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SubjectRe: [PATCH] commoncap: check return value to avoid null pointer dereference
On Mon, May 16, 2022 at 10:40:02AM -0700, Yongzhi Liu wrote:
> 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@pku.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;
> }


Huh? AFAICS, that has all of two callers - notify_change() and
dentry_needs_remove_privs(). Both of them should never be called on
negative dentries and both would have blown up well before the call of
security_inode_need_killpriv().

> @@ -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))

... and that one come from vfs_setxattr(). Again, calling that on a negative
dentry is an oopsable bug. If you have any reproducers, please post the stack
traces so it would be possible to deal with the underlying problem.

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