Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling (why is this happening?)

From: Peter Zijlstra
Date: Fri Jan 27 2023 - 04:08:48 EST

On Fri, Jan 27, 2023 at 02:56:16PM +1100, Alexey Kardashevskiy wrote:
> AMD SEV-ES guest kernels compiled without CONFIG_PARAVIRT crash when
> "perf" executes. "./perf record sleep 20" is an example.
>
> Some debugging revealed this happens when CONFIG_PARAVIRT_XXL is not
> defined. The problem seems to be that between DEFINE_IDTENTRY_RAW(exc_nmi)
> and actual reading of DB7 (which in turn causes #VC) every function is
> inlined

Very much on purpose.

> and no stack frame is created (?).

Silly compilers ;-) I think you can force it to by using inline asm with
a rsp dependency or somesuch.

> Replacing __always_inline with
> noinline in local_db_save() or native_get_debugreg() fixes the problem.

It will create others.

> Why is this crash happening and how to fix that? I am still reading
> the assembly but was hoping for a shortcut here :) Thanks,

Welcome to the wonderful shit show that is x86 exceptions :/

I thought sev_es_*() is supposed to fix this. Joerg, any clue?

> aik-Standard-PC-i440FX-PIIX-1996 login: [ 15.775303] BUG: NMI stack guard page was hit at 0000000003983d50 (stack is 000000007feb1fa4..00000000574369c2)
> [ 15.775314] stack guard page: 0000 [#1] PREEMPT SMP NOPTI
> [ 15.775316] CPU: 0 PID: 790 Comm: sleep Not tainted 6.2.0-rc4_aik-debugswap_ruby-954bhost #73
> [ 15.775322] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown unknown
> [ 15.775323] RIP: 0010:error_entry+0x17/0x140
> [ 15.775326] Code: f8 e9 98 fd ff ff 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 56 48 8b 74 24 08 48 89 7c 24 08 52 51 50 41 50 41 51 41 52 41 53 53 <55> 41 54 41 55 41 56 41 57 56 31 f6 31 d2 31 c9 45 31 c0 45 31 c9
> [ 15.775328] RSP: 0000:fffffe2446b2b000 EFLAGS: 00010097
> [ 15.775332] RAX: fffffe2446b2b0a8 RBX: 0000000000000000 RCX: ffffffffb3a00fed
> [ 15.775333] RDX: 0000000000000000 RSI: ffffffffb3a00b69 RDI: fffffe2446b2b0a8
> [ 15.775336] RBP: fffffe2446b2b0a8 R08: 0000000000000000 R09: 0000000000000000
> [ 15.775337] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> [ 15.775338] R13: 0000000000000000 R14: 000000000002dd80 R15: 0000000000000000
> [ 15.775339] FS: 0000000000000000(0000) GS:ffff94b17dc00000(0000) knlGS:ffff94b17dc00000
> [ 15.775340] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 15.775341] CR2: fffffe2446b2aff8 CR3: 00080000167b8000 CR4: 00000000003506f0
> [ 15.775342] Call Trace:
> [ 15.775352] <NMI>

<snip>

> [ 15.775495] ? asm_exc_page_fault+0x22/0x30
> [ 15.775496] ? restore_regs_and_return_to_kernel+0x22/0x22
> [ 15.775497] ? exc_page_fault+0x11/0x120
> [ 15.775499] ? asm_exc_page_fault+0x22/0x30
> [ 15.775500] ? check_preemption_disabled+0x8/0xe0
> [ 15.775502] ? __sev_es_ist_enter+0x13/0x100
> [ 15.775503] ? exc_nmi+0x10e/0x150
> [ 15.775505] ? end_repeat_nmi+0x16/0x67
> [ 15.775506] ? asm_exc_double_fault+0x30/0x30
> [ 15.775507] ? asm_exc_double_fault+0x30/0x30
> [ 15.775508] ? asm_exc_double_fault+0x30/0x30
> [ 15.775509] </NMI>
> [ 15.775509] <#VC>
> [ 15.775510] ? __show_regs.cold+0x18e/0x23d
> [ 15.775511] </#VC>
> [ 15.775511] <#DF>
> [ 15.775512] ? __die_body.cold+0x1a/0x1f
> [ 15.775513] ? die+0x26/0x40
> [ 15.775517] ? handle_stack_overflow+0x44/0x60
> [ 15.775518] ? exc_double_fault+0x14b/0x180
> [ 15.775519] ? asm_exc_double_fault+0x1f/0x30
> [ 15.775520] ? restore_regs_and_return_to_kernel+0x22/0x22
> [ 15.775521] ? asm_exc_page_fault+0x9/0x30
> [ 15.775522] ? error_entry+0x17/0x140
> [ 15.775523] </#DF>
> [ 15.775523] WARNING: stack recursion on stack type 6
> [ 15.775524] Modules linked in: msr efivarfs
> [ 15.837935] ---[ end trace 0000000000000000 ]---
>
> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
> ---
> arch/x86/include/asm/debugreg.h | 29 ++++++++++++++++++++
> arch/x86/kernel/nmi.c | 2 +-
> 2 files changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
> index b049d950612f..396e2ea114dc 100644
> --- a/arch/x86/include/asm/debugreg.h
> +++ b/arch/x86/include/asm/debugreg.h
> @@ -125,6 +125,35 @@ static __always_inline void local_db_restore(unsigned long dr7)
> set_debugreg(dr7, 7);
> }
>
> +/* noinline here inline __always_inline'd native_get_debugreg(int regno) */
> +static noinline unsigned long native_get_debugreg7(void)
> +{
> + unsigned long dr7;
> + asm("mov %%db7, %0" :"=r" (dr7));
> + return dr7;
> +}
> +
> +static __always_inline unsigned long local_db_save_exc_nmi(void)
> +{
> + unsigned long dr7;
> +
> + if (static_cpu_has(X86_FEATURE_HYPERVISOR) && !hw_breakpoint_active())
> + return 0;
> +
> + dr7 = native_get_debugreg7();
> + dr7 &= ~0x400; /* architecturally set bit */
> + if (dr7)
> + set_debugreg(0, 7);
> + /*
> + * Ensure the compiler doesn't lower the above statements into
> + * the critical section; disabling breakpoints late would not
> + * be good.
> + */
> + barrier();
> +
> + return dr7;
> +}

This is broken, and building with DEBUG_ENTRY=y would've told you.

> +
> #ifdef CONFIG_CPU_SUP_AMD
> extern void set_dr_addr_mask(unsigned long mask, int dr);
> #else
> diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> index cec0bfa3bc04..400b5b6b74f6 100644
> --- a/arch/x86/kernel/nmi.c
> +++ b/arch/x86/kernel/nmi.c
> @@ -503,7 +503,7 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
> */
> sev_es_ist_enter(regs);
>
> - this_cpu_write(nmi_dr7, local_db_save());
> + this_cpu_write(nmi_dr7, local_db_save_exc_nmi());
>
> irq_state = irqentry_nmi_enter(regs);

So what I don't get is why sev_es_ist_enter() doesn't cause us to make a
stack frame, that has an actual call in it (although admittedly
conditional).