2025-03-14T14:39:24-07:00, Deepak Gupta debug@rivosinc.com:
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h @@ -62,6 +62,9 @@ struct thread_info { long user_sp; /* User stack pointer */ int cpu; unsigned long syscall_work; /* SYSCALL_WORK_ flags */ +#ifdef CONFIG_RISCV_USER_CFI
- struct cfi_status user_cfi_state;
+#endif
I don't think it makes sense to put all the data in thread_info. kernel_ssp and user_ssp is more than enough and the rest can comfortably live elsewhere in task_struct.
thread_info is supposed to be as small as possible -- just spanning multiple cache-lines could be noticeable.
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S @@ -147,6 +147,20 @@ SYM_CODE_START(handle_exception) REG_L s0, TASK_TI_USER_SP(tp) csrrc s1, CSR_STATUS, t0
- /*
* If previous mode was U, capture shadow stack pointer and save it away* Zero CSR_SSP at the same time for sanitization.*/- ALTERNATIVE("nop; nop; nop; nop",
__stringify( \andi s2, s1, SR_SPP; \bnez s2, skip_ssp_save; \csrrw s2, CSR_SSP, x0; \REG_S s2, TASK_TI_USER_SSP(tp); \skip_ssp_save:),0,RISCV_ISA_EXT_ZICFISS,CONFIG_RISCV_USER_CFI)
(I'd prefer this closer to the user_sp and kernel_sp swap, it's breaking the flow here. We also already know if we've returned from userspace or not even without SR_SPP, but reusing the information might tangle the logic.)
csrr s2, CSR_EPC csrr s3, CSR_TVAL csrr s4, CSR_CAUSE @@ -236,6 +250,18 @@ SYM_CODE_START_NOALIGN(ret_from_exception) csrw CSR_SCRATCH, tp
- /*
* Going back to U mode, restore shadow stack pointer*/
Are we? I think we can be just as well returning back to kernel-space. Similar to how we can enter the exception handler from kernel-space.
- ALTERNATIVE("nop; nop",
__stringify( \REG_L s3, TASK_TI_USER_SSP(tp); \csrw CSR_SSP, s3),0,RISCV_ISA_EXT_ZICFISS,CONFIG_RISCV_USER_CFI)
Thanks.