Re: [PATCH bpf-next 1/2] bpf: Add arm64 JIT support for PROBE_MEM32 pseudo instructions.

[Puranjay Mohan <puranjay12@gmail.com>]

Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:

> On Thu, 14 Mar 2024 at 16:00, Puranjay Mohan <puranjay12@gmail.com> wrote:
>>
>> Add support for [LDX | STX | ST], PROBE_MEM32, [B | H | W | DW]
>> instructions.  They are similar to PROBE_MEM instructions with the
>> following differences:
>> - PROBE_MEM32 supports store.
>> - PROBE_MEM32 relies on the verifier to clear upper 32-bit of the
>>   src/dst register
>> - PROBE_MEM32 adds 64-bit kern_vm_start address (which is stored in R28
>>   in the prologue). Due to bpf_arena constructions such R28 + reg +
>>   off16 access is guaranteed to be within arena virtual range, so no
>>   address check at run-time.
>> - PROBE_MEM32 allows STX and ST. If they fault the store is a nop. When
>>   LDX faults the destination register is zeroed.
>>
>> To support these on arm64, we do tmp2 = R28 + src/dst reg and then use
>> tmp2 as the new src/dst register. This allows us to reuse most of the
>> code for normal [LDX | STX | ST].
>>
>> Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
>> ---
>
> Hi Alexei,
> Puranjay and I were discussing this stuff off list and noticed that
> atomic instructions are not handled.
> It turns out that will cause a kernel crash right now because the
> 32-bit offset into arena will be dereferenced directly.
>
> e.g. something like this:
>
> @@ -55,6 +56,7 @@ int arena_list_add(void *ctx)
>                 test_val++;
>                 n->value = i;
>                 arena_sum += i;
> +               __sync_fetch_and_add(&arena_sum, 0);
>                 list_add_head(&n->node, list_head);
>         }
>  #else
>
> I will try to prepare a fix for the x86 JIT. Puranjay will do the same
> for his set.

Yes, testing the change mentioned by Kumar on ARM64 causes a crashes as well:

bpf_testmod: loading out-of-tree module taints kernel.
 bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
 Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
 Mem abort info:
   ESR = 0x0000000096000006
   EC = 0x25: DABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
   FSC = 0x06: level 2 translation fault
 Data abort info:
   ISV = 0, ISS = 0x00000006, ISS2 = 0x00000000
   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
 user pgtable: 4k pages, 48-bit VAs, pgdp=00000004043cc000
 [0000000000000010] pgd=0800000410d8f003, p4d=0800000410d8f003, pud=0800000405972003, pmd=0000000000000000
 Internal error: Oops: 0000000096000006 [#1] SMP
 Modules linked in: bpf_testmod(OE) nls_ascii nls_cp437 sunrpc vfat fat aes_ce_blk aes_ce_cipher ghash_ce sha1_ce button sch_fq_codel dm_mod dax configfs dmi_sysfs sha2_ce sha256_arm64 efivarfs
 CPU: 8 PID: 5631 Comm: test_progs Tainted: G           OE      6.8.0+ #2
 Hardware name: Amazon EC2 c6g.16xlarge/, BIOS 1.0 11/1/2018
 pstate: 20400005 (nzCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : bpf_prog_8771c336cb6a18eb_arena_list_add+0x204/0x2b8
 lr : bpf_prog_8771c336cb6a18eb_arena_list_add+0x144/0x2b8
 sp : ffff80008b84bc30
 x29: ffff80008b84bca0 x28: ffff8000a5008000 x27: ffff80008b84bc38
 x26: 0000000000000000 x25: ffff80008b84bc60 x24: 0000000000000000
 x23: 0000000000000000 x22: 0000000000000058 x21: 0000000000000838
 x20: 0000000000000000 x19: 0000000100001fe0 x18: 0000000000000000
 x17: 0000000000000000 x16: 0000000000000000 x15: 0000ffffcc66d2c8
 x14: 0000000000000000 x13: 0000000000000000 x12: 000000000004058c
 x11: ffff8000a5008010 x10: 00000000ffffffff x9 : 00000000000002cf
 x8 : ffff800082ff4ab8 x7 : 0000000100001000 x6 : 0000000000000001
 x5 : 0000000010e5e3fd x4 : 000000003619b978 x3 : 0000000000000010
 x2 : 0000000000000000 x1 : 0000000000000000 x0 : 0000000000001fe0
 Call trace:
  bpf_prog_8771c336cb6a18eb_arena_list_add+0x204/0x2b8
  bpf_prog_test_run_syscall+0x100/0x340
  __sys_bpf+0x8e8/0xa20
  __arm64_sys_bpf+0x2c/0x48
  invoke_syscall+0x50/0x128
  el0_svc_common.constprop.0+0x48/0xf8
  do_el0_svc+0x28/0x40
  el0_svc+0x58/0x190
  el0t_64_sync_handler+0x13c/0x158
  el0t_64_sync+0x1a8/0x1b0
 Code: 8b010042 8b1c006b f9000162 d2800001 (f821307f)
 ---[ end trace 0000000000000000 ]---
 Kernel panic - not syncing: Oops: Fatal exception
 SMP: stopping secondary CPUs
 Kernel Offset: disabled
 CPU features: 0x0,00000120,7002014a,21407a0b
 Memory Limit: none
 Rebooting in 5 seconds..

I will send v2 with the arm64 JIT fix, but I guess verifier has to be modified
as well to add BPF_PROBE_MEM32 to atomic instructions.

Thanks,
Puranjay