Date:   Fri, 22 Apr 2022 22:25:32 -0700
From:   Song Liu <song@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Song Liu <songliubraving@...com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH bpf] bpf: invalidate unused part of bpf_prog_pack

On Fri, Apr 22, 2022 at 12:31 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> > > On Apr 21, 2022, at 3:30 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
> > > I actually think bpf_arch_text_copy() is another horribly badly done thing.
> > >
> > > It seems only implemented on x86 (I'm not sure how anything else is
> > > supposed to work, I didn't go look), and there it is horribly badly
> > > done, using __text_poke() that does all these magical things just to
> > > make it atomic wrt concurrent code execution.
> > >
> > > None of which is *AT*ALL* relevant for this case, since concurrent
> > > code execution simply isn't a thing (and if it were, you would already
> > > have lost).
> > >
> > > And if that wasn't pointless enough, it does all that magic "map the
> > > page writably at a different virtual address using poking_addr in
> > > poking_mm" and a different address space entirely.
> > >
> > > All of that is required for REAL KERNEL CODE.
> > >
> > > But the thing is, for bpf_prog_pack, all of that is just completely
> > > pointless and stupid complexity.
>
> I think the point is that this hole will likely share a page with active
> code, and as such there should not be a writable mapping mapping to it,
> necessitating the whole __text_poke() mess.
>
> That said; it does seem somewhat silly have a whole page worth of int3
> around just for this.
>
> Perhaps we can do something like the completely untested below?

Yeah, this looks like a better approach. I will draft v2 based on this.

Thanks,
Song

>
> ---
>  arch/x86/kernel/alternative.c | 48 +++++++++++++++++++++++++++++++++++++------
>  1 file changed, 42 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index d374cb3cf024..60afa9105307 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -994,7 +994,20 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
>  __ro_after_init struct mm_struct *poking_mm;
>  __ro_after_init unsigned long poking_addr;
>
> -static void *__text_poke(void *addr, const void *opcode, size_t len)
> +static void text_poke_memcpy(void *dst, const void *src, size_t len)
> +{
> +       memcpy(dst, src, len);
> +}
> +
> +static void text_poke_memset(void *dst, const void *src, size_t len)
> +{
> +       int c = *(int *)src;
> +       memset(dst, c, len);
> +}
> +
> +typedef void text_poke_f(void *dst, const void *src, size_t len);
> +
> +static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t len)
>  {
>         bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
>         struct page *pages[2] = {NULL};
> @@ -1059,7 +1072,7 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
>         prev = use_temporary_mm(poking_mm);
>
>         kasan_disable_current();
> -       memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len);
> +       func((void *)poking_addr + offset_in_page(addr), src, len);
>         kasan_enable_current();
>
>         /*
> @@ -1091,7 +1104,8 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
>          * If the text does not match what we just wrote then something is
>          * fundamentally screwy; there's nothing we can really do about that.
>          */
> -       BUG_ON(memcmp(addr, opcode, len));
> +       if (func == text_poke_memcpy)
> +               BUG_ON(memcmp(addr, src, len));
>
>         local_irq_restore(flags);
>         pte_unmap_unlock(ptep, ptl);
> @@ -1118,7 +1132,7 @@ void *text_poke(void *addr, const void *opcode, size_t len)
>  {
>         lockdep_assert_held(&text_mutex);
>
> -       return __text_poke(addr, opcode, len);
> +       return __text_poke(text_poke_memcpy, addr, opcode, len);
>  }
>
>  /**
> @@ -1137,7 +1151,7 @@ void *text_poke(void *addr, const void *opcode, size_t len)
>   */
>  void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
>  {
> -       return __text_poke(addr, opcode, len);
> +       return __text_poke(text_poke_memcpy, addr, opcode, len);
>  }
>
>  /**
> @@ -1167,7 +1181,29 @@ void *text_poke_copy(void *addr, const void *opcode, size_t len)
>
>                 s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - patched);
>
> -               __text_poke((void *)ptr, opcode + patched, s);
> +               __text_poke(text_poke_memcpy, (void *)ptr, opcode + patched, s);
> +               patched += s;
> +       }
> +       mutex_unlock(&text_mutex);
> +       return addr;
> +}
> +
> +void *text_poke_set(void *addr, int c, size_t len)
> +{
> +       unsigned long start = (unsigned long)addr;
> +       size_t patched = 0;
> +
> +       if (WARN_ON_ONCE(core_kernel_text(start)))
> +               return NULL;
> +
> +       mutex_lock(&text_mutex);
> +       while (patched < len) {
> +               unsigned long ptr = start + patched;
> +               size_t s;
> +
> +               s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - patched);
> +
> +               __text_poke(text_poke_memset, (void *)ptr, (void *)&c, s);
>                 patched += s;
>         }
>         mutex_unlock(&text_mutex);

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