On June 23, 2025 4:13:34 PM PDT, "Luck, Tony" <tony.luck@xxxxxxxxx> wrote: >> >> Logically there are two completely different things: >> >> >> >> 1. Touching userspace >> >> 2. Touching the lower half of the address space >> >> >> >> If it's only userspace in the lower half of the address space, then >> >> there's no controversy. But the problem obviously occurs when you want >> >> to touch kernel mappings in the lower half of the address space. >> > >> > Why does the kernel create the mappings to poke kernel text >> > for ALTERNATIVE patching in the lower half of the address space? >> > >> > Instead of special "we really to want to access the lower addresses" >> > code, wouldn't it be easier to map the "poke" virtual addresses in normal >> > kernel upper-half space? >> >> The upper half of the address space is shared kernel space, right? Every >> PGD has identical contents in the upper half. So if we create a mapping >> there,everybody get access to it. Every mm can access it. Every >> *process* can access it. It still has kernel permissions of course, but >> it's still a place that everybody can get at. >> >> The lower half is *ONLY* accessible to the local mm. In this case, only >> the text poking mm. It's a natural, safe, place to create a mapping that >> you want to be private and not be exploited. >> >> So, doing it in the upper half is risky. >> >> If we *wanted*, we could have a non-shared PGD entry in the top half of >> the address space. But we'd need to reserve its address space and all >> that jazz. I'm not sure it's any better than just disabling LASS >> enforcement for a moment. > >Maybe it’s a thing to put on the list for "when x86 drops support for 32-bit". > >Reserving a PGD entry in the kernel half of the address space for >local CPU use would be practical then. Perhaps there might be other >uses too. > >-Tony > Are we actually doing patching on more than one CPU at a time?
