[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v3 2/6] x86/HVM: split restore state checking from state loading
On Tue, Nov 28, 2023 at 11:34:04AM +0100, Jan Beulich wrote: > ..., at least as reasonably feasible without making a check hook > mandatory (in particular strict vs relaxed/zero-extend length checking > can't be done early this way). > > Note that only one of the two uses of hvm_load() is accompanied with > hvm_check(). The other directly consumes hvm_save() output, which ought > to be well-formed. This means that while input data related checks don't > need repeating in the "load" function when already done by the "check" > one (albeit assertions to this effect may be desirable), domain state > related checks (e.g. has_xyz(d)) will be required in both places. > > Suggested-by: Roger Pau Monné <roger.pau@xxxxxxxxxx> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx> > --- > Do we really need all the copying involved in use of _hvm_read_entry() > (backing hvm_load_entry()? Zero-extending loads are likely easier to > handle that way, but for strict loads all we gain is a reduced risk of > unaligned accesses (compared to simply pointing into h->data[]). See below, but I wonder whether the checks could be performed as part of hvm_load() without having to introduce a separate handler and loop over the context entries. > Would the hvm_sr_handlers[] better use array_access_nospec()? Maybe? Given this is a domctl I do wonder whether a domain already having access to such interface won't have easier ways to leak data from Xen. Maybe for a disaggregated setup. > --- > v2: New. > > --- a/xen/arch/x86/domctl.c > +++ b/xen/arch/x86/domctl.c > @@ -379,6 +379,10 @@ long arch_do_domctl( > if ( copy_from_guest(c.data, domctl->u.hvmcontext.buffer, c.size) != > 0 ) > goto sethvmcontext_out; > > + ret = hvm_check(d, &c); > + if ( ret ) > + goto sethvmcontext_out; > + > domain_pause(d); > ret = hvm_load(d, &c); > domain_unpause(d); > --- a/xen/arch/x86/hvm/save.c > +++ b/xen/arch/x86/hvm/save.c > @@ -30,7 +30,8 @@ static void arch_hvm_save(struct domain > d->arch.hvm.sync_tsc = rdtsc(); > } > > -static int arch_hvm_load(struct domain *d, const struct hvm_save_header *hdr) > +static int arch_hvm_check(const struct domain *d, > + const struct hvm_save_header *hdr) > { > uint32_t eax, ebx, ecx, edx; > > @@ -55,6 +56,11 @@ static int arch_hvm_load(struct domain * > "(%#"PRIx32") and restored on another (%#"PRIx32").\n", > d->domain_id, hdr->cpuid, eax); > > + return 0; > +} > + > +static void arch_hvm_load(struct domain *d, const struct hvm_save_header > *hdr) > +{ > /* Restore guest's preferred TSC frequency. */ > if ( hdr->gtsc_khz ) > d->arch.tsc_khz = hdr->gtsc_khz; > @@ -66,13 +72,12 @@ static int arch_hvm_load(struct domain * > > /* VGA state is not saved/restored, so we nobble the cache. */ > d->arch.hvm.stdvga.cache = STDVGA_CACHE_DISABLED; > - > - return 0; > } > > /* List of handlers for various HVM save and restore types */ > static struct { > hvm_save_handler save; > + hvm_check_handler check; > hvm_load_handler load; > const char *name; > size_t size; > @@ -88,6 +93,7 @@ void __init hvm_register_savevm(uint16_t > { > ASSERT(typecode <= HVM_SAVE_CODE_MAX); > ASSERT(hvm_sr_handlers[typecode].save == NULL); > + ASSERT(hvm_sr_handlers[typecode].check == NULL); > ASSERT(hvm_sr_handlers[typecode].load == NULL); > hvm_sr_handlers[typecode].save = save_state; > hvm_sr_handlers[typecode].load = load_state; > @@ -275,6 +281,78 @@ int hvm_save(struct domain *d, hvm_domai > return 0; > } > > +int hvm_check(const struct domain *d, hvm_domain_context_t *h) > +{ > + const struct hvm_save_header *hdr; > + int rc; > + > + if ( d->is_dying ) > + return -EINVAL; > + > + /* Get at the save header, which must be first. */ > + hdr = hvm_get_entry(HEADER, h); > + if ( !hdr ) > + return -ENODATA; > + > + rc = arch_hvm_check(d, hdr); > + if ( rc ) > + return rc; > + > + for ( ; ; ) > + { > + const struct hvm_save_descriptor *desc; > + hvm_check_handler handler; > + > + if ( h->size - h->cur < sizeof(*desc) ) > + { > + /* Run out of data */ > + printk(XENLOG_G_ERR > + "HVM restore %pd: save did not end with a null entry\n", > + d); > + return -ENODATA; > + } > + > + /* Read the typecode of the next entry and check for the end-marker. > */ > + desc = (const void *)&h->data[h->cur]; > + if ( desc->typecode == HVM_SAVE_CODE(END) ) > + { > + /* Reset cursor for hvm_load(). */ > + h->cur = 0; > + return 0; > + } > + > + /* Find the handler for this entry. */ > + if ( desc->typecode >= ARRAY_SIZE(hvm_sr_handlers) || > + !hvm_sr_handlers[desc->typecode].name || > + !hvm_sr_handlers[desc->typecode].load ) > + { > + printk(XENLOG_G_ERR "HVM restore %pd: unknown entry typecode > %u\n", > + d, desc->typecode); > + return -EINVAL; > + } > + > + /* Check the entry. */ > + handler = hvm_sr_handlers[desc->typecode].check; > + if ( !handler ) > + { > + if ( desc->length > h->size - h->cur - sizeof(*desc) ) > + return -ENODATA; > + h->cur += sizeof(*desc) + desc->length; > + } > + else if ( (rc = handler(d, h)) ) > + { > + printk(XENLOG_G_ERR > + "HVM restore %pd: failed to check %s:%u rc %d\n", > + d, hvm_sr_handlers[desc->typecode].name, desc->instance, > rc); > + return rc; > + } > + > + process_pending_softirqs(); Looking at this, won't it be better to call the check() hooks inside the hvm_load() function instead of duplicating the loop? I realize that you only perform the checks when the state is loaded from a domctl, but still seems quite a lot of code duplication for little benefit. hvm_load() could gain an extra parameter to select whether the input must be checked or not, and that would avoid having to iterate twice over the context. > + } > + > + /* Not reached */ ASSERT_UNREACHABLE() maybe? Thanks, Roger.
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