On 10/26/2022 8:37 AM, Alexei Starovoitov wrote:
On Tue, Oct 25, 2022 at 7:58 AM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
On 10/25/2022 12:43 AM, Roberto Sassu wrote:
On Mon, 2022-10-24 at 19:13 -0700, Alexei Starovoitov wrote:
I'm looking at security_inode_init_security() and it is indeed messy.
Per file system initxattrs callback that processes kmalloc-ed
strings.
Yikes.
In the short term we should denylist inode_init_security hook to
disallow attaching bpf-lsm there. set/getxattr should be done
through kfuncs instead of such kmalloc-a-string hack.
Inode_init_security is an example. It could be that the other hooks are
affected too. What happens if they get arbitrary positive values too?
TL;DR - Things will go cattywampus.
The LSM infrastructure is an interface that has "grown organically",
and isn't necessarily consistent in what it requires of the security
module implementations. There are cases where it assumes that the
security module hooks are well behaved, as you've discovered. I have
no small amount of fear that someone is going to provide an eBPF
program for security_secid_to_secctx(). There has been an assumption,
oft stated, that all security modules are going to be reviewed as
part of the upstream process. The review process ought to catch hooks
that return unacceptable values. Alas, we've lost that with BPF.
It would take a(nother) major overhaul of the LSM infrastructure to
make it safe against hooks that are not well behaved. From what I have
seen so far it wouldn't be easy/convenient/performant to do it in the
BPF security module either. I personally think that BPF needs to
ensure that the eBPF implementations don't return inappropriate values,
but I understand why that is problematic.
That's an accurate statement. Thank you.
Going back to the original question...
We fix bugs when we discover them.
Regardless of the subsystem they belong to.
No finger pointing.
I'm concerned about the following situation:
struct <something> *function()
{
ret = security_*();
if (ret)
return ERR_PTR(ret);
}
int caller()
{
ptr = function()
if (IS_ERR(ptr)
goto out;
<use of invalid pointer>
}
I quickly found an occurrence of this:
static int lookup_one_common()
{
[...]
return inode_permission();
}
struct dentry *try_lookup_one_len()
{
[...]
err = lookup_one_common(&init_user_ns, name, base, len, &this);
if (err)
return ERR_PTR(err);
Unfortunately, attaching to inode_permission causes the kernel
to crash immediately (it does not happen with negative return
values).
So, I think the fix should be broader, and not limited to the
inode_init_security hook. Will try to see how it can be fixed.
Roberto
