linux-kernel - [PATCH 4.8 121/140] arm64: Cortex-A53 errata workaround: check for kernel addresses

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Date:   Wed, 26 Oct 2016 14:23:01 +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,
        Kristina Martsenko <kristina.martsenko@....com>,
        Andre Przywara <andre.przywara@....com>,
        Will Deacon <will.deacon@....com>
Subject: [PATCH 4.8 121/140] arm64: Cortex-A53 errata workaround: check for kernel addresses

4.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andre Przywara <andre.przywara@....com>

commit 87261d19046aeaeed8eb3d2793fde850ae1b5c9e upstream.

Commit 7dd01aef0557 ("arm64: trap userspace "dc cvau" cache operation on
errata-affected core") adds code to execute cache maintenance instructions
in the kernel on behalf of userland on CPUs with certain ARM CPU errata.
It turns out that the address hasn't been checked to be a valid user
space address, allowing userland to clean cache lines in kernel space.
Fix this by introducing an address check before executing the
instructions on behalf of userland.

Since the address doesn't come via a syscall parameter, we can't just
reject tagged pointers and instead have to remove the tag when checking
against the user address limit.

Fixes: 7dd01aef0557 ("arm64: trap userspace "dc cvau" cache operation on errata-affected core")
Reported-by: Kristina Martsenko <kristina.martsenko@....com>
Signed-off-by: Andre Przywara <andre.przywara@....com>
[will: rework commit message + replace access_ok with max_user_addr()]
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/arm64/include/asm/uaccess.h |    8 ++++++++
 arch/arm64/kernel/traps.c        |   27 +++++++++++++++------------
 2 files changed, 23 insertions(+), 12 deletions(-)

--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -21,6 +21,7 @@
 /*
  * User space memory access functions
  */
+#include <linux/bitops.h>
 #include <linux/kasan-checks.h>
 #include <linux/string.h>
 #include <linux/thread_info.h>
@@ -102,6 +103,13 @@ static inline void set_fs(mm_segment_t f
 	flag;								\
 })
 
+/*
+ * When dealing with data aborts or instruction traps we may end up with
+ * a tagged userland pointer. Clear the tag to get a sane pointer to pass
+ * on to access_ok(), for instance.
+ */
+#define untagged_addr(addr)		sign_extend64(addr, 55)
+
 #define access_ok(type, addr, size)	__range_ok(addr, size)
 #define user_addr_max			get_fs
 
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -434,18 +434,21 @@ void cpu_enable_cache_maint_trap(void *_
 }
 
 #define __user_cache_maint(insn, address, res)			\
-	asm volatile (						\
-		"1:	" insn ", %1\n"				\
-		"	mov	%w0, #0\n"			\
-		"2:\n"						\
-		"	.pushsection .fixup,\"ax\"\n"		\
-		"	.align	2\n"				\
-		"3:	mov	%w0, %w2\n"			\
-		"	b	2b\n"				\
-		"	.popsection\n"				\
-		_ASM_EXTABLE(1b, 3b)				\
-		: "=r" (res)					\
-		: "r" (address), "i" (-EFAULT) )
+	if (untagged_addr(address) >= user_addr_max())		\
+		res = -EFAULT;					\
+	else							\
+		asm volatile (					\
+			"1:	" insn ", %1\n"			\
+			"	mov	%w0, #0\n"		\
+			"2:\n"					\
+			"	.pushsection .fixup,\"ax\"\n"	\
+			"	.align	2\n"			\
+			"3:	mov	%w0, %w2\n"		\
+			"	b	2b\n"			\
+			"	.popsection\n"			\
+			_ASM_EXTABLE(1b, 3b)			\
+			: "=r" (res)				\
+			: "r" (address), "i" (-EFAULT) )
 
 asmlinkage void __exception do_sysinstr(unsigned int esr, struct pt_regs *regs)
 {