Re: [RFC PATCH bpf-next v1 1/1] bpf: Enable JIT hardening by default when x86_64 CFI is enabled

[Xu Kuohai]

On 8/17/2026 9:24 PM, Jennifer Miller wrote:
> On Sun, Aug 09, 2026 at 10:30:35AM -0700, Kees Cook wrote:
> > On Fri, Jul 10, 2026 at 12:19:32PM -0700, Jennifer Miller wrote:
> > > Initializes bpf_jit_harden to 1 if x86_64 CFI is enabled. Prevents the
> > > misuse of BPF JIT code to craft CFI signatures or Endbr64 instructions
> > > by enabling constant blinding for JITted BPF code from unprivileged
> > > users.
> > >
> > > Signed-off-by: Jennifer Miller <jmill@xxxxxxx>
> > > ---
> > >   kernel/bpf/core.c | 6 +++++-
> > >   1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > > index 47fe047ad30b..ed22b90c6e4e 100644
> > > --- a/kernel/bpf/core.c
> > > +++ b/kernel/bpf/core.c
> > > @@ -548,7 +548,11 @@ void bpf_prog_kallsyms_del_all(struct bpf_prog *fp)
> > >   /* All BPF JIT sysctl knobs here. */
> > >   int bpf_jit_enable   __read_mostly = IS_BUILTIN(CONFIG_BPF_JIT_DEFAULT_ON);
> > >   int bpf_jit_kallsyms __read_mostly = IS_BUILTIN(CONFIG_BPF_JIT_DEFAULT_ON);
> > > -int bpf_jit_harden   __read_mostly;
> > > +/* Enable hardening by default when x86_64 CFI is enabled to prevent CFI
> > > + * hashes and endbr64 instructions from being crafted.
> > > + */
> > > +int bpf_jit_harden   __read_mostly = IS_ENABLED(CONFIG_X86_64) &&
> > > +					IS_ENABLED(CONFIG_CFI);
> >
> > Why not just do this for all CONFIG_CFI?
> >
> > -Kees
>
> The intention was to not enable JIT hardening by default for other
> architectures when CONFIG_CFI is enabled when we don't know if it would
> be preventing any CFI from being bypassed. It may be possible that kCFI
> on other architectures is able to be bypassed in similar ways to x86_64,
> but we haven't looked into it.
>
> ~Jennifer

Yeah, IIUC the described attacks rely heavily on x86-64's
variable-length instructions. On arm64, instructions are
strictly 4-byte aligned and jumping into the middle of an
instruction is not allowed, so the attacks seem not feasible,
at least in the current form.

> > >   long bpf_jit_limit   __read_mostly;
> > >   long bpf_jit_limit_max __read_mostly;
> > >   
> > > --
> > > 2.34.1
> >
> > --
> > Kees Cook