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.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
index 60d0bbd..1c1a1b0 100644
--- a/lib/strnlen_user.c
+++ b/lib/strnlen_user.c
@@ -114,10 +114,11 @@ 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;
}