linux-kernel - [PATCH 5.19 256/365] x86/kvm: Fix "missing ENDBR" BUG for fastop functions

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Message-Id: <20220823080128.907850538@linuxfoundation.org>
Date:   Tue, 23 Aug 2022 10:02:37 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Pengfei Xu <pengfei.xu@...el.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sasha Levin <sashal@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 5.19 256/365] x86/kvm: Fix "missing ENDBR" BUG for fastop functions

From: Josh Poimboeuf <jpoimboe@...nel.org>

[ Upstream commit 3d9606b0e0f3aed4dfb61d0853ebf432fead7bba ]

The following BUG was reported:

  traps: Missing ENDBR: andw_ax_dx+0x0/0x10 [kvm]
  ------------[ cut here ]------------
  kernel BUG at arch/x86/kernel/traps.c:253!
  invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
   <TASK>
   asm_exc_control_protection+0x2b/0x30
  RIP: 0010:andw_ax_dx+0x0/0x10 [kvm]
  Code: c3 cc cc cc cc 0f 1f 44 00 00 66 0f 1f 00 48 19 d0 c3 cc cc cc
        cc 0f 1f 40 00 f3 0f 1e fa 20 d0 c3 cc cc cc cc 0f 1f 44 00 00
        <66> 0f 1f 00 66 21 d0 c3 cc cc cc cc 0f 1f 40 00 66 0f 1f 00 21
        d0

   ? andb_al_dl+0x10/0x10 [kvm]
   ? fastop+0x5d/0xa0 [kvm]
   x86_emulate_insn+0x822/0x1060 [kvm]
   x86_emulate_instruction+0x46f/0x750 [kvm]
   complete_emulated_mmio+0x216/0x2c0 [kvm]
   kvm_arch_vcpu_ioctl_run+0x604/0x650 [kvm]
   kvm_vcpu_ioctl+0x2f4/0x6b0 [kvm]
   ? wake_up_q+0xa0/0xa0

The BUG occurred because the ENDBR in the andw_ax_dx() fastop function
had been incorrectly "sealed" (converted to a NOP) by apply_ibt_endbr().

Objtool marked it to be sealed because KVM has no compile-time
references to the function.  Instead KVM calculates its address at
runtime.

Prevent objtool from annotating fastop functions as sealable by creating
throwaway dummy compile-time references to the functions.

Fixes: 6649fa876da4 ("x86/ibt,kvm: Add ENDBR to fastops")
Reported-by: Pengfei Xu <pengfei.xu@...el.com>
Debugged-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
Message-Id: <0d4116f90e9d0c1b754bb90c585e6f0415a1c508.1660837839.git.jpoimboe@...nel.org>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/kvm/emulate.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index aa907cec0918..09fa8a94807b 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -316,7 +316,8 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
 	".align " __stringify(FASTOP_SIZE) " \n\t" \
 	".type " name ", @function \n\t" \
 	name ":\n\t" \
-	ASM_ENDBR
+	ASM_ENDBR \
+	IBT_NOSEAL(name)
 
 #define FOP_FUNC(name) \
 	__FOP_FUNC(#name)
-- 
2.35.1