BACKPORT: make 'user_access_begin()' do 'access_ok()'

Originally, the rule used to be that you'd have to do access_ok()
separately, and then user_access_begin() before actually doing the
direct (optimized) user access.

But experience has shown that people then decide not to do access_ok()
at all, and instead rely on it being implied by other operations or
similar.  Which makes it very hard to verify that the access has
actually been range-checked.

If you use the unsafe direct user accesses, hardware features (either
SMAP - Supervisor Mode Access Protection - on x86, or PAN - Privileged
Access Never - on ARM) do force you to use user_access_begin().  But
nothing really forces the range check.

By putting the range check into user_access_begin(), we actually force
people to do the right thing (tm), and the range check vill be visible
near the actual accesses.  We have way too long a history of people
trying to avoid them.

Bug 200630541
CVE-2018-20669

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Change-Id: I1d25229cd41495b782e6f2bc67a4bb638dec4540
Reviewed-on: https://git-master.nvidia.com/r/c/linux-4.9/+/2374142
(cherry picked from commit e77a94059008f0f812474d05a9f75accecaeb583)
Reviewed-on: https://git-master.nvidia.com/r/c/linux-4.9/+/2466485
Reviewed-by: Bibek Basu <bbasu@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
GVS: Gerrit_Virtual_Submit
Tested-by: Amulya Yarlagadda <ayarlagadda@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

include/linux/uaccess.h

@@ -141,7 +141,7 @@ extern long strnlen_unsafe_user(const void __user *unsafe_addr, long count);
 	probe_kernel_read(&retval, addr, sizeof(retval))
 
 #ifndef user_access_begin
-#define user_access_begin() do { } while (0)
+#define user_access_begin(ptr,len) access_ok(VERIFY_READ, ptr, len)
 #define user_access_end() do { } while (0)
 #define unsafe_get_user(x, ptr, err) do { if (unlikely(__get_user(x, ptr))) goto err; } while (0)
 #define unsafe_put_user(x, ptr, err) do { if (unlikely(__put_user(x, ptr))) goto err; } while (0)

lib/strncpy_from_user.c

@@ -114,10 +114,11 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
 
 		kasan_check_write(dst, count);
 		check_object_size(dst, count, false);
-		user_access_begin();
-		retval = do_strncpy_from_user(dst, src, count, max);
-		user_access_end();
-		return retval;
+		if (user_access_begin(src, max)) {
+			retval = do_strncpy_from_user(dst, src, count, max);
+			user_access_end();
+			return retval;
+		}
 	}
 	return -EFAULT;
 }

lib/strnlen_user.c

@@ -113,10 +113,11 @@ long strnlen_user(const char __user *str, long count)
 		unsigned long max = max_addr - src_addr;
 		long retval;
 
-		user_access_begin();
-		retval = do_strnlen_user(str, count, max);
-		user_access_end();
-		return retval;
+		if (user_access_begin(str, max)) {
+			retval = do_strnlen_user(str, count, max);
+			user_access_end();
+			return retval;
+		}
 	}
 	return 0;
 }
@@ -147,10 +148,11 @@ long strlen_user(const char __user *str)
 		unsigned long max = max_addr - src_addr;
 		long retval;
 
-		user_access_begin();
-		retval = do_strnlen_user(str, ~0ul, max);
-		user_access_end();
-		return retval;
+		if (user_access_begin(str, max)) {
+			retval = do_strnlen_user(str, ~0ul, max);
+			user_access_end();
+			return retval;
+		}
 	}
 	return 0;
 }