Re: [PATCH v4 21/30] riscv/traps: Introduce software check exception

Deepak Gupta
2024-09-17T00:00:46Z

On Fri, Sep 13, 2024 at 09:35:50PM +0200, Andy Chiu wrote:
>Hi Deepak
>
>Deepak Gupta <debug@rivosinc.com> 於 2024年9月13日 週五 上午2:32寫道：
>>
>> zicfiss / zicfilp introduces a new exception to priv isa `software check
>> exception` with cause code = 18. This patch implements software check
>> exception.
>>
>> 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@rivosinc.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 | 38 +++++++++++++++++++++++++
>> 4 files changed, 44 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 2293e535f865..4068c7e5452a 100644
>> --- a/arch/riscv/include/asm/entry-common.h
>> +++ b/arch/riscv/include/asm/entry-common.h
>> @@ -39,4 +39,6 @@ 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 ca9203e6d76d..2ec75ba864a8 100644
>> --- a/arch/riscv/kernel/entry.S
>> +++ b/arch/riscv/kernel/entry.S
>> @@ -384,6 +384,9 @@ 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 51ebfd23e007..32d1453bed72 100644
>> --- a/arch/riscv/kernel/traps.c
>> +++ b/arch/riscv/kernel/traps.c
>> @@ -354,6 +354,44 @@ 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.
>> + */
>
>It seems like this trap handler does not follow generic entry. This
>can cause problems as signal delivery is done in
>irqentry_exit_to_user_mode(). Please reference the commit f0bddf50586d
>("riscv: entry: Convert to generic entry") for more information.

Ack. will fix it.

>
>> +asmlinkage __visible __trap_section void do_trap_software_check(struct pt_regs *regs)
>> +{
>> + if (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);
>> + } 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)
>> {
>> --
>> 2.45.0
>>
>>
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>
>Cheers,
>Andy