Messages in this thread | | | Date | Wed, 2 Mar 2022 12:02:10 -0800 | From | Josh Poimboeuf <> | Subject | Re: [PATCHv5 15/30] x86/boot: Port I/O: allow to hook up alternative helpers |
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On Wed, Mar 02, 2022 at 11:41:53AM -0800, Dave Hansen wrote: > On 3/2/22 09:42, Josh Poimboeuf wrote: > > At the very least, please remove the ability for future code to> accidentally bypass 'pio_ops'. Going forward, are we really expected > to> just remember to always use pio_ops for i/o? Or else TDX will just> > silently break? That's just not acceptable. > What did you have in mind here? The in/out() instruction wrappers could > be moved to a spot where they're impossible to call directly, for instance.
I guess, though why not just put the pio_ops crud in the inb/outb wrappers themselves?
> I guess we could get really fancy and use objtool to look for any I/O > instructions that show up outside of the "official" pio_ops copies. > That would prevent anyone using inline assembly.
Yeah, there's no easy solution for asm and inline asm. We would need something like objtool to enforce the new "non-direct-i/o" policy in boot code. But objtool doesn't even validate boot code.
And it looks this patch missed an "outb"?
static inline void io_delay(void) { const u16 DELAY_PORT = 0x80; asm volatile("outb %%al,%0" : : "dN" (DELAY_PORT)); }
> In the end, though, TDX *is* a new sub-architecture. There are lots of > ways it's going to break silently and nobody will notice on bare metal. > SEV is the same way with things like the C (encryption) bit in the page > tables. Adding more safeguards sounds like a good idea but, in the end, > we're going to have to find the non-obvious issues with testing.
Right, but for this case there's no reason to destabilize TDX on purpose.
-- Josh
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