> From: Kumar Kartikeya Dwivedi [mailto:memxor@xxxxxxxxx] > Sent: Thursday, July 21, 2022 3:43 PM > Teach the verifier to detect a new KF_TRUSTED_ARGS kfunc flag, which > means each pointer argument must be trusted, which we define as a > pointer that is referenced (has non-zero ref_obj_id) and also needs to > have its offset unchanged, similar to how release functions expect their > argument. This allows a kfunc to receive pointer arguments unchanged > from the result of the acquire kfunc. > > This is required to ensure that kfunc that operate on some object only > work on acquired pointers and not normal PTR_TO_BTF_ID with same type > which can be obtained by pointer walking. The restrictions applied to > release arguments also apply to trusted arguments. This implies that > strict type matching (not deducing type by recursively following members > at offset) and OBJ_RELEASE offset checks (ensuring they are zero) are > used for trusted pointer arguments. > > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> > --- > include/linux/btf.h | 32 ++++++++++++++++++++++++++++++++ > kernel/bpf/btf.c | 17 ++++++++++++++--- > net/bpf/test_run.c | 5 +++++ > 3 files changed, 51 insertions(+), 3 deletions(-) > > diff --git a/include/linux/btf.h b/include/linux/btf.h > index 6dfc6eaf7f8c..cb63aa71e82f 100644 > --- a/include/linux/btf.h > +++ b/include/linux/btf.h > @@ -17,6 +17,38 @@ > #define KF_RELEASE (1 << 1) /* kfunc is a release function */ > #define KF_RET_NULL (1 << 2) /* kfunc returns a pointer that may be NULL */ > #define KF_KPTR_GET (1 << 3) /* kfunc returns reference to a kptr */ > +/* Trusted arguments are those which are meant to be referenced arguments > with > + * unchanged offset. It is used to enforce that pointers obtained from acquire > + * kfuncs remain unmodified when being passed to helpers taking trusted args. > + * > + * Consider > + * struct foo { > + * int data; > + * struct foo *next; > + * }; > + * > + * struct bar { > + * int data; > + * struct foo f; > + * }; > + * > + * struct foo *f = alloc_foo(); // Acquire kfunc > + * struct bar *b = alloc_bar(); // Acquire kfunc > + * > + * If a kfunc set_foo_data() wants to operate only on the allocated object, it > + * will set the KF_TRUSTED_ARGS flag, which will prevent unsafe usage like: > + * > + * set_foo_data(f, 42); // Allowed > + * set_foo_data(f->next, 42); // Rejected, non-referenced pointer > + * set_foo_data(&f->next, 42);// Rejected, referenced, but bad offset > + * set_foo_data(&b->f, 42); // Rejected, referenced, but wrong type > + * > + * In the final case, usually for the purposes of type matching, it is deduced > + * by looking at the type of the member at the offset, but due to the > + * requirement of trusted argument, this deduction will be strict and not done > + * for this case. > + */ > +#define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer > arguments */ Hi Kumar would it make sense to introduce per-parameter flags? I have a function that has several parameters, but only one is referenced. Thanks Roberto
