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Date:   Wed, 25 Oct 2023 09:22:55 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc:     Andrew Cooper <andrew.cooper3@citrix.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>, tony.luck@intel.com,
        ak@linux.intel.com, tim.c.chen@linux.intel.com,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        kvm@vger.kernel.org,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Daniel Sneddon <daniel.sneddon@linux.intel.com>,
        antonio.gomez.iglesias@linux.intel.com,
        Alyssa Milburn <alyssa.milburn@intel.com>
Subject: Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW
Message-ID: <20231025072255.GA37471@noisy.programming.kicks-ass.net>
References: <20231020-delay-verw-v1-0-cff54096326d@linux.intel.com>
 <20231020-delay-verw-v1-1-cff54096326d@linux.intel.com>
 <f620c7d4-6345-4ad0-8a45-c8089e3c34df@citrix.com>
 <20231025062818.7kaerqklaut7dg5r@desk>
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On Tue, Oct 24, 2023 at 11:28:18PM -0700, Pawan Gupta wrote:

> With .text.entry section I am getting getting below warnings and an
> error:
> 
> -----------------------------------------------------------------
>     LD      vmlinux.o
>   vmlinux.o: warning: objtool: .text.entry+0x0: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x40: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x80: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0xc0: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x100: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x140: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x180: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x1c0: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x200: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x240: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x280: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x2c0: unreachable instruction
>   vmlinux.o: warning: objtool: .text.entry+0x300: unreachable instruction
>   vmlinux.o: warning: objtool: .altinstr_replacement+0x2c: relocation to !ENDBR: .text.entry+0x0
>   vmlinux.o: warning: objtool: .altinstr_replacement+0x1c4: relocation to !ENDBR: .text.entry+0x0
>   vmlinux.o: warning: objtool: .altinstr_replacement+0x1d0: relocation to !ENDBR: .text.entry+0x0
>   vmlinux.o: warning: objtool: .altinstr_replacement+0x2d2: relocation to !ENDBR: .text.entry+0x80
>   vmlinux.o: warning: objtool: .altinstr_replacement+0x5d5: relocation to !ENDBR: .text.entry+0xc0
>     OBJCOPY modules.builtin.modinfo
>     GEN     modules.builtin
>     MODPOST vmlinux.symvers
>     UPD     include/generated/utsversion.h
>     CC      init/version-timestamp.o
>     LD      .tmp_vmlinux.kallsyms1
>   ld: error: unplaced orphan section `.text.entry' from `vmlinux.o'
>   make[2]: *** [scripts/Makefile.vmlinux:36: vmlinux] Error 1
> -----------------------------------------------------------------
> 
> ... because my config has CONFIG_LD_ORPHAN_WARN_LEVEL="error" and
> objtool needs to be told about this entry.
> 
> Do you think its worth fighting these warnings and error, or simply use
> .rodata section for verw memory operand?

I'm thinking you need to at the very least stay in a section that's
actually still mapped with PTI :-)

.entry.text is the only thing that is reliably mapped with PTI (IIRC),
some configs also get a chunk of the kernel image, but not all.

Something like the below should do I suppose.

---
 arch/x86/entry/entry.S | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
index bfb7bcb362bc..9eb2b532c92a 100644
--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -6,6 +6,8 @@
 #include <linux/linkage.h>
 #include <asm/export.h>
 #include <asm/msr-index.h>
+#include <asm/unwind_hints.h>
+#include <asm/segment.h>
 
 .pushsection .noinstr.text, "ax"
 
@@ -20,3 +22,16 @@ SYM_FUNC_END(entry_ibpb)
 EXPORT_SYMBOL_GPL(entry_ibpb);
 
 .popsection
+
+.pushsection .entry.text, "ax"
+
+.align 64
+SYM_CODE_START_NOALIGN(mds_verw_sel)
+	UNWIND_HINT_UNDEFINED
+	ANNOTATE_NOENDBR
+1:
+	.word __KERNEL_DS
+	.skip 64 - (. - 1b), 0xcc
+SYM_CODE_END(mds_verw_sel);
+
+.popsection