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From:   Tong Zhang <ztong0001@gmail.com>
To:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Tong Zhang <ztong0001@gmail.com>
Subject: [PATCH v1] binfmt_misc: fix crash when load/unload module
Date:   Sun, 23 Jan 2022 16:33:41 -0800
Message-Id: <20220124003342.1457437-1-ztong0001@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

We should unregister the table upon module unload otherwise something
horrible will happen when we load binfmt_misc module again. Also note
that we should keep value returned by register_sysctl_mount_point() and
release it later, otherwise it will leak.

reproduce:
modprobe binfmt_misc
modprobe -r binfmt_misc
modprobe binfmt_misc
modprobe -r binfmt_misc
modprobe binfmt_misc

[   18.032038] Call Trace:
[   18.032108]  <TASK>
[   18.032169]  dump_stack_lvl+0x34/0x44
[   18.032273]  __register_sysctl_table+0x6f4/0x720
[   18.032397]  ? preempt_count_sub+0xf/0xb0
[   18.032508]  ? 0xffffffffc0040000
[   18.032600]  init_misc_binfmt+0x2d/0x1000 [binfmt_misc]
[   18.042520] binfmt_misc: Failed to create fs/binfmt_misc sysctl mount point
modprobe: can't load module binfmt_misc (kernel/fs/binfmt_misc.ko): Cannot allocate memory
[   18.063549] binfmt_misc: Failed to create fs/binfmt_misc sysctl mount point
[   18.204779] BUG: unable to handle page fault for address: fffffbfff8004802

Fixes: 3ba442d5331f ("fs: move binfmt_misc sysctl to its own file")
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
---
 fs/binfmt_misc.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index ddea6acbddde..614aedb8ab2e 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -817,12 +817,16 @@ static struct file_system_type bm_fs_type = {
 };
 MODULE_ALIAS_FS("binfmt_misc");
 
+static struct ctl_table_header *binfmt_misc_header;
+
 static int __init init_misc_binfmt(void)
 {
 	int err = register_filesystem(&bm_fs_type);
 	if (!err)
 		insert_binfmt(&misc_format);
-	if (!register_sysctl_mount_point("fs/binfmt_misc")) {
+
+	binfmt_misc_header = register_sysctl_mount_point("fs/binfmt_misc");
+	if (!binfmt_misc_header) {
 		pr_warn("Failed to create fs/binfmt_misc sysctl mount point");
 		return -ENOMEM;
 	}
@@ -831,6 +835,7 @@ static int __init init_misc_binfmt(void)
 
 static void __exit exit_misc_binfmt(void)
 {
+	unregister_sysctl_table(binfmt_misc_header);
 	unregister_binfmt(&misc_format);
 	unregister_filesystem(&bm_fs_type);
 }
-- 
2.25.1