Re: [PATCH v1 RFC Zisslpcfi 7/9] target/riscv: Tracking indirect branches (fcfi) using TCG

[Deepak Gupta]

On Wed, Feb 15, 2023 at 6:44 PM LIU Zhiwei <zhiwei_liu@linux.alibaba.com> wrote:
>
>
> On 2023/2/16 8:02, Deepak Gupta wrote:
> > On Wed, Feb 15, 2023 at 12:55 AM LIU Zhiwei
> > <zhiwei_liu@linux.alibaba.com> wrote:
> >>
> >> On 2023/2/9 14:24, Deepak Gupta wrote:
> >>> zisslpcfi protects forward control flow (if enabled) by enforcing all
> >>> indirect call and jmp must land on a landing pad instruction `lpcll`
> >>> short for landing pad and check lower label value. If target of an
> >>> indirect call or jmp is not `lpcll` then cpu/hart must raise an illegal
> >>> instruction exception.
> >>>
> >>> This patch implements the mechanism using TCG. Target architecture branch
> >>> instruction must define the end of a TB. Using this property, during
> >>> translation of branch instruction, TB flag = FCFI_LP_EXPECTED can be set.
> >>> Translation of target TB can check if FCFI_LP_EXPECTED flag is set and a
> >>> flag (fcfi_lp_expected) can be set in DisasContext. If `lpcll` gets
> >>> translated, fcfi_lp_expected flag in DisasContext can be cleared. Else
> >>> it'll fault.
> >>>
> >>> This patch also also adds flag for forward and backward cfi in
> >>> DisasContext.
> >>>
> >>> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> >>> Signed-off-by: Kip Walker  <kip@rivosinc.com>
> >>> ---
> >>>    target/riscv/cpu.h        |  3 +++
> >>>    target/riscv/cpu_helper.c | 12 +++++++++
> >>>    target/riscv/translate.c  | 52 +++++++++++++++++++++++++++++++++++++++
> >>>    3 files changed, 67 insertions(+)
> >>>
> >>> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> >>> index 8803ea6426..98b272bcad 100644
> >>> --- a/target/riscv/cpu.h
> >>> +++ b/target/riscv/cpu.h
> >>> @@ -644,6 +644,9 @@ FIELD(TB_FLAGS, VMA, 25, 1)
> >>>    /* Native debug itrigger */
> >>>    FIELD(TB_FLAGS, ITRIGGER, 26, 1)
> >>>
> >>> +/* Zisslpcfi needs a TB flag to track indirect branches */
> >>> +FIELD(TB_FLAGS, FCFI_LP_EXPECTED, 27, 1)
> >>> +
> >>>    #ifdef TARGET_RISCV32
> >>>    #define riscv_cpu_mxl(env)  ((void)(env), MXL_RV32)
> >>>    #else
> >>> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> >>> index 63377abc2f..d15918f534 100644
> >>> --- a/target/riscv/cpu_helper.c
> >>> +++ b/target/riscv/cpu_helper.c
> >>> @@ -129,6 +129,18 @@ void cpu_get_tb_cpu_state(CPURISCVState *env, 
> >>> target_ulong *pc,
> >>>            flags = FIELD_DP32(flags, TB_FLAGS, VILL, 1);
> >>>        }
> >>>
> >>> +    if (cpu->cfg.ext_cfi) {
> >>> +        /*
> >>> +         * For Forward CFI, only the expectation of a lpcll at
> >>> +         * the start of the block is tracked (which can only happen
> >>> +         * when FCFI is enabled for the current processor mode). A jump
> >>> +         * or call at the end of the previous TB will have updated
> >>> +         * env->elp to indicate the expectation.
> >>> +         */
> >>> +        flags = FIELD_DP32(flags, TB_FLAGS, FCFI_LP_EXPECTED,
> >>> +                           env->elp != NO_LP_EXPECTED);
> >>> +    }
> >>> +
> >>>    #ifdef CONFIG_USER_ONLY
> >>>        flags |= TB_FLAGS_MSTATUS_FS;
> >>>        flags |= TB_FLAGS_MSTATUS_VS;
> >>> diff --git a/target/riscv/translate.c b/target/riscv/translate.c
> >>> index df38db7553..7d43d20fc3 100644
> >>> --- a/target/riscv/translate.c
> >>> +++ b/target/riscv/translate.c
> >>> @@ -41,6 +41,7 @@ static TCGv load_val;
> >>>    /* globals for PM CSRs */
> >>>    static TCGv pm_mask;
> >>>    static TCGv pm_base;
> >>> +static TCGOp *cfi_lp_check;
> >>>
> >>>    #include "exec/gen-icount.h"
> >>>
> >>> @@ -116,6 +117,10 @@ typedef struct DisasContext {
> >>>        bool itrigger;
> >>>        /* TCG of the current insn_start */
> >>>        TCGOp *insn_start;
> >>> +    /* CFI extension */
> >>> +    bool bcfi_enabled;
> >>> +    bool fcfi_enabled;
> >>> +    bool fcfi_lp_expected;
> >>>    } DisasContext;
> >>>
> >>>    static inline bool has_ext(DisasContext *ctx, uint32_t ext)
> >>> @@ -1166,11 +1171,44 @@ static void 
> >>> riscv_tr_init_disas_context(DisasContextBase *dcbase, CPUState *cs)
> >>>        ctx->pm_mask_enabled = FIELD_EX32(tb_flags, TB_FLAGS, 
> >>> PM_MASK_ENABLED);
> >>>        ctx->pm_base_enabled = FIELD_EX32(tb_flags, TB_FLAGS, 
> >>> PM_BASE_ENABLED);
> >>>        ctx->itrigger = FIELD_EX32(tb_flags, TB_FLAGS, ITRIGGER);
> >>> +    ctx->bcfi_enabled = cpu_get_bcfien(env);
> >>> +    ctx->fcfi_enabled = cpu_get_fcfien(env);
> >> This is wrong.  If you ctx->bcfi_enabled in the translation and don't
> >> put it in a tb flags field, the translated tb will
> >> be misused.
> > TLB for shadow stack index is flushed on privilege transfers.
> > All TLBs is flushed whenever enable/disable bits for shadow stack are 
> > toggled.
> > Can you elaborate a bit more how this can be misused?
> As Richard has pointed out, you need put this fields into tb flags if
> you want to use them in translation process as constants.
> Nothing to do with TLB.
>
> Tb flags will always be calculated (Except the direct block chain I
> think, is it right? @Richard) before using the translated tb. So if your
> translated tb depend on some machine states, you
> should put it in tb flags. Otherwise, the translated tb will be misused,
> as its execution environment varies from its translation machine states.
>

