proc: only require mm_struct for writing

commit 94f0b2d4a1d0c52035aef425da5e022bd2cb1c71 upstream.

Commit 591a22c14d3f ("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: 591a22c14d3f ("proc: Track /proc/$pid/attr/ opener mm_struct")
Reported-and-tested-by: Leon Romanovsky <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Andrea Righi <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

fs/proc/base.c

@@ -2386,7 +2386,9 @@ 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,