On 6/30/2022 6:47 AM, Christian Brauner wrote:

On Thu, Jun 30, 2022 at 03:29:53PM +0200, KP Singh wrote:

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On Thu, Jun 30, 2022 at 3:26 PM Christian Brauner [off-list ref] wrote:

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On Thu, Jun 30, 2022 at 02:21:56PM +0200, KP Singh wrote:

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On Thu, Jun 30, 2022 at 1:45 PM Christian Brauner [off-list ref] wrote:

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On Wed, Jun 29, 2022 at 08:02:50PM -0700, Alexei Starovoitov wrote:

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On Wed, Jun 29, 2022 at 2:56 AM Christian Brauner [off-list ref] wrote:

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Signed-off-by: KP Singh <kpsingh@kernel.org>
---
 .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++

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+SEC("lsm.s/bprm_committed_creds")
+void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
+{
+     struct task_struct *current = bpf_get_current_task_btf();
+     char dir_xattr_value[64] = {0};
+     int xattr_sz = 0;
+
+     xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
+                             bprm->file->f_path.dentry->d_inode, XATTR_NAME,
+                             dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns
nor for the idmapped mount. If this is supposed to work you will need a
variant of vfs_getxattr() that takes the mount's idmapping into account
afaict. See what needs to happen after do_getxattr().

Thanks for taking a look.

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That will not be correct.
posix_acl_fix_xattr_to_user checking current_user_ns()
is checking random tasks that happen to be running
when lsm hook got invoked.

KP,
we probably have to document clearly that neither 'current*'
should not be used here.
xattr_permission also makes little sense in this context.
If anything it can be a different kfunc if there is a use case,
but I don't see it yet.
bpf-lsm prog calling __vfs_getxattr is just like other lsm-s that
call it directly. It's the kernel that is doing its security thing.

Right, but LSMs usually only retrieve their own xattr namespace (ima,
selinux, smack) or they calculate hashes for xattrs based on the raw
filesystem xattr values (evm).

But this new bpf_getxattr() is different. It allows to retrieve _any_
xattr in any security hook it can be attached to. So someone can write a
bpf program that retrieves filesystem capabilites or posix acls. And
these are xattrs that require higher-level vfs involvement to be
sensible in most contexts.

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This hooks a bpf-lsm program to the security_bprm_committed_creds()
hook. It then retrieves the extended attributes of the file to be
executed. The hook currently always retrieves the raw filesystem values.

But for example any XATTR_NAME_CAPS filesystem capabilities that
might've been stored will be taken into account during exec. And both
the idmapping of the mount and the caller matter when determing whether
they are used or not.

But the current implementation of bpf_getxattr() just ignores both. It
will always retrieve the raw filesystem values. So if one invokes this
hook they're not actually retrieving the values as they are seen by
fs/exec.c. And I'm wondering why that is ok? And even if this is ok for
some use-cases it might very well become a security issue in others if
access decisions are always based on the raw values.

I'm not well-versed in this so bear with me, please.

If this is really just about retrieving the "security.bpf" xattr and no
other xattr then the bpf_getxattr() variant should somehow hard-code
that to ensure that no other xattrs can be retrieved, imho.

All of these restrictions look very artificial to me.
Especially the part "might very well become a security issue"
just doesn't click.
We're talking about bpf-lsm progs here that implement security.
Can somebody implement a poor bpf-lsm that doesn't enforce
any actual security? Sure. It's a code.

The point is that with the current implementation of bpf_getxattr() you
are able to retrieve any xattrs and we have way less control over a
bpf-lsm program than we do over selinux which a simple git grep
__vfs_getxattr() is all we need.

The thing is that with bpf_getxattr() as it stands it is currently
impossible to retrieve xattr values - specifically filesystem
capabilities and posix acls - and see them exactly like the code you're
trying to supervise is. And that seems very strange from a security
perspective. So if someone were to write

SEC("lsm.s/bprm_creds_from_file")
void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
{
        struct task_struct *current = bpf_get_current_task_btf();

        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_POSIX_ACL_ACCESS, ..);
        // or
        xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
                                bprm->file->f_path.dentry->d_inode,
                                XATTR_NAME_CAPS, ..);

}

they'd get the raw nscaps and the raw xattrs back. But now, as just a
tiny example, the nscaps->rootuid and the ->e_id fields in the posix
ACLs make zero sense in this context.

