Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 5 Oct 2020 11:56:05 -0700 | Subject | Re: [PATCH v2 7/9] x86: Use current USER_CS to setup correct context on vmx entry |
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On Sat, Oct 3, 2020 at 4:04 PM Andy Lutomirski <luto@kernel.org> wrote: > > On Fri, Oct 2, 2020 at 5:15 PM Andy Lutomirski <luto@kernel.org> wrote: > > > > On Fri, Oct 2, 2020 at 3:40 PM Sean Christopherson > > <sean.j.christopherson@intel.com> wrote: > > > > > > On Thu, Oct 01, 2020 at 02:52:32PM -0700, Andy Lutomirski wrote: > > > > On Thu, Oct 1, 2020 at 1:59 PM Gabriel Krisman Bertazi > > > > <krisman@collabora.com> wrote: > > > > > > > > > > vmx_prepare_switch_to_guest shouldn't use is_64bit_mm, which has a > > > > > very specific use in uprobes. Use the user_64bit_mode helper instead. > > > > > This reduces the usage of is_64bit_mm, which is awkward, since it relies > > > > > on the personality at load time, which is fine for uprobes, but doesn't > > > > > seem fine here. > > > > > > > > > > I tested this by running VMs with 64 and 32 bits payloads from 64/32 > > > > > programs. > > > > > > > > > > Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com> > > > > > --- > > > > > arch/x86/kvm/vmx/vmx.c | 2 +- > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > > > > > index 7b2a068f08c1..b5aafd9e5f5d 100644 > > > > > --- a/arch/x86/kvm/vmx/vmx.c > > > > > +++ b/arch/x86/kvm/vmx/vmx.c > > > > > @@ -1172,7 +1172,7 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu) > > > > > savesegment(es, host_state->es_sel); > > > > > > > > > > gs_base = cpu_kernelmode_gs_base(cpu); > > > > > - if (likely(is_64bit_mm(current->mm))) { > > > > > + if (likely(user_64bit_mode(current_pt_regs()))) { > > > > > current_save_fsgs(); > > > > > fs_sel = current->thread.fsindex; > > > > > gs_sel = current->thread.gsindex; > > > > > > > > I disagree with this one. This whole code path is nonsense. Can you > > > > just remove the condition entirely and use the 64-bit path > > > > unconditionally? > > > > > > I finally came back to this one with fresh eyes. I've read through the code > > > a bajllion times and typed up half a dozen responses. I think, finally, I > > > understand what's broken. > > > > > > I believe your original assertion that the bug was misdiagnosed is correct > > > (can't link because LKML wasn't in the loop). I'm pretty sure your analysis > > > that KVM's handling of things works mostly by coincidence is also correct. > > > > > > The coincidence is that "real" VMMs all use arch_prctl(), and > > > do_arch_prctl_64() ensures thread.{fs,gs}base are accurate. save_base_legacy() > > > detects sel==0 and intentionally does nothing, knowing the the base is already > > > accurate. > > > > > > Userspaces that don't use arch_prctl(), in the bug report case a 32-bit compat > > > test, may or may not have accurate thread.{fs,gs}base values. This is > > > especially true if sel!=0 as save_base_legacy() explicitly zeros the base in > > > this case, as load_seg_legacy() will restore the seg on the backend. > > > > > > KVM on the other hand assumes thread.{fs,gs}base are always fresh. When that > > > didn't hold true for userspace that didn't use arch_prctl(), the fix of > > > detecting a !64-bit mm just so happened to work because all 64-bit VMMs use > > > arch_prctl(). > > > > > > It's tempting to just open code this and use RD{FS,GS}BASE when possible, > > > i.e. avoid any guesswork. Maybe with a module param that userspace can set > > > to tell KVM it doesn't do anything fancy with FS/GS base (will userspace still > > > use arch_prctl() even if FSGSABSE is available?). > > > > > > savesegment(fs, fs_sel); > > > savesegment(gs, gs_sel); > > > if (use_current_fsgs_base) { > > > fs_base = current->thread.fsbase; > > > vmx->msr_host_kernel_gs_base = current->thread.gsbase; > > > > I don't like this. The FSGSBASE case is fast, and the !FSGSBASE case > > is mostly obsolete. I see no great reason to have a module parameter > > asking for incorrect behavior. There have been too many bugs in this > > area -- let's not add more please. > > > > > } else if (static_cpu_has(X86_FEATURE_FSGSBASE)) { > > > fs_base = rdfsbase() > > > vmx->msr_host_kernel_gs_base = __rdgsbase_inactive(); > > > } else { > > > fs_base = read_msr(MSR_FS_BASE); > > > vmx->msr_host_kernel_gs_base = read_msr(MSR_KERNEL_GS_BASE); > > > } > > > > I'm okay with this, but I think you should fix the corresponding bugs > > on the restore side as well. The current code is: > > > > if (host_state->ldt_sel || (host_state->gs_sel & 7)) { > > kvm_load_ldt(host_state->ldt_sel); > > #ifdef CONFIG_X86_64 > > load_gs_index(host_state->gs_sel); > > #else > > loadsegment(gs, host_state->gs_sel); > > #endif > > } > > if (host_state->fs_sel & 7) > > loadsegment(fs, host_state->fs_sel); > > > > which is blatantly wrong in the case where fs_set.TI == 1, gs_set.TI > > == 0, and ldt_sel != 0. But it's also more subtly wrong -- this > > corrupts all the segment attributes in the case where a segment points > > to the GDT and the GDT attributes are non-default. So I would suggest > > making the code radically simpler and more correct: > > > > if (host_state->idt_sel) > > kvm_load_idt(host_state->idt_sel); // see below > > if (host_state->ds_sel) > > loadsegment(ds, host_state->ds_sel); > > if (host_state->es_sel) > > loadsegment(es, host_state->es_sel); > > if (host_state->fs_sel) { > > loadsegment(fs, host_state->fs_sel); > > x86_fsbase_write_cpu(host_state->fs_base); > > } > > if (host_state->gs_sel) { > > load_gs_index(host_state->gs_sel); > > x86_gsbase_write_cpu_inactive(host_state->msr_host_kernel_gs_base); > > } > > > > In the IMO unlikely event that any performance-critical KVM userspace > > runs with these selectors != 0, you could also skip the load if they > > are set to __USER_DS. > > > > You can also simplify this crud: > > > > if (unlikely(fs_sel != host->fs_sel)) { > > if (!(fs_sel & 7)) > > vmcs_write16(HOST_FS_SELECTOR, fs_sel); > > else > > vmcs_write16(HOST_FS_SELECTOR, 0); > > host->fs_sel = fs_sel; > > } > > > > There is nothing special about segments with TI set according to the > > SDM (AFAICT) nor is anything particularly special about them in > > Linux's segment tables, so this code makes little sense. It could > > just be: > > > > host->fs_sel = fs_sel; > > > > and leave the VMCS field set to 0. Or if you do the __USER_DS > > optimization above, you could do: > > > > if (unlikely(fs_sel != host->fs_sel)) { > > vmcs_write16(HOST_FS_SELECTOR, fs_sel == __USER_DS ? __USER_DS : 0); > > host->fs_sel = fs_sel; > > } > > > > I suspect that the only users who care about performance (or for whom > > we should care about performance) are normal userspace, and normal > > 64-bit userspace has DS == ES == FS == GS == 0. > > > > > > I would also be okay with making the KVM code match the context switch > > code, but this may be distinctly nontrivial. > > If you're okay waiting for a couple days, I'll just do this. I have > it 2/3-done already, except I'm running into the utter catastrophe > that is 32-bit stackprotector, so I'm going to fix that first. (Or > delete it if I get toosick of it.)
Rough draft. This needs the rest of that branch applied to have any chance of being correct.
I was contemplating whether we could get a meaningful speedup if we declared that entering a VM would wipe segments, but I don't think there's actually much speedup available.
https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/unify_stack_canary&id=f5a2dd07cd222bb674bd947f5d9c698ab50ccb88
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