Re: [PULL v3 24/42] target/riscv: Implementation of enhanced PMP (ePMP)

[Alistair Francis]

On Thu, May 20, 2021 at 11:51 PM Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Tue, 11 May 2021 at 11:21, Alistair Francis <alistair.francis@wdc.com> 
> wrote:
> >
> > From: Hou Weiying <weiying_hou@outlook.com>
> >
> > This commit adds support for ePMP v0.9.1.
> >
> > The ePMP spec can be found in:
> > https://docs.google.com/document/d/1Mh_aiHYxemL0umN3GTTw8vsbmzHZ_nxZXgjgOUzbvc8
> >
> > Signed-off-by: Hongzheng-Li <Ethan.Lee.QNL@gmail.com>
> > Signed-off-by: Hou Weiying <weiying_hou@outlook.com>
> > Signed-off-by: Myriad-Dreamin <camiyoru@gmail.com>
> > Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> > Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
> > Message-id: 
> > fef23b885f9649a4d54e7c98b168bdec5d297bb1.1618812899.git.alistair.francis@wdc.com
> > [ Changes by AF:
> >  - Rebase on master
> >  - Update to latest spec
> >  - Use a switch case to handle ePMP MML permissions
> >  - Fix a few bugs
> > ]
> > Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
>
> Hi; this code confuses Coverity into thinking that the pmp_hart_has_privs()
> function might read the value pointed to by 'allowed_privs' when
> it is uninitialized (CID 1453108):
>
>
> > @@ -294,13 +351,94 @@ bool pmp_hart_has_privs(CPURISCVState *env, 
> > target_ulong addr,
> >              pmp_get_a_field(env->pmp_state.pmp[i].cfg_reg);
> >
> >          /*
> > -         * If the PMP entry is not off and the address is in range, do the 
> > priv
> > -         * check
> > +         * Convert the PMP permissions to match the truth table in the
> > +         * ePMP spec.
> >           */
> > +        const uint8_t epmp_operation =
> > +            ((env->pmp_state.pmp[i].cfg_reg & PMP_LOCK) >> 4) |
> > +            ((env->pmp_state.pmp[i].cfg_reg & PMP_READ) << 2) |
> > +            (env->pmp_state.pmp[i].cfg_reg & PMP_WRITE) |
> > +            ((env->pmp_state.pmp[i].cfg_reg & PMP_EXEC) >> 2);
>
> Here we construct a value which can only be in the range [0,15],
> but we do it in a way that Coverity isn't clever enough to figure out...
>
> > +
> >          if (((s + e) == 2) && (PMP_AMATCH_OFF != a_field)) {
> > -            *allowed_privs = PMP_READ | PMP_WRITE | PMP_EXEC;
> > -            if ((mode != PRV_M) || pmp_is_locked(env, i)) {
> > -                *allowed_privs &= env->pmp_state.pmp[i].cfg_reg;
> > +            /*
> > +             * If the PMP entry is not off and the address is in range,
> > +             * do the priv check
> > +             */
> > +            if (!MSECCFG_MML_ISSET(env)) {
> > +                /*
> > +                 * If mseccfg.MML Bit is not set, do pmp priv check
> > +                 * This will always apply to regular PMP.
> > +                 */
> > +                *allowed_privs = PMP_READ | PMP_WRITE | PMP_EXEC;
> > +                if ((mode != PRV_M) || pmp_is_locked(env, i)) {
> > +                    *allowed_privs &= env->pmp_state.pmp[i].cfg_reg;
> > +                }
> > +            } else {
> > +                /*
> > +                 * If mseccfg.MML Bit set, do the enhanced pmp priv check
> > +                 */
> > +                if (mode == PRV_M) {
> > +                    switch (epmp_operation) {
> > +                    case 0:
> > +                    case 1:
> > +                    case 4:
> > +                    case 5:
> > +                    case 6:
> > +                    case 7:
> > +                    case 8:
> > +                        *allowed_privs = 0;
> > +                        break;
> > +                    case 2:
> > +                    case 3:
> > +                    case 14:
> > +                        *allowed_privs = PMP_READ | PMP_WRITE;
> > +                        break;
> > +                    case 9:
> > +                    case 10:
> > +                        *allowed_privs = PMP_EXEC;
> > +                        break;
> > +                    case 11:
> > +                    case 13:
> > +                        *allowed_privs = PMP_READ | PMP_EXEC;
> > +                        break;
> > +                    case 12:
> > +                    case 15:
> > +                        *allowed_privs = PMP_READ;
> > +                        break;
>
> ...so coverity thinks that "via the 'default' case" is a valid flow
> of control in these switch() statements...
>
> > +                    }
> > +                } else {
> > +                    switch (epmp_operation) {
> > +                    case 0:
> > +                    case 8:
> > +                    case 9:
> > +                    case 12:
> > +                    case 13:
> > +                    case 14:
> > +                        *allowed_privs = 0;
> > +                        break;
> > +                    case 1:
> > +                    case 10:
> > +                    case 11:
> > +                        *allowed_privs = PMP_EXEC;
> > +                        break;
> > +                    case 2:
> > +                    case 4:
> > +                    case 15:
> > +                        *allowed_privs = PMP_READ;
> > +                        break;
> > +                    case 3:
> > +                    case 6:
> > +                        *allowed_privs = PMP_READ | PMP_WRITE;
> > +                        break;
> > +                    case 5:
> > +                        *allowed_privs = PMP_READ | PMP_EXEC;
> > +                        break;
> > +                    case 7:
> > +                        *allowed_privs = PMP_READ | PMP_WRITE | PMP_EXEC;
> > +                        break;
> > +                    }
> > +                }
> >              }
> >
> >              ret = ((privs & *allowed_privs) == privs);
>
> ...and that we can get to here without having ever set *allowed_privs.
>
>
> Adding
>    default:
>        g_assert_not_reached();
>
> to both switches should clarify to both Coverity and human readers that
> the cases in the switch are a complete enumeration of the possibilities.

Thanks Peter, I'll send a fix.

Alistair

>
> thanks
> -- PMM