linux-kernel - [PATCH 09/20] x86/uaccess,kasan: Fix KASAN vs SMAP

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Date:   Thu, 07 Mar 2019 12:45:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     torvalds@...ux-foundation.org, tglx@...utronix.de, hpa@...or.com,
        julien.thierry@....com, will.deacon@....com, luto@...capital.net,
        mingo@...nel.org, catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        dvyukov@...gle.com, rostedt@...dmis.org
Subject: [PATCH 09/20] x86/uaccess,kasan: Fix KASAN vs SMAP

KASAN inserts extra code for every LOAD/STORE emitted by te compiler.
Much of this code is simple and safe to run with AC=1, however the
kasan_report() function, called on error, is most certainly not safe
to call with AC=1.

Therefore wrap kasan_report() in user_access_{save,restore}; which for
x86 SMAP, saves/restores EFLAGS and clears AC before calling the real
function.

Also ensure all the functions are without __fentry__ hook. The
function tracer is also not safe.

Cc: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 mm/kasan/Makefile |    3 +++
 mm/kasan/common.c |   10 ++++++++++
 mm/kasan/report.c |    3 +--
 3 files changed, 14 insertions(+), 2 deletions(-)

--- a/mm/kasan/Makefile
+++ b/mm/kasan/Makefile
@@ -2,11 +2,13 @@
 KASAN_SANITIZE := n
 UBSAN_SANITIZE_common.o := n
 UBSAN_SANITIZE_generic.o := n
+UBSAN_SANITIZE_generic_report.o := n
 UBSAN_SANITIZE_tags.o := n
 KCOV_INSTRUMENT := n
 
 CFLAGS_REMOVE_common.o = -pg
 CFLAGS_REMOVE_generic.o = -pg
+CFLAGS_REMOVE_generic_report.o = -pg
 CFLAGS_REMOVE_tags.o = -pg
 
 # Function splitter causes unnecessary splits in __asan_load1/__asan_store1
@@ -14,6 +16,7 @@ CFLAGS_REMOVE_tags.o = -pg
 
 CFLAGS_common.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector)
 CFLAGS_generic.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector)
+CFLAGS_generic_report.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector)
 CFLAGS_tags.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector)
 
 obj-$(CONFIG_KASAN) := common.o init.o report.o
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -34,6 +34,7 @@
 #include <linux/types.h>
 #include <linux/vmalloc.h>
 #include <linux/bug.h>
+#include <linux/uaccess.h>
 
 #include "kasan.h"
 #include "../slab.h"
@@ -612,6 +613,15 @@ void kasan_free_shadow(const struct vm_s
 		vfree(kasan_mem_to_shadow(vm->addr));
 }
 
+extern void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip);
+
+void kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
+{
+	unsigned long flags = user_access_save();
+	__kasan_report(addr, size, is_write, ip);
+	user_access_restore(flags);
+}
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 static bool shadow_mapped(unsigned long addr)
 {
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -281,8 +281,7 @@ void kasan_report_invalid_free(void *obj
 	end_report(&flags);
 }
 
-void kasan_report(unsigned long addr, size_t size,
-		bool is_write, unsigned long ip)
+void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
 {
 	struct kasan_access_info info;
 	void *tagged_addr;