proc: only require mm_struct for writing

Commit 591a22c1 ("proc: Track /proc/$pid/attr/ opener mm_struct") we
started using __mem_open() to track the mm_struct at open-time, so that
we could then check it for writes.

But that also ended up making the permission checks at open time much
stricter - and not just for writes, but for reads too.  And that in turn
caused a regression for at least Fedora 29, where NIC interfaces fail to
start when using NetworkManager.

Since only the write side wanted the mm_struct test, ignore any failures
by __mem_open() at open time, leaving reads unaffected.  The write()
time verification of the mm_struct pointer will then catch the failure
case because a NULL pointer will not match a valid 'current->mm'.

Link: https://lore.kernel.org/netdev/YMjTlp2FSJYvoyFa@unreal/
Fixes: 591a22c1 ("proc: Track /proc/$pid/attr/ opener mm_struct")
Reported-and-tested-by: Leon Romanovsky <leon@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -2676,7 +2676,9 @@ static int proc_pident_readdir(struct file *file, struct dir_context *ctx,
 #ifdef CONFIG_SECURITY
 static int proc_pid_attr_open(struct inode *inode, struct file *file)
 {
-	return __mem_open(inode, file, PTRACE_MODE_READ_FSCREDS);
+	file->private_data = NULL;
+	__mem_open(inode, file, PTRACE_MODE_READ_FSCREDS);
+	return 0;
 }
 
 static ssize_t proc_pid_attr_read(struct file * file, char __user * buf,