linux-kernel - [PATCH v7 2/3] x86/entry_32: Clear CPU buffers after register restore in NMI return

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Message-ID: <20240925-fix-dosemu-vm86-v7-2-1de0daca2d42@linux.intel.com>
Date: Wed, 25 Sep 2024 15:25:44 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	Robert Gill <rtgill82@...il.com>,
	Jari Ruusu <jariruusu@...tonmail.com>,
	Brian Gerst <brgerst@...il.com>,
	"Linux regression tracking (Thorsten Leemhuis)" <regressions@...mhuis.info>,
	antonio.gomez.iglesias@...ux.intel.com,
	daniel.sneddon@...ux.intel.com, stable@...r.kernel.org
Subject: [PATCH v7 2/3] x86/entry_32: Clear CPU buffers after register
 restore in NMI return

CPU buffers are currently cleared after call to exc_nmi, but before
register state is restored. This may be okay for MDS mitigation but not for
RDFS. Because RDFS mitigation requires CPU buffers to be cleared when
registers don't have any sensitive data.

Move CLEAR_CPU_BUFFERS after RESTORE_ALL_NMI.

Fixes: a0e2dab44d22 ("x86/entry_32: Add VERW just before userspace transition")
Cc: stable@...r.kernel.org # 5.10+
Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
---
 arch/x86/entry/entry_32.S | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 9ad6cd89b7ac..20be5758c2d2 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -1145,7 +1145,6 @@ SYM_CODE_START(asm_exc_nmi)
 
 	/* Not on SYSENTER stack. */
 	call	exc_nmi
-	CLEAR_CPU_BUFFERS
 	jmp	.Lnmi_return
 
 .Lnmi_from_sysenter_stack:
@@ -1166,6 +1165,7 @@ SYM_CODE_START(asm_exc_nmi)
 
 	CHECK_AND_APPLY_ESPFIX
 	RESTORE_ALL_NMI cr3_reg=%edi pop=4
+	CLEAR_CPU_BUFFERS
 	jmp	.Lirq_return
 
 #ifdef CONFIG_X86_ESPFIX32
@@ -1207,6 +1207,7 @@ SYM_CODE_START(asm_exc_nmi)
 	 *  1 - orig_ax
 	 */
 	lss	(1+5+6)*4(%esp), %esp			# back to espfix stack
+	CLEAR_CPU_BUFFERS
 	jmp	.Lirq_return
 #endif
 SYM_CODE_END(asm_exc_nmi)

-- 
2.34.1