And what's more there's no way for the bpf-lsm program to turn them into
something that makes sense in the context of the hook they are retrieved
in. It lacks all the necessary helpers to do so afaict.

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No one complains about the usage of EXPORT_SYMBOL(__vfs_getxattr)
in the existing LSMs like selinux.

Selinux only cares about its own xattr namespace. It doesn't retrieve
fscaps or posix acls and it's not possible to write selinux programs
that do so. With the bpf-lsm that's very much possible.

And if we'd notice selinux would start retrieving random xattrs we'd ask
the same questions we do here.

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No one complains about its usage in out of tree LSMs.
Is that a security issue? Of course not.
__vfs_getxattr is a kernel mechanism that LSMs use to implement
the security features they need.
__vfs_getxattr as kfunc here is pretty much the same as EXPORT_SYMBOL
with a big difference that it's EXPORT_SYMBOL_GPL.
BPF land doesn't have an equivalent of non-gpl export and is not going
to get one.

I want to reiterate what Alexei is saying here:

*Please* consider this as a simple wrapper around __vfs_getxattr
with a limited attach surface and extra verification checks and
and nothing else.

What you are saying is __vfs_getxattr does not make sense in some
contexts. But kernel modules can still use it right?

The user is implementing an LSM, if they chose to do things that don't make
sense, then they can surely cause a lot more harm:

SEC("lsm/bprm_check_security")
int BPF_PROG(bprm_check, struct linux_binprm *bprm)
{
     return -EPERM;
}

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This discussion would probably be a lot shorter if this series were sent
with a proper explanation of how this supposed to work and what it's
used for.

It's currently scoped to BPF LSM (albeit limited to LSM for now)
but it won't just be used in LSM programs but some (allow-listed)
tracing programs too.

We want to leave the flexibility to the implementer of the LSM hooks. If the
implementer choses to retrieve posix_acl_* we can also expose
posix_acl_fix_xattr_to_user or a different kfunc that adds this logic too
but that would be a separate kfunc (and a separate use-case).

No, sorry. That's what I feared and that's why I think this low-level
exposure of __vfs_getxattr() is wrong:
The posix_acl_fix_xattr_*() helpers, as well as the helpers like
get_file_caps() will not be exported. We're not going to export that

I don't want to expose them and I don't want any others to be
exposed either.

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deeply internal vfs machinery. So I would NACK that. If you want that -
and that's what I'm saying here - you need to encapsulate this into your
vfs_*xattr() helper that you can call from your kfuncs.

It seems like __vfs_getxattr is already exposed and does the wrong thing in
some contexts, why can't we just "fix" __vfs_getxattr then?

To me having either a version of bpf_getxattr() that restricts access to
certain xattrs or a version that takes care to perform the neccesary
translations is what seems to make the most sense. I suggested that in
one of my first mails.

The one thing where the way the xattrs are retrieved really matters is
for vfscaps (see get_vfs_caps_from_disk()) you really need something
like that function in order for vfs caps to make any sense and be
interpretable by the user of the hook.

But again, I might just misunderstand the context here and for the
bpf-lsm all of this isn't really a concern. If your new series comes out
I'll try to get more into the wider context.
If the security folks are happy with this then I won't argue.

A security module (BPF) using another security module's (Smack)
xattrs without that module's (Smack) explicit approval would be
considered extremely rude.  Smack and SELinux use published interfaces
of the capability security module, but never access the capability
attributes directly. The details of a security module's implementation
are not a factor. The fact that BPF uses loadable programs as opposed
to loadable policy is not relevant. The only security.xattr values
that the BPF security module should allow the programs it runs to
access are the ones it is managing. If you decided to create an eBPF
implementation of SELinux you would still have to use attributes
specific to the BPF security module. If, on the other hand, you wanted
to extend Smack using eBPF programs, and the Smack maintainer liked
the idea, it would be OK for the BPF security module to access some
of the security.SMACK64 attributes.

I want it to be clear that BPF is a Linux Security Module (LSM) and
a collection of eBPF programs is *not* an LSM. BPF is responsible
for being a good kernel citizen, and must ensure that it does not
allow a set of configuration data that violates proper behavior.
You can't write an SELinux policy that monster-mashes an ACL.
You can't allow BPF to permit that either. You can't count on the
good intentions, wisdom or skill of the author of an unreviewed,
out of tree, eBPF program. I believe that this was understood during
the review process of the BPF LSM.