procfs: make /proc/*/pagemap 0400

procfs: make /proc/*/pagemap 0400

The /proc/*/pagemap contain sensitive information and currently its mode
is 0444.  Change this to 0400, so the VFS will prevent unprivileged
processes from getting file descriptors on arbitrary privileged
/proc/*/pagemap files.

This reduces the scope of address space leaking and bypasses by protecting
already running processes.

Signed-off-by: Djalal Harouni <[email protected]>
Acked-by: Kees Cook <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Ingo Molnar <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>