linux-kernel - [PATCH] binderfs: Fix the maximum minor value in binderfs_binder_device_create() and binderfs_binder_ctl

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Message-Id: <ba880255b91b4682c21c62ae0bc5673e34a119aa.1648379891.git.christophe.jaillet@wanadoo.fr>
Date:   Sun, 27 Mar 2022 13:18:17 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
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>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Todd Kjos <tkjos@...gle.com>
Subject: [PATCH] binderfs: Fix the maximum minor value in binderfs_binder_device_create() and binderfs_binder_ctl_create()

ida_alloc_max(..., max, ...) returns values from 0 to max, inclusive.

So, BINDERFS_MAX_MINOR is a valid value for 'minor'.

BINDERFS_MAX_MINOR is '1U << MINORBITS' and we have:
	#define MKDEV(ma,mi)	(((ma) << MINORBITS) | (mi))

So, When this value is used in MKDEV() and it will overflow.

Fixes: 3ad20fe393b3 ("binder: implement binderfs")
Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
This patch is completely speculative.

The 'BINDERFS_MAX_MINOR_CAPPED - 1' is here only for symmetry with the
BINDERFS_MAX_MINOR case. I'm not sure at all that is is needed and, more
importantly, that it is correct.
---
 drivers/android/binderfs.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
index e3605cdd4335..47df1f381150 100644
--- a/drivers/android/binderfs.c
+++ b/drivers/android/binderfs.c
@@ -130,8 +130,8 @@ static int binderfs_binder_device_create(struct inode *ref_inode,
 	mutex_lock(&binderfs_minors_mutex);
 	if (++info->device_count <= info->mount_opts.max)
 		minor = ida_alloc_max(&binderfs_minors,
-				      use_reserve ? BINDERFS_MAX_MINOR :
-						    BINDERFS_MAX_MINOR_CAPPED,
+				      use_reserve ? BINDERFS_MAX_MINOR - 1:
+						    BINDERFS_MAX_MINOR_CAPPED - 1,
 				      GFP_KERNEL);
 	else
 		minor = -ENOSPC;
@@ -433,8 +433,8 @@ static int binderfs_binder_ctl_create(struct super_block *sb)
 	/* Reserve a new minor number for the new device. */
 	mutex_lock(&binderfs_minors_mutex);
 	minor = ida_alloc_max(&binderfs_minors,
-			      use_reserve ? BINDERFS_MAX_MINOR :
-					    BINDERFS_MAX_MINOR_CAPPED,
+			      use_reserve ? BINDERFS_MAX_MINOR - 1 :
+					    BINDERFS_MAX_MINOR_CAPPED - 1,
 			      GFP_KERNEL);
 	mutex_unlock(&binderfs_minors_mutex);
 	if (minor < 0) {
-- 
2.32.0