proc: Track /proc/$pid/attr/ opener mm_struct

proc: Track /proc/$pid/attr/ opener mm_struct

Commit bfb819ea20ce ("proc: Check /proc/$pid/attr/ writes against file opener")
tried to make sure that there could not be a confusion between the opener of
a /proc/$pid/attr/ file and the writer. It used struct cred to make sure
the privileges didn't change. However, there were existing cases where a more
privileged thread was passing the opened fd to a differently privileged thread
(during container setup). Instead, use mm_struct to track whether the opener
and writer are still the same process. (This is what several other proc files
already do, though for different reasons.)

Reported-by: Christian Brauner <[email protected]>
Reported-by: Andrea Righi <[email protected]>
Tested-by: Andrea Righi <[email protected]>
Fixes: bfb819ea20ce ("proc: Check /proc/$pid/attr/ writes against file opener")
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>