Messages in this thread | | | Subject | Re: [PATCH] riscv: Ensure BPF_JIT_REGION_START aligned with PMD size | From | Alex Ghiti <> | Date | Thu, 17 Jun 2021 10:09:22 +0200 |
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Le 17/06/2021 à 09:30, Palmer Dabbelt a écrit : > On Tue, 15 Jun 2021 17:03:28 PDT (-0700), jszhang3@mail.ustc.edu.cn wrote: >> On Tue, 15 Jun 2021 20:54:19 +0200 >> Alex Ghiti <alex@ghiti.fr> wrote: >> >>> Hi Jisheng, >> >> Hi Alex, >> >>> >>> Le 14/06/2021 à 18:49, Jisheng Zhang a écrit : >>> > 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. >>> Good catch, we make sure no physical allocation happens between _end >>> and the next PMD aligned address, but I missed this one. >>> >>> > > 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. >>> But I disagree with your solution: I made sure modules and BPF >>> programs get their own virtual regions to avoid worst case scenario >>> where one could allocate all the space and leave nothing to the other >>> (we are limited to +- 2GB offset). Why don't just align >>> BPF_JIT_REGION_START to the next PMD aligned address? >> >> Originally, I planed to fix the issue by aligning >> BPF_JIT_REGION_START, but >> IIRC, BPF experts are adding (or have added) "Calling kernel functions >> from BPF" >> feature, there's a risk that BPF JIT region is beyond the 2GB of >> module region: >> >> ------ >> module >> ------ >> kernel >> ------ >> BPF_JIT >> >> So I made this patch finally. In this patch, we let BPF JIT region sit >> between module and kernel. >> >> To address "make sure modules and BPF programs get their own virtual >> regions", >> what about something as below (applied against this patch)? >> >> diff --git a/arch/riscv/include/asm/pgtable.h >> b/arch/riscv/include/asm/pgtable.h >> index 380cd3a7e548..da1158f10b09 100644 >> --- a/arch/riscv/include/asm/pgtable.h >> +++ b/arch/riscv/include/asm/pgtable.h >> @@ -31,7 +31,7 @@ >> #define BPF_JIT_REGION_SIZE (SZ_128M) >> #ifdef CONFIG_64BIT >> #define BPF_JIT_REGION_START (BPF_JIT_REGION_END - >> BPF_JIT_REGION_SIZE) >> -#define BPF_JIT_REGION_END (MODULES_END) >> +#define BPF_JIT_REGION_END (PFN_ALIGN((unsigned long)&_start)) >> #else >> #define BPF_JIT_REGION_START (PAGE_OFFSET - BPF_JIT_REGION_SIZE) >> #define BPF_JIT_REGION_END (VMALLOC_END) >> @@ -40,7 +40,7 @@ >> /* Modules always live before the kernel */ >> #ifdef CONFIG_64BIT >> #define MODULES_VADDR (PFN_ALIGN((unsigned long)&_end) - SZ_2G) >> -#define MODULES_END (PFN_ALIGN((unsigned long)&_start)) >> +#define MODULES_END (BPF_JIT_REGION_END) >> #endif >> >> >> >>> >>> Again, good catch, thanks, >>> >>> Alex >>> >>> > > 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) >>> > > > This, when applied onto fixes, is breaking early boot on KASAN > configurations for me.
Not surprising, I took a shortcut when initializing KASAN for modules, kernel and BPF:
kasan_populate(kasan_mem_to_shadow((const void *)MODULES_VADDR), kasan_mem_to_shadow((const void *)BPF_JIT_REGION_END));
The kernel is then not covered, I'm taking a look at how to fix that properly.
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