Re: [PATCH bpf-next v3 12/17] bpf: Disallow fentry/fexit/freplace for exception callbacks

Kumar Kartikeya Dwivedi
2023-09-14T12:13:43Z

On Wed, 13 Sept 2023 at 17:24, Puranjay Mohan <puranjay12@gmail.com> wrote:
>
> On Wed, Sep 13 2023, Kumar Kartikeya Dwivedi wrote:
>
> > During testing, it was discovered that extensions to exception callbacks
> > had no checks, upon running a testcase, the kernel ended up running off
> > the end of a program having final call as bpf_throw, and hitting int3
> > instructions.
> >
> > The reason is that while the default exception callback would have reset
> > the stack frame to return back to the main program's caller, the
> > replacing extension program will simply return back to bpf_throw, which
> > will instead return back to the program and the program will continue
> > execution, now in an undefined state where anything could happen.
> >
> > The way to support extensions to an exception callback would be to mark
> > the BPF_PROG_TYPE_EXT main subprog as an exception_cb, and prevent it
> > from calling bpf_throw. This would make the JIT produce a prologue that
> > restores saved registers and reset the stack frame. But let's not do
> > that until there is a concrete use case for this, and simply disallow
> > this for now.
> >
> > Similar issues will exist for fentry and fexit cases, where trampoline
> > saves data on the stack when invoking exception callback, which however
> > will then end up resetting the stack frame, and on return, the fexit
> > program will never will invoked as the return address points to the main
> > program's caller in the kernel. Instead of additional complexity and
> > back and forth between the two stacks to enable such a use case, simply
> > forbid it.
> >
> > One key point here to note is that currently X86_TAIL_CALL_OFFSET didn't
> > require any modifications, even though we emit instructions before the
> > corresponding endbr64 instruction. This is because we ensure that a main
> > subprog never serves as an exception callback, and therefore the
> > exception callback (which will be a global subprog) can never serve as
> > the tail call target, eliminating any discrepancies. However, once we
> > support a BPF_PROG_TYPE_EXT to also act as an exception callback, it
> > will end up requiring change to the tail call offset to account for the
> > extra instructions. For simplicitly, tail calls could be disabled for
> > such targets.
> >
> > Noting the above, it appears better to wait for a concrete use case
> > before choosing to permit extension programs to replace exception
> > callbacks.
> >
> > As a precaution, we disable fentry and fexit for exception callbacks as
> > well.
> >
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> > ---
> > kernel/bpf/helpers.c | 1 +
> > kernel/bpf/verifier.c | 11 +++++++++++
> > 2 files changed, 12 insertions(+)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 2c8e1ee97b71..7ff2a42f1996 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -2490,6 +2490,7 @@ __bpf_kfunc void bpf_throw(u64 cookie)
> > */
> > kasan_unpoison_task_stack_below((void *)ctx.sp);
> > ctx.aux->bpf_exception_cb(cookie, ctx.sp, ctx.bp);
> > + WARN(1, "A call to BPF exception callback should never return\n");
> > }
> >
> > __diag_pop();
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 0ba32b626320..21e37e46d792 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -19750,6 +19750,12 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
> > bpf_log(log, "Subprog %s doesn't exist\n", tname);
> > return -EINVAL;
> > }
> > + if (aux->func && aux->func[subprog]->aux->exception_cb) {
> > + bpf_log(log,
> > + "%s programs cannot attach to exception callback\n",
> > + prog_extension ? "Extension" : "FENTRY/FEXIT");
> > + return -EINVAL;
> > + }
> > conservative = aux->func_info_aux[subprog].unreliable;
> > if (prog_extension) {
> > if (conservative) {
> > @@ -19762,6 +19768,11 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
> > "Extension programs should be JITed\n");
> > return -EINVAL;
> > }
> > + if (aux->func && aux->func[subprog]->aux->exception_cb) {
> > + bpf_log(log,
> > + "Extension programs cannot replace exception callback\n");
> > + return -EINVAL;
> > + }
>
> This check is redundant because you already did this check above if (prog_extension branch)
> Remove this as it will never be reached.
>

Good catch, will fix it in v4.

>
> > }
> > if (!tgt_prog->jited) {
> > bpf_log(log, "Can attach to only JITed progs\n");
>
>
> Thanks,
> Puranjay