Subject: Re: [REPORT] BPF: Reproducible triggering of BUG() from userspace PoC

From: Lee Jones <[email-masked]>
Date: Tue, 21 Nov 2023 16:53:51 +0000

On Thu, 09 Nov 2023, Jiri Olsa wrote:

> On Wed, Nov 08, 2023 at 03:46:26PM +0000, Lee Jones wrote:
> > Good afternoon,
> > 
> > After coming across a recent Syzkaller report [0] I thought I'd take
> > some time to firstly reproduce the issue, then see if there was a
> > trivial way to mitigate it.  The report suggests that a BUG() in
> > prog_array_map_poke_run() [1] can be trivially and reliably triggered
> > from userspace using the PoC provided [2].
> > 
> >         ret = bpf_arch_text_poke(poke->tailcall_bypass,
> >                                  BPF_MOD_JUMP,
> >                                  old_bypass_addr,
> >                                  poke->bypass_addr);
> >         BUG_ON(ret < 0 && ret != -EINVAL);
> > 
> > Indeed the PoC does seem to be able to consistently trigger the BUG(),
> > not only on the reported kernel (v6.1), but also on linux-next.  I went
> > to the trouble of checking LORE, but failed to find any patches which
> > may be attempting to fix this.
> > 
> >     kernel BUG at kernel/bpf/arraymap.c:1094!
> >     invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> >     CPU: 5 PID: 45 Comm: kworker/5:0 Not tainted 6.6.0-rc3-next-20230929-dirty #74
> >     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> >     Workqueue: events prog_array_map_clear_deferred
> >     RIP: 0010:prog_array_map_poke_run+0x6b4/0x6d0
> >     Code: ff 0f 0b e8 1e 27 e1 ff 48 c7 c7 60 80 93 85 48 c7 c6 00 7f 93 85 48 c7 c2 bb c2 39 86 b9 45 04 00 00 45 89 f8 e8 9c 890
> >     RSP: 0018:ffffc9000036fb50 EFLAGS: 00010246
> >     RAX: 0000000000000044 RBX: ffff88811f337490 RCX: 63af48a1314f9900
> >     RDX: 0000000000000000 RSI: 0000000080000000 RDI: 0000000000000000
> >     RBP: ffffc9000036fbe8 R08: ffffffff815c23c5 R09: 1ffff11084c14eba
> >     R10: dfffe91084c14ebc R11: ffffed1084c14ebb R12: ffff888116517800
> >     R13: dffffc0000000000 R14: ffff888125a1a400 R15: 00000000fffffff0
> >     FS:  0000000000000000(0000) GS:ffff888426080000(0000) knlGS:0000000000000000
> >     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >     CR2: 00000000004ab678 CR3: 0000000122ac4000 CR4: 0000000000350eb0
> >     DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> >     DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >     Call Trace:
> >      <TASK>
> >      ? __die_body+0x92/0xf0
> >      ? die+0xa2/0xe0
> >      ? do_trap+0x12f/0x370
> >      ? handle_invalid_op+0xa6/0x140
> >      ? handle_invalid_op+0xdf/0x140
> >      ? prog_array_map_poke_run+0x6b4/0x6d0
> >      ? prog_array_map_poke_run+0x6b4/0x6d0
> >      ? exc_invalid_op+0x32/0x50
> >      ? asm_exc_invalid_op+0x1b/0x20
> >      ? __wake_up_klogd+0xd5/0x110
> >      ? prog_array_map_poke_run+0x6b4/0x6d0
> >      ? bpf_prog_6781ebc2dae4bad9+0xb/0x53
> >      fd_array_map_delete_elem+0x152/0x250
> >      prog_array_map_clear_deferred+0xf6/0x210
> >      ? __bpf_array_map_seq_show+0xa40/0xa40
> >      ? kick_pool+0x164/0x350
> >      ? process_one_work+0x57a/0xd00
> >      process_one_work+0x5e4/0xd00
> >      worker_thread+0x9cf/0xea0
> >      kthread+0x2b4/0x350
> >      ? pr_cont_work+0x580/0x580
> >      ? kthread_blkcg+0xd0/0xd0
> >      ret_from_fork+0x4a/0x80
> >      ? kthread_blkcg+0xd0/0xd0
> >      ret_from_fork_asm+0x11/0x20
> >      </TASK>
> >     Modules linked in:
> >     ---[ end trace 0000000000000000 ]---
> > 
> > However, with my very limited BPF subsystem knowledge I was unable to
> > trivially fix the issue.  Hopefully some knowledgable person would be
> > kind enough to provide me with some pointers.
> > 
> > bpf_arch_text_poke() seems to be returning -EBUSY due to a negative
> > memcmp() result from [3].
> > 
> >         ret = -EBUSY;
> >         mutex_lock(&text_mutex);
> >         if (memcmp(ip, old_insn, X86_PATCH_SIZE)) {
> >                 goto out;
> >         [...]
> > 
> > When spitting out the memory at those locations, this is the result:
> > 
> >     ip:        e9 06 00 00 00
> >     old_insn:  0f 1f 44 00 00
> >     nop_insn:  0f 1f 44 00 00
> > 
> > As you can see, the information stored in 'ip' does not match that of
> > the data stored in 'old_insn', causing bpf_arch_text_poke() to return
> > early with the error -EBUSY, suggesting that the data pointed to by
> > 'old_insn', and by extension 'prog' should have been changed when
> > emit_call()ing, to the value of 'ip', but wasn't.
> 
> hi,
> thanks for the report.. I can reproduce that easily with [2]
> 
> AFAICS it looks like previous update fails because we use bpf_arch_text_poke,
> which can't find poke->tailcall_bypass value as bpf program symbol and fails
> with -EINVAL
> 
> then the following update fails to find expected jmp/nop because it was never
> updated.. I think we should use __bpf_arch_text_poke like we do in
> bpf_tail_call_direct_fixup and skip the bpf symbol check
> 
> with the patch below I can't reproduce the issue anymore, I'll do some more
> checking though

