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

(off-list ancestor, not in this archive)

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 [off-list ref] 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 <redacted>
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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