linux-kernel - [PATCH] commoncap: check return value to avoid null pointer dereference

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Message-Id: <1652722802-66170-1-git-send-email-lyz_cs@pku.edu.cn>
Date:   Mon, 16 May 2022 10:40:02 -0700
From:   Yongzhi Liu <lyz_cs@....edu.cn>
To:     serge@...lyn.com, jmorris@...ei.org, viro@...iv.linux.org.uk,
        dhowells@...hat.com, ebiederm@...ssion.com
Cc:     linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, Yongzhi Liu <lyz_cs@....edu.cn>
Subject: [PATCH] commoncap: check return value to avoid null pointer dereference

The pointer inode is dereferenced before a null pointer
check on inode, hence if inode is actually null we will
get a null pointer dereference. Fix this by only dereferencing
inode after the null pointer check on inode.

Fixes: c6f493d631c ("VFS: security/: d_backing_inode() annotations")
Fixes: 8db6c34 ("Introduce v3 namespaced file capabilities")

Signed-off-by: Yongzhi Liu <lyz_cs@....edu.cn>
---
 security/commoncap.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index 5fc8986..978f011 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -298,6 +298,8 @@ int cap_inode_need_killpriv(struct dentry *dentry)
 	struct inode *inode = d_backing_inode(dentry);
 	int error;
 
+	if (!inode)
+		return 0;
 	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
 	return error > 0;
 }
@@ -545,11 +547,13 @@ int cap_convert_nscap(struct user_namespace *mnt_userns, struct dentry *dentry,
 	const struct vfs_cap_data *cap = *ivalue;
 	__u32 magic, nsmagic;
 	struct inode *inode = d_backing_inode(dentry);
-	struct user_namespace *task_ns = current_user_ns(),
-		*fs_ns = inode->i_sb->s_user_ns;
+	struct user_namespace *task_ns = current_user_ns(), *fs_ns;
 	kuid_t rootid;
 	size_t newsize;
 
+	if (!inode)
+		return -EINVAL;
+	fs_ns = inode->i_sb->s_user_ns;
 	if (!*ivalue)
 		return -EINVAL;
 	if (!validheader(size, cap))
-- 
2.7.4