +807,7 @@ void binder_alloc_deferred_release(struct binder_alloc *alloc)
 
 	buffers = 0;
 	mutex_lock(&alloc->mutex);
-	BUG_ON(alloc->vma_addr &&
-	       vma_lookup(alloc->mm, alloc->vma_addr));
+	BUG_ON(alloc->vma);
 
 	while ((n = rb_first(&alloc->allocated_buffers))) {
 		buffer = rb_entry(n, struct binder_buffer, rb_node);
@@ -958,7 +959,7 @@ int binder_alloc_get_allocated_count(struct binder_alloc *alloc)
  */
 void binder_alloc_vma_close(struct binder_alloc *alloc)
 {
-	alloc->vma_addr = 0;
+	alloc->vma = 0;
 }
 
 /**
diff --git a/drivers/android/binder_alloc.h b/drivers/android/binder_alloc.h
index 0f811ac4bcff..138d1d5af9ce 100644
--- a/drivers/android/binder_alloc.h
+++ b/drivers/android/binder_alloc.h
@@ -75,7 +75,7 @@ struct binder_lru_page {
 /**
  * struct binder_alloc - per-binder proc state for binder allocator
  * @mutex:              protects binder_alloc fields
- * @vma_addr:           vm_area_struct->vm_start passed to mmap_handler
+ * @vma:                vm_area_struct passed to mmap_handler
  *                      (invariant after mmap)
  * @mm:                 copy of task->mm (invariant after open)
  * @buffer:             base of per-proc address space mapped via mmap
@@ -99,7 +99,7 @@ struct binder_lru_page {
  */
 struct binder_alloc {
 	struct mutex mutex;
-	unsigned long vma_addr;
+	struct vm_area_struct *vma;
 	struct mm_struct *mm;
 	void __user *buffer;
 	struct list_head buffers;
diff --git a/drivers/android/binder_alloc_selftest.c b/drivers/android/binder_alloc_selftest.c
index 43a881073a42..c2b323bc3b3a 100644
--- a/drivers/android/binder_alloc_selftest.c
+++ b/drivers/android/binder_alloc_selftest.c
@@ -287,7 +287,7 @@ void binder_selftest_alloc(struct binder_alloc *alloc)
 	if (!binder_selftest_run)
 		return;
 	mutex_lock(&binder_selftest_lock);
-	if (!binder_selftest_run || !alloc->vma_addr)
+	if (!binder_selftest_run || !alloc->vma)
 		goto done;
 	pr_info("STARTED\n");
 	binder_selftest_alloc_offset(alloc, end_offset, 0);
-- 
2.40.0.634.g4ca3ef3211-goog