The patch below seems to be working for me also:

Tested-by: Lee Jones <[email-masked]>

> ---
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 8c10d9abc239..35c2988caf29 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -391,8 +391,8 @@ static int emit_jump(u8 **pprog, void *func, void *ip)
>  	return emit_patch(pprog, func, ip, 0xE9);
>  }
>  
> -static int __bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
> -				void *old_addr, void *new_addr)
> +int __bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
> +			 void *old_addr, void *new_addr)
>  {
>  	const u8 *nop_insn = x86_nops[5];
>  	u8 old_insn[X86_PATCH_SIZE];
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index eb84caf133df..0d7b8311fada 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -3172,6 +3172,8 @@ enum bpf_text_poke_type {
>  
>  int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
>  		       void *addr1, void *addr2);
> +int __bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
> +			 void *old_addr, void *new_addr);
>  
>  void *bpf_arch_text_copy(void *dst, void *src, size_t len);
>  int bpf_arch_text_invalidate(void *dst, size_t len);
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 2058e89b5ddd..4ab5864746ce 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -1073,33 +1073,33 @@ static void prog_array_map_poke_run(struct bpf_map *map, u32 key,
>  			new_addr = new ? (u8 *)new->bpf_func + poke->adj_off : NULL;
>  
>  			if (new) {
> -				ret = bpf_arch_text_poke(poke->tailcall_target,
> +				ret = __bpf_arch_text_poke(poke->tailcall_target,
>  							 BPF_MOD_JUMP,
>  							 old_addr, new_addr);
>  				BUG_ON(ret < 0 && ret != -EINVAL);
>  				if (!old) {
> -					ret = bpf_arch_text_poke(poke->tailcall_bypass,
> +					ret = __bpf_arch_text_poke(poke->tailcall_bypass,
>  								 BPF_MOD_JUMP,
>  								 poke->bypass_addr,
>  								 NULL);
> -					BUG_ON(ret < 0 && ret != -EINVAL);
> +					BUG_ON(ret < 0);
>  				}
>  			} else {
> -				ret = bpf_arch_text_poke(poke->tailcall_bypass,
> +				ret = __bpf_arch_text_poke(poke->tailcall_bypass,
>  							 BPF_MOD_JUMP,
>  							 old_bypass_addr,
>  							 poke->bypass_addr);
> -				BUG_ON(ret < 0 && ret != -EINVAL);
> +				BUG_ON(ret < 0);
>  				/* let other CPUs finish the execution of program
>  				 * so that it will not possible to expose them
>  				 * to invalid nop, stack unwind, nop state
>  				 */
>  				if (!ret)
>  					synchronize_rcu();
> -				ret = bpf_arch_text_poke(poke->tailcall_target,
> +				ret = __bpf_arch_text_poke(poke->tailcall_target,
>  							 BPF_MOD_JUMP,
>  							 old_addr, NULL);
> -				BUG_ON(ret < 0 && ret != -EINVAL);
> +				BUG_ON(ret < 0);
>  			}
>  		}
>  	}

-- 
Lee Jones [李琼斯]