Re: [PATCH v4 3/3] sysfs: Unconditionally use vmalloc for buffer

[Christoph Hellwig]

On Thu, Apr 01, 2021 at 03:13:20PM -0700, Kees Cook wrote:
> The sysfs interface to seq_file continues to be rather fragile
> (seq_get_buf() should not be used outside of seq_file), as seen with
> some recent exploits[1]. Move the seq_file buffer to the vmap area
> (while retaining the accounting flag), since it has guard pages that will
> catch and stop linear overflows. This seems justified given that sysfs's
> use of seq_file almost always already uses PAGE_SIZE allocations, has
> normally short-lived allocations, and is not normally on a performance
> critical path.

This looks completely weird to me.  In the end sysfs uses nothing
of the seq_file infrastructure, so why do we even pretend to use it?
Just switch sysfs_file_kfops_ro and sysfs_file_kfops_rw from using
->seq_show to ->read and do the vmalloc there instead of pretending
this is a seq_file.

> Once seq_get_buf() has been removed (and all sysfs callbacks using
> seq_file directly), this change can also be removed.

And with sysfs out of the way I think kiling off the other few users
should be pretty easy as well.