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Message-ID: <fec3b7be-4259-4eef-87f9-b2cee5718cae@rivosinc.com>
Date: Thu, 6 Feb 2025 14:49:09 +0100
From: Clément Léger <cleger@...osinc.com>
To: Deepak Gupta <debug@...osinc.com>, 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>,
Jann Horn <jannh@...gle.com>, Conor Dooley <conor+dt@...nel.org>
Cc: 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, alexghiti@...osinc.com,
samitolvanen@...gle.com, broonie@...nel.org, rick.p.edgecombe@...el.com
Subject: Re: [PATCH v9 14/26] riscv/traps: Introduce software check exception
On 05/02/2025 02:22, Deepak Gupta wrote:
> zicfiss / zicfilp introduces a new exception to priv isa `software check
> exception` with cause code = 18. This patch implements software check
> exception.
Hey Deepak,
While not directly related to this patch, is the exception 18 delegation
documented in the SBI doc ? I mean, should we specify that it is always
delegated when implementing FWFT LANDING_PAD/SHADOW_STACK ?
Thanks,
Clément
>
> Additionally it implements a cfi violation handler which checks for code
> in xtval. If xtval=2, it means that sw check exception happened because of
> an indirect branch not landing on 4 byte aligned PC or not landing on
> `lpad` instruction or label value embedded in `lpad` not matching label
> value setup in `x7`. If xtval=3, it means that sw check exception happened
> because of mismatch between link register (x1 or x5) and top of shadow
> stack (on execution of `sspopchk`).
>
> In case of cfi violation, SIGSEGV is raised with code=SEGV_CPERR.
> SEGV_CPERR was introduced by x86 shadow stack patches.
>
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
> arch/riscv/include/asm/asm-prototypes.h | 1 +
> arch/riscv/include/asm/entry-common.h | 2 ++
> arch/riscv/kernel/entry.S | 3 +++
> arch/riscv/kernel/traps.c | 43 +++++++++++++++++++++++++++++++++
> 4 files changed, 49 insertions(+)
>
> diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h
> index cd627ec289f1..5a27cefd7805 100644
> --- a/arch/riscv/include/asm/asm-prototypes.h
> +++ b/arch/riscv/include/asm/asm-prototypes.h
> @@ -51,6 +51,7 @@ DECLARE_DO_ERROR_INFO(do_trap_ecall_u);
> DECLARE_DO_ERROR_INFO(do_trap_ecall_s);
> DECLARE_DO_ERROR_INFO(do_trap_ecall_m);
> DECLARE_DO_ERROR_INFO(do_trap_break);
> +DECLARE_DO_ERROR_INFO(do_trap_software_check);
>
> asmlinkage void handle_bad_stack(struct pt_regs *regs);
> asmlinkage void do_page_fault(struct pt_regs *regs);
> diff --git a/arch/riscv/include/asm/entry-common.h b/arch/riscv/include/asm/entry-common.h
> index b28ccc6cdeea..34ed149af5d1 100644
> --- a/arch/riscv/include/asm/entry-common.h
> +++ b/arch/riscv/include/asm/entry-common.h
> @@ -40,4 +40,6 @@ static inline int handle_misaligned_store(struct pt_regs *regs)
> }
> #endif
>
> +bool handle_user_cfi_violation(struct pt_regs *regs);
> +
> #endif /* _ASM_RISCV_ENTRY_COMMON_H */
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 00494b54ff4a..9c00cac3f6f2 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -472,6 +472,9 @@ SYM_DATA_START_LOCAL(excp_vect_table)
> RISCV_PTR do_page_fault /* load page fault */
> RISCV_PTR do_trap_unknown
> RISCV_PTR do_page_fault /* store page fault */
> + RISCV_PTR do_trap_unknown /* cause=16 */
> + RISCV_PTR do_trap_unknown /* cause=17 */
> + RISCV_PTR do_trap_software_check /* cause=18 is sw check exception */
> SYM_DATA_END_LABEL(excp_vect_table, SYM_L_LOCAL, excp_vect_table_end)
>
> #ifndef CONFIG_MMU
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 8ff8e8b36524..3f7709f4595a 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -354,6 +354,49 @@ void do_trap_ecall_u(struct pt_regs *regs)
>
> }
>
> +#define CFI_TVAL_FCFI_CODE 2
> +#define CFI_TVAL_BCFI_CODE 3
> +/* handle cfi violations */
> +bool handle_user_cfi_violation(struct pt_regs *regs)
> +{
> + bool ret = false;
> + unsigned long tval = csr_read(CSR_TVAL);
> +
> + if ((tval == CFI_TVAL_FCFI_CODE && cpu_supports_indirect_br_lp_instr()) ||
> + (tval == CFI_TVAL_BCFI_CODE && cpu_supports_shadow_stack())) {
> + do_trap_error(regs, SIGSEGV, SEGV_CPERR, regs->epc,
> + "Oops - control flow violation");
> + ret = true;
> + }
> +
> + return ret;
> +}
> +
> +/*
> + * software check exception is defined with risc-v cfi spec. Software check
> + * exception is raised when:-
> + * a) An indirect branch doesn't land on 4 byte aligned PC or `lpad`
> + * instruction or `label` value programmed in `lpad` instr doesn't
> + * match with value setup in `x7`. reported code in `xtval` is 2.
> + * b) `sspopchk` instruction finds a mismatch between top of shadow stack (ssp)
> + * and x1/x5. reported code in `xtval` is 3.
> + */
> +asmlinkage __visible __trap_section void do_trap_software_check(struct pt_regs *regs)
> +{
> + if (user_mode(regs)) {
> + irqentry_enter_from_user_mode(regs);
> +
> + /* not a cfi violation, then merge into flow of unknown trap handler */
> + if (!handle_user_cfi_violation(regs))
> + do_trap_unknown(regs);
> +
> + irqentry_exit_to_user_mode(regs);
> + } else {
> + /* sw check exception coming from kernel is a bug in kernel */
> + die(regs, "Kernel BUG");
> + }
> +}
> +
> #ifdef CONFIG_MMU
> asmlinkage __visible noinstr void do_page_fault(struct pt_regs *regs)
> {
>
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