Re: [PATCH v2 04/22] x86/kvm: Don't call kvm_spurious_fault() from .fixup

[Paolo Bonzini]

On 18/07/19 03:36, 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>
> Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
> v2: Fix ____kvm_handle_fault_on_reboot() comment [Paolo]
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h | 34 ++++++++++++++++++---------------
>  1 file changed, 19 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0cc5b611a113..8282b8d41209 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1496,25 +1496,29 @@ enum {
>  #define kvm_arch_vcpu_memslots_id(vcpu) ((vcpu)->arch.hflags & HF_SMM_MASK ? 1 : 0)
>  #define kvm_memslots_for_spte_role(kvm, role) __kvm_memslots(kvm, (role).smm)
>  
> +asmlinkage void __noreturn kvm_spurious_fault(void);
> +
>  /*
>   * 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.
> + * Usually after catching the fault we just panic; during reboot
> + * instead the instruction is ignored.
>   */
> -asmlinkage void kvm_spurious_fault(void);
> -
> -#define ____kvm_handle_fault_on_reboot(insn, cleanup_insn)	\
> -	"666: " insn "\n\t" \
> -	"668: \n\t"                           \
> -	".pushsection .fixup, \"ax\" \n" \
> -	"667: \n\t" \
> -	cleanup_insn "\n\t"		      \
> -	"cmpb $0, kvm_rebooting \n\t"	      \
> -	"jne 668b \n\t"      		      \
> -	__ASM_SIZE(push) " $666b \n\t"	      \
> -	"jmp kvm_spurious_fault \n\t"	      \
> -	".popsection \n\t" \
> -	_ASM_EXTABLE(666b, 667b)
> +#define ____kvm_handle_fault_on_reboot(insn, cleanup_insn)		\
> +	"666: \n\t"							\
> +	insn "\n\t"							\
> +	"jmp	668f \n\t"						\
> +	"667: \n\t"							\
> +	"call	kvm_spurious_fault \n\t"				\
> +	"668: \n\t"							\
> +	".pushsection .fixup, \"ax\" \n\t"				\
> +	"700: \n\t"							\
> +	cleanup_insn "\n\t"						\
> +	"cmpb	$0, kvm_rebooting\n\t"					\
> +	"je	667b \n\t"						\
> +	"jmp	668b \n\t"						\
> +	".popsection \n\t"						\
> +	_ASM_EXTABLE(666b, 700b)
>  
>  #define __kvm_handle_fault_on_reboot(insn)		\
>  	____kvm_handle_fault_on_reboot(insn, "")
> 

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

This has a side effect of adding a jump in a generally hot path, but
let's hope that the speculation gods for once help us.

Paolo