On 19/05/2020 15:48, Jan Beulich wrote: > On 19.05.2020 16:11, Andrew Cooper wrote: >> On 19/05/2020 09:34, Jan Beulich wrote: >>> On 18.05.2020 17:38, Andrew Cooper wrote: >>>> @@ -1439,6 +1418,18 @@ void do_page_fault(struct cpu_user_regs *regs) >>>> if ( unlikely(fixup_page_fault(addr, regs) != 0) ) >>>> return; >>>> >>>> + /* >>>> + * Xen have reserved bits in its pagetables, nor do we permit PV >>>> guests to >>>> + * write any. Such entries would be vulnerable to the L1TF >>>> sidechannel. >>>> + * >>>> + * The only logic which intentionally sets reserved bits is the shadow >>>> + * MMIO fastpath (SH_L1E_MMIO_*), which is careful not to be >>>> + * L1TF-vulnerable, and handled via the VMExit #PF intercept path, >>>> rather >>>> + * than here. >>> What about SH_L1E_MAGIC and sh_l1e_gnp()? The latter gets used by >>> _sh_propagate() without visible restriction to HVM. >> SH_L1E_MAGIC looks to be redundant with SH_L1E_MMIO_MAGIC. >> sh_l1e_mmio() is the only path which ever creates an entry like that. >> >> sh_l1e_gnp() is a very well hidden use of reserved bits, but surely >> can't be used for PV guests, as there doesn't appear to be anything to >> turn the resulting fault back into a plain not-present. > Well, in this case the implied question remains: How does this fit > with what _sh_propagate() does?
I'm in the process of investigating. >>> And of course every time I look at this code I wonder how we can >>> get away with (quoting a comment) "We store 28 bits of GFN in >>> bits 4:32 of the entry." Do we have a hidden restriction >>> somewhere guaranteeing that guests won't have (emulated MMIO) >>> GFNs above 1Tb when run in shadow mode? >> I've raised that several times before. Its broken. >> >> Given that shadow frames are limited to 44 bits anyway (and not yet >> levelled safely in the migration stream), my suggestion for fixing this >> was just to use one extra nibble for the extra 4 bits and call it done. > Would you remind(?) me of where this 44-bit restriction is coming > from? >From paging_max_paddr_bits(), /* Shadowed superpages store GFNs in 32-bit page_info fields. */ ~Andrew
