s390/uaccess: fix kernel ds access for page table walk

When the kernel resides in home space and the mvcos instruction is not
available uaccesses for kernel ds happen via simple strnlen() or memcpy()
calls.
This however can break badly, since uaccesses in kernel space may fail as
well, especially if CONFIG_DEBUG_PAGEALLOC is turned on.

To fix this implement strnlen_kernel() and copy_in_kernel() functions
which can only be used by the page table uaccess functions. These two
functions detect invalid memory accesses and return the correct length
of processed data.. Both functions are more or less a copy of the std
variants without sacf calls.

Fixes ipl crashes on 31 bit machines as well on 64 bit machines without
mvcos. Caused by changing the default address space of the kernel being
home space.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/lib/uaccess_pt.c

@@ -14,6 +14,63 @@
 #include <asm/futex.h>
 #include "uaccess.h"
 
+#ifndef CONFIG_64BIT
+#define AHI	"ahi"
+#define SLR	"slr"
+#else
+#define AHI	"aghi"
+#define SLR	"slgr"
+#endif
+
+static size_t strnlen_kernel(size_t count, const char __user *src)
+{
+	register unsigned long reg0 asm("0") = 0UL;
+	unsigned long tmp1, tmp2;
+
+	asm volatile(
+		"   la	  %2,0(%1)\n"
+		"   la	  %3,0(%0,%1)\n"
+		"  "SLR"  %0,%0\n"
+		"0: srst  %3,%2\n"
+		"   jo	  0b\n"
+		"   la	  %0,1(%3)\n"	/* strnlen_kernel results includes \0 */
+		"  "SLR"  %0,%1\n"
+		"1:\n"
+		EX_TABLE(0b,1b)
+		: "+a" (count), "+a" (src), "=a" (tmp1), "=a" (tmp2)
+		: "d" (reg0) : "cc", "memory");
+	return count;
+}
+
+static size_t copy_in_kernel(size_t count, void __user *to,
+			     const void __user *from)
+{
+	unsigned long tmp1;
+
+	asm volatile(
+		"  "AHI"  %0,-1\n"
+		"   jo	  5f\n"
+		"   bras  %3,3f\n"
+		"0:"AHI"  %0,257\n"
+		"1: mvc	  0(1,%1),0(%2)\n"
+		"   la	  %1,1(%1)\n"
+		"   la	  %2,1(%2)\n"
+		"  "AHI"  %0,-1\n"
+		"   jnz	  1b\n"
+		"   j	  5f\n"
+		"2: mvc	  0(256,%1),0(%2)\n"
+		"   la	  %1,256(%1)\n"
+		"   la	  %2,256(%2)\n"
+		"3:"AHI"  %0,-256\n"
+		"   jnm	  2b\n"
+		"4: ex	  %0,1b-0b(%3)\n"
+		"5:"SLR"  %0,%0\n"
+		"6:\n"
+		EX_TABLE(1b,6b) EX_TABLE(2b,0b) EX_TABLE(4b,0b)
+		: "+a" (count), "+a" (to), "+a" (from), "=a" (tmp1)
+		: : "cc", "memory");
+	return count;
+}
 
 /*
  * Returns kernel address for user virtual address. If the returned address is
@@ -123,10 +180,8 @@ size_t copy_from_user_pt(size_t n, const void __user *from, void *to)
 {
 	size_t rc;
 
-	if (segment_eq(get_fs(), KERNEL_DS)) {
-		memcpy(to, (void __kernel __force *) from, n);
-		return 0;
-	}
+	if (segment_eq(get_fs(), KERNEL_DS))
+		return copy_in_kernel(n, (void __user *) to, from);
 	rc = __user_copy_pt((unsigned long) from, to, n, 0);
 	if (unlikely(rc))
 		memset(to + n - rc, 0, rc);
@@ -135,30 +190,28 @@ size_t copy_from_user_pt(size_t n, const void __user *from, void *to)
 
 size_t copy_to_user_pt(size_t n, void __user *to, const void *from)
 {
-	if (segment_eq(get_fs(), KERNEL_DS)) {
-		memcpy((void __kernel __force *) to, from, n);
-		return 0;
-	}
+	if (segment_eq(get_fs(), KERNEL_DS))
+		return copy_in_kernel(n, to, (void __user *) from);
 	return __user_copy_pt((unsigned long) to, (void *) from, n, 1);
 }
 
 static size_t clear_user_pt(size_t n, void __user *to)
 {
+	void *zpage = &empty_zero_page;
 	long done, size, ret;
 
-	if (segment_eq(get_fs(), KERNEL_DS)) {
-		memset((void __kernel __force *) to, 0, n);
-		return 0;
-	}
 	done = 0;
 	do {
 		if (n - done > PAGE_SIZE)
 			size = PAGE_SIZE;
 		else
 			size = n - done;
-		ret = __user_copy_pt((unsigned long) to + done,
-				      &empty_zero_page, size, 1);
+		if (segment_eq(get_fs(), KERNEL_DS))
+			ret = copy_in_kernel(n, to, (void __user *) zpage);
+		else
+			ret = __user_copy_pt((unsigned long) to, zpage, size, 1);
 		done += size;
+		to += size;
 		if (ret)
 			return ret + n - done;
 	} while (done < n);
@@ -175,7 +228,7 @@ static size_t strnlen_user_pt(size_t count, const char __user *src)
 	if (unlikely(!count))
 		return 0;
 	if (segment_eq(get_fs(), KERNEL_DS))
-		return strnlen((const char __kernel __force *) src, count) + 1;
+		return strnlen_kernel(count, src);
 	done = 0;
 retry:
 	spin_lock(&mm->page_table_lock);
@@ -206,19 +259,17 @@ static size_t strncpy_from_user_pt(size_t count, const char __user *src,
 
 	if (unlikely(!count))
 		return 0;
-	if (segment_eq(get_fs(), KERNEL_DS)) {
-		len = strnlen((const char __kernel __force *) src, count) + 1;
-		if (len > count)
-			len = count;
-		memcpy(dst, (const char __kernel __force *) src, len);
-		return (dst[len - 1] == '\0') ? len - 1 : len;
-	}
 	done = 0;
 	do {
 		offset = (size_t)src & ~PAGE_MASK;
 		len = min(count - done, PAGE_SIZE - offset);
-		if (__user_copy_pt((unsigned long) src, dst, len, 0))
-			return -EFAULT;
+		if (segment_eq(get_fs(), KERNEL_DS)) {
+			if (copy_in_kernel(len, (void __user *) dst, src))
+				return -EFAULT;
+		} else {
+			if (__user_copy_pt((unsigned long) src, dst, len, 0))
+				return -EFAULT;
+		}
 		len_str = strnlen(dst, len);
 		done += len_str;
 		src += len_str;
@@ -237,10 +288,8 @@ static size_t copy_in_user_pt(size_t n, void __user *to,
 	unsigned long kaddr_to, kaddr_from;
 	int write_user;
 
-	if (segment_eq(get_fs(), KERNEL_DS)) {
-		memcpy((void __force *) to, (void __force *) from, n);
-		return 0;
-	}
+	if (segment_eq(get_fs(), KERNEL_DS))
+		return copy_in_kernel(n, to, from);
 	done = 0;
 retry:
 	spin_lock(&mm->page_table_lock);