RE: [PATCH v9 06/10] bpf: Add bpf_lookup_*_key() and bpf_key_put() kfuncs

[Roberto Sassu]

> From: Daniel Borkmann [mailto:daniel@xxxxxxxxxxxxx]
> Sent: Wednesday, August 10, 2022 12:53 AM
> On 8/9/22 3:45 PM, Roberto Sassu wrote:
> > Add the bpf_lookup_user_key(), bpf_lookup_system_key() and bpf_key_put()
> > kfuncs, to respectively search a key with a given serial and flags, obtain
> > a key from a pre-determined ID defined in include/linux/verification.h, and
> > cleanup.
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> >   include/linux/bpf.h      |   6 ++
> >   kernel/trace/bpf_trace.c | 151
> +++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 157 insertions(+)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 0d56c23cc504..564b9e0b8c16 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -2572,4 +2572,10 @@ static inline void bpf_cgroup_atype_get(u32
> attach_btf_id, int cgroup_atype) {}
> >   static inline void bpf_cgroup_atype_put(int cgroup_atype) {}
> >   #endif /* CONFIG_BPF_LSM */
> >
> > +#ifdef CONFIG_KEYS
> > +struct bpf_key {
> > +	struct key *key;
> > +	bool valid_ptr;
> > +};
> > +#endif /* CONFIG_KEYS */
> >   #endif /* _LINUX_BPF_H */
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 68e5cdd24cef..33ca4cfe6e26 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -20,6 +20,7 @@
> >   #include <linux/fprobe.h>
> >   #include <linux/bsearch.h>
> >   #include <linux/sort.h>
> > +#include <linux/key.h>
> >
> >   #include <net/bpf_sk_storage.h>
> >
> > @@ -1181,6 +1182,156 @@ static const struct bpf_func_proto
> bpf_get_func_arg_cnt_proto = {
> >   	.arg1_type	= ARG_PTR_TO_CTX,
> >   };
> >
> > +#ifdef CONFIG_KEYS
> > +__diag_push();
> > +__diag_ignore_all("-Wmissing-prototypes",
> > +		  "kfuncs which will be used in BPF programs");
> > +
> > +/**
> > + * bpf_lookup_user_key - lookup a key by its serial
> > + * @serial: key serial
> > + * @flags: lookup-specific flags
> > + *
> > + * Search a key with a given *serial* and the provided *flags*. The
> > + * returned key, if found, has the reference count incremented by
> > + * one, and is stored in a bpf_key structure, returned to the caller.
> > + * The bpf_key structure must be passed to bpf_key_put() when done
> > + * with it, so that the key reference count is decremented and the
> > + * bpf_key structure is freed.
> > + *
> > + * Permission checks are deferred to the time the key is used by
> > + * one of the available key-specific kfuncs.
> > + *
> > + * Set *flags* with 1, to attempt creating a requested special
> > + * keyring (e.g. session keyring), if it doesn't yet exist. Set
> > + * *flags* with 2 to lookup a key without waiting for the key
> > + * construction, and to retrieve uninstantiated keys (keys without
> > + * data attached to them).
> > + *
> > + * Return: a bpf_key pointer with a valid key pointer if the key is found, a
> > + *         NULL pointer otherwise.
> > + */
> > +noinline __weak struct bpf_key *bpf_lookup_user_key(u32 serial, u64 flags)
> 
> Why the need for noinline and the __weak here and below? (If indeed needed
> this
> should probably be explained in the commit desc.)

Oh, I took from v3 of KP's patch set. It is gone in v5. Will do
the same as well.

> > +{
> > +	key_ref_t key_ref;
> > +	struct bpf_key *bkey;
> > +
> > +	/* Keep in sync with include/linux/key.h. */
> > +	if (flags > (KEY_LOOKUP_PARTIAL << 1) - 1)
> 
> Can't we just simplify and test flags &
> ~(KEY_LOOKUP_CREATE|KEY_LOOKUP_PARTIAL)?

I thought as if we have many flags.

