Re: [PATCH] commoncap: check return value to avoid null pointer dereference

From: Eric W. Biederman
Date: Mon May 16 2022 - 14:14:52 EST


Yongzhi Liu <lyz_cs@xxxxxxxxxx> writes:

> 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")

I don't see how this fixes anything.

The dentry should be non-negative so d_backing_inode should always
return true.


> Signed-off-by: Yongzhi Liu <lyz_cs@xxxxxxxxxx>
> ---
> 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;

How can dentry->d_inode be valid and d_backing_inode not be valid?

That would seem to be a bug elsewhere in the code if it actually happens.

> 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))

Same with this one. Short of a negative dentry I don't see how
d_backing_inode can be NULL, and we are talking about negative
dentries that should have been handled long before these two functions
are called.

Eric