Re: [PATCH v2 7/7] virtio_balloon: Stop calling page_address() in free_pages()

[Michael S. Tsirkin]

On Thu, Aug 28, 2025 at 03:53:56PM +0100, Matthew Wilcox wrote:
> On Wed, Aug 27, 2025 at 11:29:22AM -0700, Vishal Moola (Oracle) wrote:
> > I imagine theres more of these lingering in the kernel, but theres so
> > many callers and I only looked for the ones that were calling
> > page_address() inline :(.
> 
> There's only 841 callers of free_page() and free_pages()!
> 
> It's a bit of a disease we have, to be honest,  Almost all of
> them should be using kmalloc() instead.  To pick on one at random,
> sel_read_bool() in security/selinux/selinuxfs.c is the implementation
> of read() for some file in selinux.  All it's trying to do is output two
> numbers, so it allocates an entire page of memory, prints two numbers
> to it (while being VERY CAREFUL not to overflow the buffer!) and copies
> the buffer to userspace.
> 
> It should just use kmalloc.

Why even kmalloc? Why not have a small array on stack?

>  Oh, and it should avoid leaking the buffer
> if security_get_bool_value() returns an error.