secretmem: Prevent secretmem_users from wrapping to zero

Commit 1108605 ("mm/secretmem: use refcount_t instead of atomic_t")
attempted to fix the problem of secretmem_users wrapping to zero and
allowing suspend once again.  Prevent secretmem_users from wrapping to
zero by forbidding new users if the number of users has wrapped from
positive to negative.  This stops a long way short of reaching the
necessary 4 billion users, so there's no need to be clever with special
anti-wrap types or checking the return value from atomic_inc().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Jordy Zomer <jordy@pwning.systems>
Cc: Kees Cook <keescook@chromium.org>,
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>

mm/secretmem.c

@@ -204,6 +204,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)
 
 	if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC))
 		return -EINVAL;
+	if (atomic_read(&secretmem_users) < 0)
+		return -ENFILE;
 
 	fd = get_unused_fd_flags(flags & O_CLOEXEC);
 	if (fd < 0)