linux-kernel - [PATCH] sysctl: Make sure proc handlers can't expose heap memory

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Message-ID: <202005041205.C7AF4AF@keescook>
Date:   Mon, 4 May 2020 12:08:55 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Alexey Dobriyan <adobriyan@...il.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] sysctl: Make sure proc handlers can't expose heap memory

Just as a precaution, make sure that proc handlers don't accidentally
grow "count" beyond the allocated kbuf size.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
This applies to hch's sysctl cleanup tree...
---
 fs/proc/proc_sysctl.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 15030784566c..535ab26473af 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 	struct inode *inode = file_inode(filp);
 	struct ctl_table_header *head = grab_header(inode);
 	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+	size_t count_max = count;
 	void *kbuf;
 	ssize_t error;
 
@@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	if (!write) {
 		error = -EFAULT;
+		if (WARN_ON(count > count_max))
+			count = count_max;
 		if (copy_to_user(ubuf, kbuf, count))
 			goto out_free_buf;
 	}
-- 
2.20.1


-- 
Kees Cook