Re: [PATCH 04/22] x86/kvm: Don't call kvm_spurious_fault() from .fixup

[Josh Poimboeuf]

On Mon, Jul 15, 2019 at 11:07:28AM +0200, Paolo Bonzini wrote:
> On 15/07/19 02:36, Josh Poimboeuf wrote:
> > After making a change to improve objtool's sibling call detection, it
> > started showing the following warning:
> > 
> >   arch/x86/kvm/vmx/nested.o: warning: objtool: .fixup+0x15: sibling call from callable instruction with modified stack frame
> > 
> > The problem is the ____kvm_handle_fault_on_reboot() macro.  It does a
> > fake call by pushing a fake RIP and doing a jump.  That tricks the
> > unwinder into printing the function which triggered the exception,
> > rather than the .fixup code.
> > 
> > Instead of the hack to make it look like the original function made the
> > call, just change the macro so that the original function actually does
> > make the call.  This allows removal of the hack, and also makes objtool
> > happy.
> > 
> > I triggered a vmx instruction exception and verified that the stack
> > trace is still sane:
> > 
> >   kernel BUG at arch/x86/kvm/x86.c:358!
> >   invalid opcode: 0000 [#1] SMP PTI
> >   CPU: 28 PID: 4096 Comm: qemu-kvm Not tainted 5.2.0+ #16
> >   Hardware name: Lenovo THINKSYSTEM SD530 -[7X2106Z000]-/-[7X2106Z000]-, BIOS -[TEE113Z-1.00]- 07/17/2017
> >   RIP: 0010:kvm_spurious_fault+0x5/0x10
> >   Code: 00 00 00 00 00 8b 44 24 10 89 d2 45 89 c9 48 89 44 24 10 8b 44 24 08 48 89 44 24 08 e9 d4 40 22 00 0f 1f 40 00 0f 1f 44 00 00 <0f> 0b 66 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 41 55 49 89 fd 41
> >   RSP: 0018:ffffbf91c683bd00 EFLAGS: 00010246
> >   RAX: 000061f040000000 RBX: ffff9e159c77bba0 RCX: ffff9e15a5c87000
> >   RDX: 0000000665c87000 RSI: ffff9e15a5c87000 RDI: ffff9e159c77bba0
> >   RBP: 0000000000000000 R08: 0000000000000000 R09: ffff9e15a5c87000
> >   R10: 0000000000000000 R11: fffff8f2d99721c0 R12: ffff9e159c77bba0
> >   R13: ffffbf91c671d960 R14: ffff9e159c778000 R15: 0000000000000000
> >   FS:  00007fa341cbe700(0000) GS:ffff9e15b7400000(0000) knlGS:0000000000000000
> >   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >   CR2: 00007fdd38356804 CR3: 00000006759de003 CR4: 00000000007606e0
> >   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> >   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >   PKRU: 55555554
> >   Call Trace:
> >    loaded_vmcs_init+0x4f/0xe0
> >    alloc_loaded_vmcs+0x38/0xd0
> >    vmx_create_vcpu+0xf7/0x600
> >    kvm_vm_ioctl+0x5e9/0x980
> >    ? __switch_to_asm+0x40/0x70
> >    ? __switch_to_asm+0x34/0x70
> >    ? __switch_to_asm+0x40/0x70
> >    ? __switch_to_asm+0x34/0x70
> >    ? free_one_page+0x13f/0x4e0
> >    do_vfs_ioctl+0xa4/0x630
> >    ksys_ioctl+0x60/0x90
> >    __x64_sys_ioctl+0x16/0x20
> >    do_syscall_64+0x55/0x1c0
> >    entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >   RIP: 0033:0x7fa349b1ee5b
> > 
> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > ---
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/kvm_host.h | 33 ++++++++++++++++++---------------
> >  1 file changed, 18 insertions(+), 15 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 0cc5b611a113..af7e18c05f98 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1499,22 +1499,25 @@ enum {
> >  /*
> >   * Hardware virtualization extension instructions may fault if a
> >   * reboot turns off virtualization while processes are running.
> > - * Trap the fault and ignore the instruction if that happens.
> > + * If that happens, trap the fault and panic (unless we're rebooting).
> 
> Not sure the comment is better than before, but apar from that

The previous comment didn't seem to match the code, since we only ignore
the instruction if we're rebooting.

> Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Thanks!

-- 
Josh