Re: [RESEND][RFC][PATCH 2/3] bpf-lsm: Limit values that can be returned by security modules

[Roberto Sassu]

On Thu, 2022-11-03 at 16:09 +0100, KP Singh wrote:
> On Fri, Oct 28, 2022 at 6:55 PM Roberto Sassu
> <roberto.sassu@xxxxxxxxxxxxxxx> wrote:
> > From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > 
> > BPF LSM defines a bpf_lsm_*() function for each LSM hook, so that
> > security modules can define their own implementation for the desired hooks.
> > 
> > Unfortunately, BPF LSM does not restrict which values security modules can
> > return (for non-void LSM hooks). Security modules might follow the
> > conventions stated in include/linux/lsm_hooks.h, or put arbitrary values.
> > 
> > This could cause big troubles, as the kernel is not ready to handle
> > possibly malicious return values from LSMs. Until now, it was not the
> 
> I am not sure I would call this malicious. This would be incorrect, if
> someone is writing a BPF LSM program they already have the powers
> to willingly do a lot of malicious stuff.
> 
> It's about unknowingly returning values that can break the system.

Maybe it is possible to return specific values that lead to acquire
more information/do actions that the eBPF program is not supposed to
cause.

I don't have a concrete example, so I will use the word you suggested.

> > case, as each LSM is carefully reviewed and it won't be accepted if it
> > does not meet the return value conventions.
> > 
> > The biggest problem is when an LSM returns a positive value, instead of a
> > negative value, as it could be converted to a pointer. Since such pointer
> > escapes the IS_ERR() check, its use later in the code can cause
> > unpredictable consequences (e.g. invalid memory access).
> > 
> > Another problem is returning zero when an LSM is supposed to have done some
> > operations. For example, the inode_init_security hook expects that their
> > implementations return zero only if they set the name and value of the new
> > xattr to be added to the new inode. Otherwise, other kernel subsystems
> > might encounter unexpected conditions leading to a crash (e.g.
> > evm_protected_xattr_common() getting NULL as argument).
> > 
> > Finally, there are LSM hooks which are supposed to return just one as
> > positive value, or non-negative values. Also in these cases, although it
> > seems less critical, it is safer to return to callers of the LSM
> > infrastructure more precisely what they expect.
> > 
> > As eBPF allows code outside the kernel to run, it is its responsibility
> > to ensure that only expected values are returned to LSM infrastructure
> > callers.
> > 
> > Create four new BTF ID sets, respectively for hooks that can return
> > positive values, only one as positive value, that cannot return zero, and
> > that cannot return negative values. Create also corresponding functions to
> > check if the hook a security module is attached to belongs to one of the
> > defined sets.
> > 
> > Finally, check in the eBPF verifier the value returned by security modules
> > for each attached LSM hook, and return -EINVAL (the security module cannot
> > run) if the hook implementation does not satisfy the hook return value
> > policy.
> > 
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: 9d3fdea789c8 ("bpf: lsm: Provide attachment points for BPF LSM programs")
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> >  include/linux/bpf_lsm.h | 24 ++++++++++++++++++
> >  kernel/bpf/bpf_lsm.c    | 56 +++++++++++++++++++++++++++++++++++++++++
> >  kernel/bpf/verifier.c   | 35 +++++++++++++++++++++++---
> >  3 files changed, 112 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> > index 4bcf76a9bb06..cd38aca4cfc0 100644
> > --- a/include/linux/bpf_lsm.h
> > +++ b/include/linux/bpf_lsm.h
> > @@ -28,6 +28,10 @@ int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> >                         const struct bpf_prog *prog);
> > 
> >  bool bpf_lsm_is_sleepable_hook(u32 btf_id);
> > +bool bpf_lsm_can_ret_pos_value(u32 btf_id);
> > +bool bpf_lsm_can_ret_only_one_as_pos_value(u32 btf_id);
> > +bool bpf_lsm_cannot_ret_zero(u32 btf_id);
> > +bool bpf_lsm_cannot_ret_neg_value(u32 btf_id);
> > 
> 
> This does not need to be exported to the rest of the kernel. Please
> have this logic in bpf_lsm.c and export a single verify function.
> 
> Also, these really don't need to be such scattered logic, Could we
> somehow encode this into the LSM_HOOK definition?

