On Thu, Feb 07, 2019 at 10:24:20AM -0800, Alexander Duyck wrote: > On Thu, Feb 7, 2019 at 9:56 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: > > > > > > On 2/7/19 12:23 PM, Alexander Duyck wrote: > > > On Mon, Feb 4, 2019 at 2:11 PM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: > > >> This patch disables page poisoning if guest page hinting is enabled. > > >> It is required to avoid possible guest memory corruption errors. > > >> Page Poisoning is a feature in which the page is filled with a specific > > >> pattern of (0x00 or 0xaa) after arch_free_page and the same is verified > > >> before arch_alloc_page to prevent following issues: > > >> *information leak from the freed data > > >> *use after free bugs > > >> *memory corruption > > >> Selection of the pattern depends on the CONFIG_PAGE_POISONING_ZERO > > >> Once the guest pages which are supposed to be freed are sent to the > > >> hypervisor it frees them. After freeing the pages in the global list > > >> following things may happen: > > >> *Hypervisor reallocates the freed memory back to the guest > > >> *Hypervisor frees the memory and maps a different physical memory > > >> In order to prevent any information leak hypervisor before allocating > > >> memory to the guest fills it with zeroes. > > >> The issue arises when the pattern used for Page Poisoning is 0xaa while > > >> the newly allocated page received from the hypervisor by the guest is > > >> filled with the pattern 0x00. This will result in memory corruption errors. > > >> > > >> Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx> > > > This seems kind of backwards to me. Why disable page poisoning instead > > > of just not hinting about the free pages? There shouldn't be that many > > > instances when page poisoning is enabled, and when it is it would make > > > more sense to leave it enabled rather than silently disable it. > > As I have mentioned in the cover email, I intend to reuse Wei's already > > merged work. > > > > This will enable the guest to communicate the poison value which is in > > use to the host. > > That is far from being reliable given that you are having to buffer > the pages for some period of time. I really think it would be better > to just allow page poisoning to function and when you can support > applying poison to a newly allocated page then you could look at > re-enabling it. > > What I am getting at is that those that care about poisoning won't > likely care about performance and I would lump the memory hinting in > with other performance features. It's not just a performance issue. There is an issue is with the host/guest API. Once host discards pages it currently always gives you back zero-filled pages. So guest that looks for the poison using e.g. unpoison_page will crash unless the poison value is 0. Idea behind current code is just to let host know: it can either be more careful with these pages, or skip them completely. -- MST
