netdev - Re: [PATCH -next,v2] bpf, test_run: fix alignment problem in bpf_prog_test_run

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Date:   Mon, 7 Nov 2022 14:17:56 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Baisong Zhong <zhongbaisong@...wei.com>, elver@...gle.com,
        Catalin Marinas <catalin.marinas@....com>, edumazet@...gle.com,
        keescook@...omium.org, kuba@...nel.org, ast@...nel.org,
        daniel@...earbox.net, davem@...emloft.net, pabeni@...hat.com,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH -next,v2] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

On Mon, Nov 7, 2022 at 11:33 AM Mark Rutland <mark.rutland@....com> wrote:
>
> On Fri, Nov 04, 2022 at 06:06:05PM +0100, Alexander Potapenko wrote:
> > On Wed, Nov 2, 2022 at 9:16 AM Baisong Zhong <zhongbaisong@...wei.com> wrote:
> > >
> > > we got a syzkaller problem because of aarch64 alignment fault
> > > if KFENCE enabled.
> > >
> > > When the size from user bpf program is an odd number, like
> > > 399, 407, etc, it will cause the struct skb_shared_info's
> > > unaligned access. As seen below:
> > >
> > > BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
> >
> > It's interesting that KFENCE is reporting a UAF without a deallocation
> > stack here.
> >
> > Looks like an unaligned access to 0xffff6254fffac077 causes the ARM
> > CPU to throw a fault handled by __do_kernel_fault()
>
> Importantly, an unaligned *atomic*, which is a bug regardless of KFENCE.
>
> > This isn't technically a page fault, but anyway the access address
> > gets passed to kfence_handle_page_fault(), which defaults to a
> > use-after-free, because the address belongs to the object page, not
> > the redzone page.
> >
> > Catalin, Mark, what is the right way to only handle traps caused by
> > reading/writing to a page for which `set_memory_valid(addr, 1, 0)` was
> > called?
>
> That should appear as a translation fault, so we could add an
> is_el1_translation_fault() helper for that. I can't immediately recall how
> misaligned atomics are presented, but I presume as something other than a
> translation fault.
>
> If the below works for you, I can go spin that as a real patch.

Thanks!
It works for me in QEMU (doesn't report UAF for an unaligned atomic
access and doesn't break the original KFENCE tests), and matches my
reading of https://developer.arm.com/documentation/ddi0595/2020-12/AArch64-Registers/ESR-EL1--Exception-Syndrome-Register--EL1-

Feel free to add:
  Reviewed-by: Alexander Potapenko <glider@...gle.com>
  Tested-by: Alexander Potapenko <glider@...gle.com>

> Mark.
>
> ---->8----
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 5b391490e045b..1de4b6afa8515 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -239,6 +239,11 @@ static bool is_el1_data_abort(unsigned long esr)
>         return ESR_ELx_EC(esr) == ESR_ELx_EC_DABT_CUR;
>  }
>
> +static bool is_el1_translation_fault(unsigned long esr)
> +{
> +       return (esr & ESR_ELx_FSC_TYPE) == ESR_ELx_FSC_FAULT;

Should we also introduce ESR_ELx_FSC(esr) for this?

> +}
> +
>  static inline bool is_el1_permission_fault(unsigned long addr, unsigned long esr,
>                                            struct pt_regs *regs)
>  {
> @@ -385,7 +390,8 @@ static void __do_kernel_fault(unsigned long addr, unsigned long esr,
>         } else if (addr < PAGE_SIZE) {
>                 msg = "NULL pointer dereference";
>         } else {
> -               if (kfence_handle_page_fault(addr, esr & ESR_ELx_WNR, regs))
> +               if (is_el1_translation_fault(esr) &&
> +                   kfence_handle_page_fault(addr, esr & ESR_ELx_WNR, regs))
>                         return;
>
>                 msg = "paging request";
--
Alexander Potapenko
Software Engineer

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