The problem is that a new LSM_HOOK definition would apply to every LSM
hook, while we need the ability to select subsets.

I was thinking, but I didn't check yet, what about using BTF_ID_FLAGS,
introducing a flag for each interval (<0, 0, 1, >1) and setting the
appropriate flags for each LSM hook?

Roberto

> >  static inline struct bpf_storage_blob *bpf_inode(
> >         const struct inode *inode)
> > @@ -51,6 +55,26 @@ static inline bool bpf_lsm_is_sleepable_hook(u32 btf_id)
> >         return false;
> >  }
> > 
> > +static inline bool bpf_lsm_can_ret_pos_value(u32 btf_id)
> > +{
> > +       return false;
> > +}
> > +
> > +static inline bool bpf_lsm_can_ret_only_one_as_pos_value(u32 btf_id)
> > +{
> > +       return false;
> > +}
> > +
> > +static inline bool bpf_lsm_cannot_ret_zero(u32 btf_id)
> > +{
> > +       return false;
> > +}
> > +
> > +static inline bool bpf_lsm_cannot_ret_neg_value(u32 btf_id)
> > +{
> > +       return false;
> > +}
> > +
> >  static inline int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> >                                       const struct bpf_prog *prog)
> >  {
> > diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> > index d6c9b3705f24..3dcb70b2f978 100644
> > --- a/kernel/bpf/bpf_lsm.c
> > +++ b/kernel/bpf/bpf_lsm.c
> > @@ -348,6 +348,62 @@ bool bpf_lsm_is_sleepable_hook(u32 btf_id)
> >         return btf_id_set_contains(&sleepable_lsm_hooks, btf_id);
> >  }
> > 
> > +/* The set of hooks which are allowed to return a positive value. */
> > +BTF_SET_START(pos_ret_value_lsm_hooks)
> > +BTF_ID(func, bpf_lsm_vm_enough_memory)
> > +BTF_ID(func, bpf_lsm_inode_getsecurity)
> > +BTF_ID(func, bpf_lsm_inode_listsecurity)
> > +BTF_ID(func, bpf_lsm_inode_need_killpriv)
> > +BTF_ID(func, bpf_lsm_inode_copy_up_xattr)
> > +BTF_ID(func, bpf_lsm_getprocattr)
> > +BTF_ID(func, bpf_lsm_setprocattr)
> > +BTF_ID(func, bpf_lsm_xfrm_state_pol_flow_match)
> > +BTF_ID(func, bpf_lsm_key_getsecurity)
> > +BTF_ID(func, bpf_lsm_ismaclabel)
> > +BTF_ID(func, bpf_lsm_audit_rule_known)
> > +BTF_ID(func, bpf_lsm_audit_rule_match)
> > +BTF_SET_END(pos_ret_value_lsm_hooks)
> > +
> > +bool bpf_lsm_can_ret_pos_value(u32 btf_id)
> > +{
> > +       return btf_id_set_contains(&pos_ret_value_lsm_hooks, btf_id);
> > +}
> > +
> > +BTF_SET_START(one_ret_value_lsm_hooks)
> > +BTF_ID(func, bpf_lsm_vm_enough_memory)
> > +BTF_ID(func, bpf_lsm_inode_copy_up_xattr)
> > +BTF_ID(func, bpf_lsm_xfrm_state_pol_flow_match)
> > +BTF_ID(func, bpf_lsm_ismaclabel)
> > +BTF_ID(func, bpf_lsm_audit_rule_known)
> > +BTF_ID(func, bpf_lsm_audit_rule_match)
> > +BTF_SET_END(one_ret_value_lsm_hooks)
> > +
> > +bool bpf_lsm_can_ret_only_one_as_pos_value(u32 btf_id)
> > +{
> > +       return btf_id_set_contains(&one_ret_value_lsm_hooks, btf_id);
> > +}
> > +
> > +/* The set of hooks which are not allowed to return zero. */
> > +BTF_SET_START(not_zero_ret_value_lsm_hooks)
> > +BTF_ID(func, bpf_lsm_inode_init_security)
> > +BTF_SET_END(not_zero_ret_value_lsm_hooks)
> > +
> > +bool bpf_lsm_cannot_ret_zero(u32 btf_id)
> > +{
> > +       return btf_id_set_contains(&not_zero_ret_value_lsm_hooks, btf_id);
> > +}
> > +
