fs/proc/kcore.c: use probe_kernel_read() instead of memcpy()

Commit df04abfd ("fs/proc/kcore.c: Add bounce buffer for ktext
data") added a bounce buffer to avoid hardened usercopy checks.  Copying
to the bounce buffer was implemented with a simple memcpy() assuming
that it is always valid to read from kernel memory iff the
kern_addr_valid() check passed.

A simple, but pointless, test case like "dd if=/proc/kcore of=/dev/null"
now can easily crash the kernel, since the former execption handling on
invalid kernel addresses now doesn't work anymore.

Also adding a kern_addr_valid() implementation wouldn't help here.  Most
architectures simply return 1 here, while a couple implemented a page
table walk to figure out if something is mapped at the address in
question.

With DEBUG_PAGEALLOC active mappings are established and removed all the
time, so that relying on the result of kern_addr_valid() before
executing the memcpy() also doesn't work.

Therefore simply use probe_kernel_read() to copy to the bounce buffer.
This also allows to simplify read_kcore().

At least on s390 this fixes the observed crashes and doesn't introduce
warnings that were removed with df04abfd ("fs/proc/kcore.c: Add
bounce buffer for ktext data"), even though the generic
probe_kernel_read() implementation uses uaccess functions.

While looking into this I'm also wondering if kern_addr_valid() could be
completely removed...(?)

Link: http://lkml.kernel.org/r/20171202132739.99971-1-heiko.carstens@de.ibm.com
Fixes: df04abfd ("fs/proc/kcore.c: Add bounce buffer for ktext data")
Fixes: f5509cc1 ("mm: Hardened usercopy")
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/kcore.c

@@ -512,23 +512,15 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 				return -EFAULT;
 		} else {
 			if (kern_addr_valid(start)) {
-				unsigned long n;
-
 				/*
 				 * Using bounce buffer to bypass the
 				 * hardened user copy kernel text checks.
 				 */
-				memcpy(buf, (char *) start, tsz);
-				n = copy_to_user(buffer, buf, tsz);
-				/*
-				 * We cannot distinguish between fault on source
-				 * and fault on destination. When this happens
-				 * we clear too and hope it will trigger the
-				 * EFAULT again.
-				 */
-				if (n) { 
-					if (clear_user(buffer + tsz - n,
-								n))
+				if (probe_kernel_read(buf, (void *) start, tsz)) {
+					if (clear_user(buffer, tsz))
+						return -EFAULT;
+				} else {
+					if (copy_to_user(buffer, buf, tsz))
 						return -EFAULT;
 				}
 			} else {