linux-kernel - Re: [PATCH -tip 2/2] kprobes: Set unoptimized flag after unoptimizing code

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Message-ID: <20191127061910.nbfmzds4k5wxorwz@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 26 Nov 2019 22:19:11 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, bristot@...hat.com,
        jbaron@...mai.com, torvalds@...ux-foundation.org,
        tglx@...utronix.de, namit@...are.com, hpa@...or.com,
        luto@...nel.org, ard.biesheuvel@...aro.org, jpoimboe@...hat.com,
        jeyu@...nel.org
Subject: Re: [PATCH -tip 2/2] kprobes: Set unoptimized flag after
 unoptimizing code

On Wed, Nov 27, 2019 at 02:57:04PM +0900, Masami Hiramatsu wrote:
> Fix to set unoptimized flag after confirming the code is completely
> unoptimized. Without this fix, when a kprobe hits the intermediate
> modified instruction (the first byte is replaced by int3, but
> latter bytes still be a jump address operand) while unoptimizing,
> it can return to the middle byte of the modified code. And it causes
> an invalid instruction exception in the kernel.
> 
> Usually, this is a rare case, but if we put a probe on the function
> called while text patching, it always causes a kernel panic as below.
> (text_poke() is used for patching the code in optprobe)
> 
>  # echo p text_poke+5 > kprobe_events
>  # echo 1 > events/kprobes/enable
>  # echo 0 > events/kprobes/enable
>  invalid opcode: 0000 [#1] PREEMPT SMP PTI
>  CPU: 7 PID: 137 Comm: kworker/7:1 Not tainted 5.4.0-rc8+ #29
>  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
>  Workqueue: events kprobe_optimizer
>  RIP: 0010:text_poke+0x9/0x50
>  Code: 01 00 00 5b 5d 41 5c 41 5d c3 89 c0 0f b7 4c 02 fe 66 89 4c 05 fe e9 31 ff ff ff e8 71 ac 03 00 90 55 48 89 f5 53 cc 30 cb fd <1e> ec 08 8b 05 72 98 31 01 85 c0 75 11 48 83 c4 08 48 89 ee 48 89
>  RSP: 0018:ffffc90000343df0 EFLAGS: 00010686
>  RAX: 0000000000000000 RBX: ffffffff81025796 RCX: 0000000000000000
>  RDX: 0000000000000004 RSI: ffff88807c983148 RDI: ffffffff81025796
>  RBP: ffff88807c983148 R08: 0000000000000001 R09: 0000000000000000
>  R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff82284fe0
>  R13: ffff88807c983138 R14: ffffffff82284ff0 R15: 0ffff88807d9eee0
>  FS:  0000000000000000(0000) GS:ffff88807d9c0000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 000000000058158b CR3: 000000007b372000 CR4: 00000000000006a0
>  Call Trace:
>   arch_unoptimize_kprobe+0x22/0x28
>   arch_unoptimize_kprobes+0x39/0x87
>   kprobe_optimizer+0x6e/0x290
>   process_one_work+0x2a0/0x610
>   worker_thread+0x28/0x3d0
>   ? process_one_work+0x610/0x610
>   kthread+0x10d/0x130
>   ? kthread_park+0x80/0x80
>   ret_from_fork+0x3a/0x50
>  Modules linked in:
>  ---[ end trace 83b34b22a228711b ]---
> 
> This can happen even if we blacklist text_poke() and other functions,
> because there is a small time window which showing the intermediate
> code to other CPUs.
> 
> Fixes: 6274de4984a6 ("kprobes: Support delayed unoptimizing")
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>

Awesome. It fixes the crash for me.
Tested-by: Alexei Starovoitov <ast@...nel.org>