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

Alexei Starovoitov
2019-11-27T06:19:19Z

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@kernel.org>

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