Yes it's TIL day.
Thanks a lot for the review and comments. Really appreciate it.

> Zhiwei
>
> >
> >>
> >> Zhiwei
> >>
> >>> +    ctx->fcfi_lp_expected = FIELD_EX32(tb_flags, TB_FLAGS, 
> >>> FCFI_LP_EXPECTED);
> >>>        ctx->zero = tcg_constant_tl(0);
> >>>    }
> >>>
> >>>    static void riscv_tr_tb_start(DisasContextBase *db, CPUState *cpu)
> >>>    {
> >>> +    DisasContext *ctx = container_of(db, DisasContext, base);
> >>> +
> >>> +    if (ctx->fcfi_lp_expected) {
> >>> +        /*
> >>> +         * Since we can't look ahead to confirm that the first
> >>> +         * instruction is a legal landing pad instruction, emit
> >>> +         * compare-and-branch sequence that will be fixed-up in
> >>> +         * riscv_tr_tb_stop() to either statically hit or skip an
> >>> +         * illegal instruction exception depending on whether the
> >>> +         * flag was lowered by translation of a CJLP or JLP as
> >>> +         * the first instruction in the block.
> >>> +         */
> >>> +        TCGv_i32 immediate;
> >>> +        TCGLabel *l;
> >>> +        l = gen_new_label();
> >>> +        immediate = tcg_temp_local_new_i32();
> >>> +        tcg_gen_movi_i32(immediate, 0);
> >>> +        cfi_lp_check = tcg_last_op();
> >>> +        tcg_gen_brcondi_i32(TCG_COND_EQ, immediate, 0, l);
> >>> +        tcg_temp_free_i32(immediate);
> >>> +        gen_exception_illegal(ctx);
> >>> +        gen_set_label(l);
> >>> +        /*
> >>> +         * Despite the use of gen_exception_illegal(), the rest of
> >>> +         * the TB needs to be generated. The TCG optimizer will
> >>> +         * clean things up depending on which path ends up being
> >>> +         * active.
> >>> +         */
> >>> +        ctx->base.is_jmp = DISAS_NEXT;
> >>> +    }
> >>>    }
> >>>
> >>>    static void riscv_tr_insn_start(DisasContextBase *dcbase, CPUState 
> >>> *cpu)
> >>> @@ -1225,6 +1263,7 @@ static void 
> >>> riscv_tr_translate_insn(DisasContextBase *dcbase, CPUState *cpu)
> >>>    static void riscv_tr_tb_stop(DisasContextBase *dcbase, CPUState *cpu)
> >>>    {
> >>>        DisasContext *ctx = container_of(dcbase, DisasContext, base);
> >>> +    CPURISCVState *env = cpu->env_ptr;
> >>>
> >>>        switch (ctx->base.is_jmp) {
> >>>        case DISAS_TOO_MANY:
> >>> @@ -1235,6 +1274,19 @@ static void riscv_tr_tb_stop(DisasContextBase 
> >>> *dcbase, CPUState *cpu)
> >>>        default:
> >>>            g_assert_not_reached();
> >>>        }
> >>> +
> >>> +    if (ctx->fcfi_lp_expected) {
> >>> +        /*
> >>> +         * If the "lp expected" flag is still up, the block needs to 
> >>> take an
> >>> +         * illegal instruction exception.
> >>> +         */
> >>> +        tcg_set_insn_param(cfi_lp_check, 1, 
> >>> tcgv_i32_arg(tcg_constant_i32(1)));
> >>> +    } else {
> >>> +        /*
> >>> +        * LP instruction requirement was met, clear up LP expected
> >>> +        */
> >>> +        env->elp = NO_LP_EXPECTED;
> >>> +    }
> >>>    }
> >>>
> >>>    static void riscv_tr_disas_log(const DisasContextBase *dcbase,