Message-ID: <CAKddAkDbGYeONaksq6fzLzx47BHZo3Ar7Sog3MOgf7Y+Birovw@mail.gmail.com>
Date: Thu, 14 Nov 2024 09:20:14 +0800
From: Nick Hu <nick.hu@...ive.com>
To: Deepak Gupta <debug@...osinc.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Christian Brauner <brauner@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Oleg Nesterov <oleg@...hat.com>, Eric Biederman <ebiederm@...ssion.com>, Kees Cook <kees@...nel.org>, 
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, alistair.francis@....com, 
	richard.henderson@...aro.org, jim.shu@...ive.com, andybnac@...il.com, 
	kito.cheng@...ive.com, charlie@...osinc.com, atishp@...osinc.com, 
	evan@...osinc.com, cleger@...osinc.com, alexghiti@...osinc.com, 
	samitolvanen@...gle.com, broonie@...nel.org, rick.p.edgecombe@...el.com
Subject: Re: [PATCH v8 24/29] riscv: enable kernel access to shadow stack
 memory via FWFT sbi call

Hi Deepak

On Thu, Nov 14, 2024 at 9:06 AM Deepak Gupta <debug@...osinc.com> wrote:
>
> On Thu, Nov 14, 2024 at 12:13:38AM +0800, Nick Hu wrote:
> >Hi Deepak
> >
> >On Tue, Nov 12, 2024 at 5:08 AM Deepak Gupta <debug@...osinc.com> wrote:
> >>
> >> Kernel will have to perform shadow stack operations on user shadow stack.
> >> Like during signal delivery and sigreturn, shadow stack token must be
> >> created and validated respectively. Thus shadow stack access for kernel
> >> must be enabled.
> >>
> >> In future when kernel shadow stacks are enabled for linux kernel, it must
> >> be enabled as early as possible for better coverage and prevent imbalance
> >> between regular stack and shadow stack. After `relocate_enable_mmu` has
> >> been done, this is as early as possible it can enabled.
> >>
> >> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> >> ---
> >>  arch/riscv/kernel/asm-offsets.c |  4 ++++
> >>  arch/riscv/kernel/head.S        | 12 ++++++++++++
> >>  2 files changed, 16 insertions(+)
> >>
> >> diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> >> index 766bd33f10cb..a22ab8a41672 100644
> >> --- a/arch/riscv/kernel/asm-offsets.c
> >> +++ b/arch/riscv/kernel/asm-offsets.c
> >> @@ -517,4 +517,8 @@ void asm_offsets(void)
> >>         DEFINE(FREGS_A6,            offsetof(struct ftrace_regs, a6));
> >>         DEFINE(FREGS_A7,            offsetof(struct ftrace_regs, a7));
> >>  #endif
> >> +       DEFINE(SBI_EXT_FWFT, SBI_EXT_FWFT);
> >> +       DEFINE(SBI_EXT_FWFT_SET, SBI_EXT_FWFT_SET);
> >> +       DEFINE(SBI_FWFT_SHADOW_STACK, SBI_FWFT_SHADOW_STACK);
> >> +       DEFINE(SBI_FWFT_SET_FLAG_LOCK, SBI_FWFT_SET_FLAG_LOCK);
> >>  }
> >> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> >> index 356d5397b2a2..6244408ca917 100644
> >> --- a/arch/riscv/kernel/head.S
> >> +++ b/arch/riscv/kernel/head.S
> >> @@ -164,6 +164,12 @@ secondary_start_sbi:
> >>         call relocate_enable_mmu
> >>  #endif
> >>         call .Lsetup_trap_vector
> >> +       li a7, SBI_EXT_FWFT
> >> +       li a6, SBI_EXT_FWFT_SET
> >> +       li a0, SBI_FWFT_SHADOW_STACK
> >> +       li a1, 1 /* enable supervisor to access shadow stack access */
> >> +       li a2, SBI_FWFT_SET_FLAG_LOCK
> >> +       ecall
> >>         scs_load_current
> >>         call smp_callin
> >>  #endif /* CONFIG_SMP */
> >> @@ -320,6 +326,12 @@ SYM_CODE_START(_start_kernel)
> >>         la tp, init_task
> >>         la sp, init_thread_union + THREAD_SIZE
> >>         addi sp, sp, -PT_SIZE_ON_STACK
> >> +       li a7, SBI_EXT_FWFT
> >> +       li a6, SBI_EXT_FWFT_SET
> >> +       li a0, SBI_FWFT_SHADOW_STACK
> >> +       li a1, 1 /* enable supervisor to access shadow stack access */
> >> +       li a2, SBI_FWFT_SET_FLAG_LOCK
> >> +       ecall
> >>         scs_load_current
> >>
> >>  #ifdef CONFIG_KASAN
> >>
> >> --
> >> 2.45.0
> >>
> >Should we clear the SBI_FWFT_SET_FLAG_LOCK before the cpu hotplug
> >otherwise the menvcfg.sse won't be set by the fwft set sbi call when
> >the hotplug cpu back to kernel?
>
> Hmm...
>
> An incoming hotplug CPU has no features setup on it.
> I see that `sbi_cpu_start` will supply `secondary_start_sbi` as start
> up code for incoming CPU. `secondary_start_sbi` is in head.S which converges
> in `.Lsecondary_start_common`. And thus hotplugged CPU should be
> issuing shadow stack set FWFT sbi as well.
>
> Am I missing something ?
>
This is the correct flow. However the opensbi will deny it due to the
SBI_FWFT_SET_FLAG_LOCK already being set.
So the menvcfg.sse will not set by this flow.

if (conf->flags & SBI_FWFT_SET_FLAG_LOCK)
                return SBI_EDENIED;

Regards,
Nick
> >
> >Regards,
> >Nick
> >>
> >> _______________________________________________
> >> linux-riscv mailing list
> >> linux-riscv@...ts.infradead.org
> >> http://lists.infradead.org/mailman/listinfo/linux-riscv

Powered by blists - more mailing lists