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Date:   Fri, 1 May 2020 00:17:06 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC][PATCH] x86/ftrace: Have ftrace trampolines turn read-only
 at the end of system boot up

On Thu, Apr 30, 2020 at 11:47:33PM -0500, Josh Poimboeuf wrote:
> On Thu, Apr 30, 2020 at 08:21:47PM -0400, Steven Rostedt wrote:
> > The cause is the "ftrace=function" would register the function tracer
> > and create a trampoline, and it will set it as executable and
> > read-only. Then the "trace_options=func_stack_trace" would then update
> > the same trampoline to include the stack tracer version of the function
> > tracer. But since the trampoline already exists, it updates it with
> > text_poke_bp(). The problem is that text_poke_bp() called while
> > system_state == SYSTEM_BOOTING, it will simply do a memcpy() and not
> > the page mapping, as it would think that the text is still read-write.
> > But in this case it is not, and we take a fault and crash.
> > 
> > Instead, lets keep the ftrace trampolines read-write during boot up,
> > and then when the kernel executable text is set to read-only, the
> > ftrace trampolines get set to read-only as well.
> 
> Would it be easier to just call a new __text_poke_bp() which skips the
> SYSTEM_BOOTING check, since you know the trampoline will always be
> read-only?
> 
> Like:

early_trace_init() is called after mm_init(), so I thought it might
work, but I guess not:

[    0.206271] Starting tracer 'function'
[    0.232032] BUG: kernel NULL pointer dereference, address: 0000000000000050
[    0.232035] #PF: supervisor read access in kernel mode
[    0.232036] #PF: error_code(0x0000) - not-present page
[    0.232037] PGD 0 P4D 0 
[    0.232040] Oops: 0000 [#1] PREEMPT SMP PTI
[    0.232042] CPU: 0 PID: 0 Comm: swapper Not tainted 5.7.0-rc2-next-20200424+ #127
[    0.232043] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-2.fc30 04/01/2014
[    0.232047] RIP: 0010:walk_to_pmd+0x11/0x140
[    0.232048] Code: 78 c1 32 82 e8 a0 68 ff ff 0f 0b 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 0f 1f 44 00 00 48 89 f0 48 c1 e8 24 25 f8 0f 00 00 <48> 03 47 50 0f 84 12 01 00 00 41 54 49 89 fc 55 48 89 c5 53 48 8b
[    0.232050] RSP: 0000:ffffffff82603c68 EFLAGS: 00010046
[    0.232051] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[    0.232052] RDX: ffffffff82603cc8 RSI: 0000000000000000 RDI: 0000000000000000
[    0.232053] RBP: 8000000000000063 R08: 0000000000000001 R09: fff0000000000fff
[    0.232054] R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff82603cc8
[    0.232055] R13: 0000000000000000 R14: 0000000000000066 R15: fffffffffffffeff
[    0.232056] FS:  0000000000000000(0000) GS:ffff88813b600000(0000) knlGS:0000000000000000
[    0.232057] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.232058] CR2: 0000000000000050 CR3: 0000000002612001 CR4: 00000000000606b0
[    0.232061] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    0.232062] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[    0.232063] Call Trace:
[    0.232066]  __get_locked_pte+0x19/0x100
[    0.232069]  ? 0xffffffffa0000065
[    0.232071]  __text_poke+0x14d/0x640
[    0.232073]  ? 0xffffffffa0000065
[    0.232076]  text_poke_bp_batch+0x8b/0x1c0
[    0.232078]  ? 0xffffffffa0000065
[    0.232080]  __text_poke_bp+0x3a/0x60
[    0.232083]  arch_ftrace_update_trampoline+0xac/0x300
[    0.232087]  __register_ftrace_function+0x7c/0xc0
[    0.232089]  ftrace_startup+0x1e/0xf0
[    0.232091]  register_ftrace_function+0x24/0x70
[    0.232093]  func_set_flag+0x6f/0x90
[    0.232096]  __set_tracer_option.isra.0+0x24/0x50
[    0.232098]  trace_set_options+0x149/0x160
[    0.232102]  apply_trace_boot_options+0x44/0x6d
[    0.232105]  register_tracer+0x1d8/0x1e9
[    0.232107]  early_trace_init+0x266/0x378
[    0.232109]  start_kernel+0x337/0x614
[    0.232112]  ? x86_family+0x5/0x20
[    0.232114]  secondary_startup_64+0xa4/0xb0
[    0.232119] Modules linked in:
[    0.232120] CR2: 0000000000000050
[    0.232126] random: get_random_bytes called from print_oops_end_marker+0x26/0x40 with crng_init=0
[    0.232128] ---[ end trace 71e23a89b9b5224f ]---
[    0.232130] RIP: 0010:walk_to_pmd+0x11/0x140
[    0.232131] Code: 78 c1 32 82 e8 a0 68 ff ff 0f 0b 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 0f 1f 44 00 00 48 89 f0 48 c1 e8 24 25 f8 0f 00 00 <48> 03 47 50 0f 84 12 01 00 00 41 54 49 89 fc 55 48 89 c5 53 48 8b
[    0.232133] RSP: 0000:ffffffff82603c68 EFLAGS: 00010046
[    0.232134] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[    0.232135] RDX: ffffffff82603cc8 RSI: 0000000000000000 RDI: 0000000000000000
[    0.232136] RBP: 8000000000000063 R08: 0000000000000001 R09: fff0000000000fff
[    0.232137] R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff82603cc8
[    0.232138] R13: 0000000000000000 R14: 0000000000000066 R15: fffffffffffffeff
[    0.232139] FS:  0000000000000000(0000) GS:ffff88813b600000(0000) knlGS:0000000000000000
[    0.232140] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.232141] CR2: 0000000000000050 CR3: 0000000002612001 CR4: 00000000000606b0
[    0.232142] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    0.232143] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[    0.232144] Kernel panic - not syncing: Attempted to kill the idle task!
[    0.232184] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---

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