Re: KASAN: use-after-free Read in ___bpf_prog_run

[Yonghong Song <yhs@meta.com>]

On 12/16/22 10:54 PM, Hao Sun wrote:
> 
> 
>> On 17 Dec 2022, at 1:07 PM, Yonghong Song <yhs@meta.com> wrote:
>>
>>
>>
>> On 12/14/22 11:49 PM, Hao Sun wrote:
>>> Hi,
>>> The following KASAN report can be triggered by loading and test
>>> running this simple BPF prog with a random data/ctx:
>>> 0: r0 = bpf_get_current_task_btf      ;
>>> R0_w=trusted_ptr_task_struct(off=0,imm=0)
>>> 1: r0 = *(u32 *)(r0 +8192)       ;
>>> R0_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
>>> 2: exit
>>> I've simplified the C reproducer but didn't find the root cause.
>>> JIT was disabled, and the interpreter triggered UAF when executing
>>> the load insn. A slab-out-of-bound read can also be triggered:
>>> https://pastebin.com/raw/g9zXr8jU
>>> This can be reproduced on:
>>> HEAD commit: b148c8b9b926 selftests/bpf: Add few corner cases to test
>>> padding handling of btf_dump
>>> git tree: bpf-next
>>> console log: https://pastebin.com/raw/1EUi9tJe
>>> kernel config: https://pastebin.com/raw/rgY3AJDZ
>>> C reproducer: https://pastebin.com/raw/cfVGuCBm
>>
>> I I tried with your above kernel config and C reproducer and cannot reproduce the kasan issue you reported.
>>
>> [root@arch-fb-vm1 bpf-next]# ./a.out
>> func#0 @0
>> 0: R1=ctx(off=0,imm=0) R10=fp0
>> 0: (85) call bpf_get_current_task_btf#158     ; R0_w=trusted_ptr_task_struct(off=0,imm=0)
>> 1: (61) r0 = *(u32 *)(r0 +8192)       ; R0_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
>> 2: (95) exit
>> processed 3 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
>>
>> prog fd: 3
>> [root@arch-fb-vm1 bpf-next]#
>>
>> Your config indeed has kasan on.
> 
> Hi,
> 
> I can still reproduce this on a latest bpf-next build: 0e43662e61f25
> (“tools/resolve_btfids: Use pkg-config to locate libelf”).
> The simplified C reproducer sometime need to be run twice to trigger
> the UAF. Also note that interpreter is required. Here is the original
> C reproducer that loads and runs the BPF prog continuously for your
> convenience:
> https://pastebin.com/raw/WSJuNnVU
> 

I still cannot reproduce with more than 10 runs. The config has jit off 
so it already uses interpreter. It has kasan on as well.
# CONFIG_BPF_JIT is not set

Since you can reproduce it, I guess it would be great if you can 
continue to debug this.

>>
>>> ==================================================================
>>> BUG: KASAN: use-after-free in ___bpf_prog_run+0x7f35/0x8fd0
>>> kernel/bpf/core.c:1937
>>> Read of size 4 at addr ffff88801f1f2000 by task a.out/7137
>>> CPU: 3 PID: 7137 Comm: a.out Not tainted
>>> 6.1.0-rc8-02212-gef3911a3e4d6-dirty #137
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux
>>> 1.16.1-1-1 04/01/2014
>>> Call Trace:
[...]