> > +/* The set of hooks which are not allowed to return a negative value. */
> > +BTF_SET_START(not_neg_ret_value_lsm_hooks)
> > +BTF_ID(func, bpf_lsm_vm_enough_memory)
> > +BTF_ID(func, bpf_lsm_audit_rule_known)
> > +BTF_SET_END(not_neg_ret_value_lsm_hooks)
> > +
> > +bool bpf_lsm_cannot_ret_neg_value(u32 btf_id)
> > +{
> > +       return btf_id_set_contains(&not_neg_ret_value_lsm_hooks, btf_id);
> > +}
> > +
> >  const struct bpf_prog_ops lsm_prog_ops = {
> >  };
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 7f0a9f6cb889..099c1bf88fed 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -10623,9 +10623,38 @@ static int check_return_code(struct bpf_verifier_env *env)
> > 
> >         case BPF_PROG_TYPE_LSM:
> >                 if (env->prog->expected_attach_type != BPF_LSM_CGROUP) {
> > -                       /* Regular BPF_PROG_TYPE_LSM programs can return
> > -                        * any value.
> > -                        */
> > +                       /* < 0 */
> > +                       if (tnum_in(tnum_range((u64)(~0) << 31, (u64)(~0)), reg->var_off)) {
> > +                               if (bpf_lsm_cannot_ret_neg_value(env->prog->aux->attach_btf_id)) {
> > +                                       verbose(env, "Invalid R0, cannot return negative value\n");
> > +                                       return -EINVAL;
> > +                               }
> > +                       /* = 0 */
> > +                       } else if (tnum_equals_const(reg->var_off, 0)) {
> > +                               if (bpf_lsm_cannot_ret_zero(env->prog->aux->attach_btf_id)) {
> > +                                       verbose(env, "Invalid R0, cannot return zero value\n");
> > +                                       return -EINVAL;
> > +                               }
> > +                       /* = 1 */
> > +                       } else if (tnum_equals_const(reg->var_off, 1)) {
> > +                               if (!bpf_lsm_can_ret_pos_value(env->prog->aux->attach_btf_id)) {
> > +                                       verbose(env, "Invalid R0, cannot return positive value\n");
> > +                                       return -EINVAL;
> > +                               }
> > +                       /* > 1 */
> > +                       } else {
> > +                               if (!bpf_lsm_can_ret_pos_value(env->prog->aux->attach_btf_id)) {
> > +                                       verbose(env, "Invalid R0, cannot return positive value\n");
> > +                                       return -EINVAL;
> > +                               }
> > +
> > +                               if (bpf_lsm_can_ret_only_one_as_pos_value(env->prog->aux->attach_btf_id)) {
> > +                                       verbose(env,
> > +                                               "Invalid R0, can return only one as positive value\n");
> > +                                       return -EINVAL;
> > +                               }
> > +                       }
> > +
> >                         return 0;
> >                 }
> >                 if (!env->prog->aux->attach_func_proto->type) {
> > --
> > 2.25.1
> >