Re: [PATCH] riscv: Ensure BPF_JIT_REGION_START aligned with PMD size

Daniel Borkmann
2021-06-15T12:30:15Z

On 6/14/21 6:49 PM, Jisheng Zhang wrote:
> From: Jisheng Zhang <jszhang@kernel.org>
>
> Andreas reported commit fc8504765ec5 ("riscv: bpf: Avoid breaking W^X")
> breaks booting with one kind of config file, I reproduced a kernel panic
> with the config:
>
> [ 0.138553] Unable to handle kernel paging request at virtual address ffffffff81201220
> [ 0.139159] Oops [#1]
> [ 0.139303] Modules linked in:
> [ 0.139601] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.13.0-rc5-default+ #1
> [ 0.139934] Hardware name: riscv-virtio,qemu (DT)
> [ 0.140193] epc : __memset+0xc4/0xfc
> [ 0.140416] ra : skb_flow_dissector_init+0x1e/0x82
> [ 0.140609] epc : ffffffff8029806c ra : ffffffff8033be78 sp : ffffffe001647da0
> [ 0.140878] gp : ffffffff81134b08 tp : ffffffe001654380 t0 : ffffffff81201158
> [ 0.141156] t1 : 0000000000000002 t2 : 0000000000000154 s0 : ffffffe001647dd0
> [ 0.141424] s1 : ffffffff80a43250 a0 : ffffffff81201220 a1 : 0000000000000000
> [ 0.141654] a2 : 000000000000003c a3 : ffffffff81201258 a4 : 0000000000000064
> [ 0.141893] a5 : ffffffff8029806c a6 : 0000000000000040 a7 : ffffffffffffffff
> [ 0.142126] s2 : ffffffff81201220 s3 : 0000000000000009 s4 : ffffffff81135088
> [ 0.142353] s5 : ffffffff81135038 s6 : ffffffff8080ce80 s7 : ffffffff80800438
> [ 0.142584] s8 : ffffffff80bc6578 s9 : 0000000000000008 s10: ffffffff806000ac
> [ 0.142810] s11: 0000000000000000 t3 : fffffffffffffffc t4 : 0000000000000000
> [ 0.143042] t5 : 0000000000000155 t6 : 00000000000003ff
> [ 0.143220] status: 0000000000000120 badaddr: ffffffff81201220 cause: 000000000000000f
> [ 0.143560] [<ffffffff8029806c>] __memset+0xc4/0xfc
> [ 0.143859] [<ffffffff8061e984>] init_default_flow_dissectors+0x22/0x60
> [ 0.144092] [<ffffffff800010fc>] do_one_initcall+0x3e/0x168
> [ 0.144278] [<ffffffff80600df0>] kernel_init_freeable+0x1c8/0x224
> [ 0.144479] [<ffffffff804868a8>] kernel_init+0x12/0x110
> [ 0.144658] [<ffffffff800022de>] ret_from_exception+0x0/0xc
> [ 0.145124] ---[ end trace f1e9643daa46d591 ]---
>
> After some investigation, I think I found the root cause: commit
> 2bfc6cd81bd ("move kernel mapping outside of linear mapping") moves
> BPF JIT region after the kernel:
>
> The &_end is unlikely aligned with PMD size, so the front bpf jit
> region sits with part of kernel .data section in one PMD size mapping.
> But kernel is mapped in PMD SIZE, when bpf_jit_binary_lock_ro() is
> called to make the first bpf jit prog ROX, we will make part of kernel
> .data section RO too, so when we write to, for example memset the
> .data section, MMU will trigger a store page fault.
>
> To fix the issue, we need to ensure the BPF JIT region is PMD size
> aligned. This patch acchieve this goal by restoring the BPF JIT region
> to original position, I.E the 128MB before kernel .text section.
>
> Reported-by: Andreas Schwab <schwab@linux-m68k.org>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
> arch/riscv/include/asm/pgtable.h | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 9469f464e71a..380cd3a7e548 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -30,9 +30,8 @@
>
> #define BPF_JIT_REGION_SIZE (SZ_128M)
> #ifdef CONFIG_64BIT
> -/* KASLR should leave at least 128MB for BPF after the kernel */
> -#define BPF_JIT_REGION_START PFN_ALIGN((unsigned long)&_end)
> -#define BPF_JIT_REGION_END (BPF_JIT_REGION_START + BPF_JIT_REGION_SIZE)
> +#define BPF_JIT_REGION_START (BPF_JIT_REGION_END - BPF_JIT_REGION_SIZE)
> +#define BPF_JIT_REGION_END (MODULES_END)
> #else
> #define BPF_JIT_REGION_START (PAGE_OFFSET - BPF_JIT_REGION_SIZE)
> #define BPF_JIT_REGION_END (VMALLOC_END)

I presume this fix will be routed via riscv tree?

Thanks,
Daniel