Re: [PATCH v2 bpf-next 0/2] enforce W^X for trampoline and dispatcher

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 26/09/2022 20.47, Song Liu wrote:
> Changes v1 => v2:
> 1. Update arch_prepare_bpf_dispatcher to use a RO image and a RW buffer.
>     (Alexei) Note: I haven't found an existing test to cover this part, so
>     this part was tested manually (comparing the generated dispatcher is
>     the same).
> 
> Jeff Layton reported CPA W^X warning linux-next [1]. It turns out to be
> W^X issue with bpf trampoline and bpf dispatcher. Fix these by:
> 
> 1. Use bpf_prog_pack for bpf_dispatcher;
> 2. Set memory permission properly with bpf trampoline.

Indirectly related to your patchset[0].
  - TL;DR calling set_memory_x() have side-effects

We are getting reports that loading BPF-progs (jit stage) cause issues 
for RT in the form of triggering work on isolated CPUs.  It looks like 
BTF JIT stage cause a TLB flush on all CPUs, including isolated CPUs.

The triggering function is set_memory_x() (see call-stack[2]).

We have noticed (and appreciate) you have previously improved the 
situation in this patchset[3]:
  [3] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=80123f0ac4a6

Is this patchset also part of improving the situation, or does it 
introduce more calls to set_memory_x() ?


> [1] https://lore.kernel.org/lkml/c84cc27c1a5031a003039748c3c099732a718aec.camel@kernel.org/


[2] Call stack triggering issue:

         smp_call_function_many_cond+0x1
         smp_call_function+0x39
         on_each_cpu+0x2a
         cpa_flush+0x11a
         change_page_attr_set_clr+0x129
         set_memory_x+0x37
         bpf_int_jit_compile+0x36f
         bpf_prog_select_runtime+0xc6
         bpf_prepare_filter+0x523
         sk_attach_filter+0x13
         sock_setsockopt+0x920
         __sys_setsockopt+0x16a
         __x64_sys_setsockopt+0x20
         do_syscall_64+0x87
         entry_SYSCALL_64_after_hwframe+0x65


[0] https://lore.kernel.org/all/20220926184739.3512547-1-song@kernel.org/#r

--Jesper