> > +		return NULL;
> > +
> > +	/* Permission check is deferred until actual kfunc using the key. */
> > +	key_ref = lookup_user_key(serial, flags, KEY_DEFER_PERM_CHECK);
> > +	if (IS_ERR(key_ref))
> > +		return NULL;
> > +
> > +	bkey = kmalloc(sizeof(*bkey), GFP_KERNEL);
> > +	if (!bkey) {
> > +		key_put(key_ref_to_ptr(key_ref));
> > +		return bkey;
> 
> nit: just return NULL probably cleaner

Ok.

> > +	}
> > +
> > +	bkey->key = key_ref_to_ptr(key_ref);
> > +	bkey->valid_ptr = true;
> 
> nit: I'd probably rename s/valid_ptr/has_ref/.

Ok.

> > +	return bkey;
> > +}
> > +
> > +/**
> > + * bpf_lookup_system_key - lookup a key by a system-defined ID
> > + * @id: key ID
> > + *
> > + * Obtain a bpf_key structure with a key pointer set to the passed key ID.
> > + * The key pointer is marked as invalid, to prevent bpf_key_put() from
> > + * attempting to decrement the key reference count on that pointer. The key
> > + * pointer set in such way is currently understood only by
> > + * verify_pkcs7_signature().
> > + *
> > + * Set *id* to one of the values defined in include/linux/verification.h:
> > + * 0 for the primary keyring (immutable keyring of system keys); 1 for both
> > + * the primary and secondary keyring (where keys can be added only if they
> > + * are vouched for by existing keys in those keyrings); 2 for the platform
> > + * keyring (primarily used by the integrity subsystem to verify a kexec'ed
> > + * kerned image and, possibly, the initramfs signature).
> > + *
> > + * Return: a bpf_key pointer with an invalid key pointer set from the
> > + *         pre-determined ID on success, a NULL pointer otherwise
> > + */
> > +noinline __weak struct bpf_key *bpf_lookup_system_key(u64 id)
> > +{
> > +	struct bpf_key *bkey;
> > +
> > +	/* Keep in sync with defs in include/linux/verification.h. */
> > +	if (id > (unsigned long)VERIFY_USE_PLATFORM_KEYRING)
> > +		return NULL;
> > +
> > +	bkey = kmalloc(sizeof(*bkey), GFP_KERNEL);
> 
> nit: Can't this be GFP_ATOMIC? Then bpf_lookup_system_key doesn't need
> KF_SLEEPABLE
> attribute, fwiw. Overall, the bpf_lookup_{system,user}_key() look reasonable.

Ok, will do.

Thanks

Roberto

> > +	if (!bkey)
> > +		return bkey;
> > +
> > +	bkey->key = (struct key *)(unsigned long)id;
> > +	bkey->valid_ptr = false;
> > +
> > +	return bkey;
> > +}
> > +
> > +/**
> > + * bpf_key_put - decrement key reference count if key is valid and free
> bpf_key
> > + * @bkey: bpf_key structure
> > + *
> > + * Decrement the reference count of the key inside *bkey*, if the pointer
> > + * is valid, and free *bkey*.
> > + */
> > +noinline __weak void bpf_key_put(struct bpf_key *bkey)
> > +{
> > +	if (bkey->valid_ptr)
> > +		key_put(bkey->key);
> > +
> > +	kfree(bkey);
> > +}
> > +
> > +__diag_pop();
> > +
> > +BTF_SET8_START(key_kfunc_set)
> > +BTF_ID_FLAGS(func, bpf_lookup_user_key, KF_ACQUIRE | KF_RET_NULL |
> KF_SLEEPABLE)
> > +BTF_ID_FLAGS(func, bpf_lookup_system_key,
> > +	     KF_ACQUIRE | KF_RET_NULL | KF_SLEEPABLE)
> > +BTF_ID_FLAGS(func, bpf_key_put, KF_RELEASE)
> > +BTF_SET8_END(key_kfunc_set)
> > +
> > +static const struct btf_kfunc_id_set bpf_key_kfunc_set = {
> > +	.owner = THIS_MODULE,
> > +	.set = &key_kfunc_set,
> > +};
> > +#endif /* CONFIG_KEYS */
> > +
> > +const struct btf_kfunc_id_set *kfunc_sets[] = {
> > +#ifdef CONFIG_KEYS
> > +	&bpf_key_kfunc_set,
> > +#endif /* CONFIG_KEYS */
> > +};