From bb4435255df53118082b3567db6da100962b3822 Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Sat, 22 May 2021 22:48:36 +0300
Subject: [PATCH] amd64: clear PSL.AC in the right frame

If copyin family of routines fault, kernel does clear PSL.AC on the
fault entry, but the AC flag of the faulted frame is kept intact.  Since
onfault handler is effectively jump, AC survives until syscall exit.

Approved by:	so
Security:	FreeBSD-SA-21:11.smap
Security:	CVE-2021-29628
Reported by:	m00nbsd, via Sony
Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
admbugs:	975

(cherry picked from commit 91aae953cb807d6fb7a70782b323bf9beb60d7c9)
(cherry picked from commit 4590f0345956329d414611c6cee300f486732b53)
---
 sys/amd64/amd64/support.S           | 18 ++++++++++++------
 sys/amd64/linux/linux_support.s     |  5 ++++-
 sys/amd64/linux32/linux32_support.s |  5 ++++-
 3 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/sys/amd64/amd64/support.S b/sys/amd64/amd64/support.S
index f4d3f5c124c..023ca66f088 100644
--- a/sys/amd64/amd64/support.S
+++ b/sys/amd64/amd64/support.S
@@ -868,9 +868,11 @@ ENTRY(copyin_smap_erms)
 END(copyin_smap_erms)
 
 	ALIGN_TEXT
-	/* Trap entry clears PSL.AC */
 copy_fault:
-	movq	$0,PCB_ONFAULT(%r11)
+	testl	$CPUID_STDEXT_SMAP,cpu_stdext_feature(%rip)
+	je	1f
+	clac
+1:	movq	$0,PCB_ONFAULT(%r11)
 	movl	$EFAULT,%eax
 	POP_FRAME_POINTER
 	ret
@@ -1311,9 +1313,11 @@ ENTRY(subyte_smap)
 END(subyte_smap)
 
 	ALIGN_TEXT
-	/* Fault entry clears PSL.AC */
 fusufault:
-	movq	PCPU(CURPCB),%rcx
+	testl	$CPUID_STDEXT_SMAP,cpu_stdext_feature(%rip)
+	je	1f
+	clac
+1:	movq	PCPU(CURPCB),%rcx
 	xorl	%eax,%eax
 	movq	%rax,PCB_ONFAULT(%rcx)
 	decq	%rax
@@ -1396,8 +1400,10 @@ ENTRY(copyinstr_smap)
 END(copyinstr_smap)
 
 cpystrflt:
-	/* Fault entry clears PSL.AC */
-	movl	$EFAULT,%eax
+	testl	$CPUID_STDEXT_SMAP,cpu_stdext_feature(%rip)
+	je	1f
+	clac
+1:	movl	$EFAULT,%eax
 cpystrflt_x:
 	/* set *lencopied and return %eax */
 	movq	$0,PCB_ONFAULT(%r9)
diff --git a/sys/amd64/linux/linux_support.s b/sys/amd64/linux/linux_support.s
index 391f76414f2..2de778e151b 100644
--- a/sys/amd64/linux/linux_support.s
+++ b/sys/amd64/linux/linux_support.s
@@ -34,7 +34,10 @@
 #include "assym.inc"
 
 futex_fault:
-	movq	$0,PCB_ONFAULT(%r8)
+	testl	$CPUID_STDEXT_SMAP,cpu_stdext_feature(%rip)
+	je	1f
+	clac
+1:	movq	$0,PCB_ONFAULT(%r8)
 	movl	$-EFAULT,%eax
 	ret
 
diff --git a/sys/amd64/linux32/linux32_support.s b/sys/amd64/linux32/linux32_support.s
index 981bba9f582..7ff3e2293f6 100644
--- a/sys/amd64/linux32/linux32_support.s
+++ b/sys/amd64/linux32/linux32_support.s
@@ -34,7 +34,10 @@
 #include "assym.inc"
 
 futex_fault:
-	movq	$0,PCB_ONFAULT(%r8)
+	testl	$CPUID_STDEXT_SMAP,cpu_stdext_feature(%rip)
+	je	1f
+	clac
+1:	movq	$0,PCB_ONFAULT(%r8)
 	movl	$-EFAULT,%eax
 	ret
 
-- 
2.45.0