linux-kernel - [PATCH] x86/mm: Do not verify W^X at boot up

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Message-ID: <20221024114536.44686c83@gandalf.local.home>
Date:   Mon, 24 Oct 2022 11:45:36 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH] x86/mm: Do not verify W^X at boot up

From: "Steven Rostedt (Google)" <rostedt@...dmis.org>

Adding on the kernel command line "ftrace=function" triggered:

------------[ cut here ]------------
CPA detected W^X violation: 8000000000000063 -> 0000000000000063 range:
0xffffffffc0013000 - 0xffffffffc0013fff PFN 10031b
WARNING: CPU: 0 PID: 0 at arch/x86/mm/pat/set_memory.c:609
verify_rwx+0x61/0x6d
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 6.1.0-rc1-test+ #3
Hardware name: MSI MS-7823/CSM-H87M-G43 (MS-7823), BIOS V1.6 02/22/2014
RIP: 0010:verify_rwx+0x61/0x6d
Code: e5 01 00 75 27 49 c1 e0 0c 48 89 d1 48 89 fe 48 c7 c7 5b b3 92 84 4e
8d 44 02 ff 48 89 da c6 05 71 29 e5 01 01 e8 35 90 e2 00 <0f> 0b 48 89 d8
5b 5d e9 6f 95 1a 01 0f 1f 44 00 00 55 48 89 e5 53
RSP: 0000:ffffffff84c03b08 EFLAGS: 00010086
RAX: 0000000000000000 RBX: 0000000000000063 RCX: 0000000000000003
RDX: 0000000000000003 RSI: ffffffff84c039b0 RDI: 0000000000000001
RBP: ffffffff84c03b10 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000025 R12: ffff8e730031c098
R13: 000000000010031b R14: 800000010031b063 R15: 8000000000000063
FS:  0000000000000000(0000) GS:ffff8e7416a00000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff8e73fd801000 CR3: 00000001fcc22001 CR4: 00000000000606f0
Call Trace:
 <TASK>
 __change_page_attr_set_clr+0x146/0x8a6
 ? __mutex_unlock_slowpath+0x41/0x213
 ? mutex_unlock+0x12/0x18
 ? _vm_unmap_aliases+0x126/0x136
 change_page_attr_set_clr+0x135/0x268
 ? find_vmap_area+0x32/0x3e
 ? __fentry__+0x10/0x10
 change_page_attr_clear.constprop.0+0x16/0x1c
 set_memory_x+0x2c/0x32
 arch_ftrace_update_trampoline+0x218/0x2db
 ? ftrace_caller_op_ptr+0x17/0x17
 ftrace_update_trampoline+0x16/0xa1
 ? tracing_gen_ctx+0x1c/0x1c
 __register_ftrace_function+0x93/0xb2
 ftrace_startup+0x21/0xf0
 ? tracing_gen_ctx+0x1c/0x1c
 register_ftrace_function_nolock+0x26/0x40
 register_ftrace_function+0x4e/0x143
 ? mutex_unlock+0x12/0x18
 ? tracing_gen_ctx+0x1c/0x1c
 function_trace_init+0x7d/0xc3
 tracer_init+0x23/0x2c
 tracing_set_tracer+0x1d5/0x206
 register_tracer+0x1c0/0x1e4
 init_function_trace+0x90/0x96
 early_trace_init+0x25c/0x352
 start_kernel+0x424/0x6e4
 x86_64_start_reservations+0x24/0x2a
 x86_64_start_kernel+0x8c/0x95
 secondary_startup_64_no_verify+0xe0/0xeb
 </TASK>
---[ end trace 0000000000000000 ]---

This is because at boot up, kernel text is writable, and there's no reason
to do tricks to updated it. But the verifier does not distinguish updates
at boot up and at run time, and causes a warning at time of boot.

Add a check for system_state == SYSTEM_BOOTING and allow it if that is the
case.

Link: https://lore.kernel.org/r/20221024112730.180916b3@gandalf.local.home

Fixes: 652c5bf380ad0 ("x86/mm: Refuse W^X violations")
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 arch/x86/mm/pat/set_memory.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 97342c42dda8..2e5a045731de 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -587,6 +587,10 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
 {
 	unsigned long end;
 
+	/* Kernel text is rw at boot up */
+	if (system_state == SYSTEM_BOOTING)
+		return new;
+
 	/*
 	 * 32-bit has some unfixable W+X issues, like EFI code
 	 * and writeable data being in the same page.  Disable
-- 
2.35.1