Re: [PATCH 2/2] target/riscv: fixes a bug against `ssamoswap` behavior in M-mode

[Deepak Gupta <debug@rivosinc.com>]

On Thu, Mar 06, 2025 at 03:29:00PM +1000, Alistair Francis wrote:
>On Tue, Feb 18, 2025 at 12:57 PM Deepak Gupta <debug@rivosinc.com> wrote:
>>
>> Commit f06bfe3dc38c ("target/riscv: implement zicfiss instructions") adds
>> `ssamoswap` instruction. `ssamoswap` takes the code-point from existing
>> reserved encoding (and not a zimop like other shadow stack instructions).
>> If shadow stack is not enabled (via xenvcfg.SSE), then `ssamoswap` must
>> result in an illegal instruction exception. However there is a slightly
>> modified behavior for M-mode.
>>
>> Shadow stack are not available in M-mode and all shadow stack instructions
>> in M-mode exhibit zimop behavior. However, `ssamoswap` can still succeed
>> if MPRV=1 and MPP is non-zero (see section 2.7 of zicfiss specification).
>> This patch corrects that behavior for `ssamoswap`.
>
>Section "22.2.3. Shadow Stack Memory Protection " of the latest priv
>spec [1] seems to say: "When the effective privilege mode is M, any
>memory access by an SSAMOSWAP.W/D
>instruction will result in a store/AMO access-fault exception."

Hmm I didn't look at priv spec. Let me fix this one.

>
>1: https://github.com/riscv/riscv-isa-manual/releases/tag/riscv-isa-release-9cfaf37-2025-03-06
>
>Alistair
>
>>
>> Fixes: f06bfe3dc38c ("target/riscv: implement zicfiss instructions")
>>
>> Reported-by: Ved Shanbhogue <ved@rivosinc.com>
>> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>> ---
>>  target/riscv/insn_trans/trans_rvzicfiss.c.inc | 13 +++++++++++--
>>  1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/target/riscv/insn_trans/trans_rvzicfiss.c.inc b/target/riscv/insn_trans/trans_rvzicfiss.c.inc
>> index e3ebc4977c..ec016cd70f 100644
>> --- a/target/riscv/insn_trans/trans_rvzicfiss.c.inc
>> +++ b/target/riscv/insn_trans/trans_rvzicfiss.c.inc
>> @@ -15,6 +15,13 @@
>>   * You should have received a copy of the GNU General Public License along with
>>   * this program.  If not, see <http://www.gnu.org/licenses/>.
>>   */
>> +
>> + #define REQUIRE_ZICFISS(ctx) do {       \
>> +    if (!ctx->cfg_ptr->ext_zicfiss) {    \
>> +        return false;                    \
>> +    }                                    \
>> +} while (0)
>> +
>>  static bool trans_sspopchk(DisasContext *ctx, arg_sspopchk *a)
>>  {
>>      if (!ctx->bcfi_enabled) {
>> @@ -77,7 +84,8 @@ static bool trans_ssrdp(DisasContext *ctx, arg_ssrdp *a)
>>  static bool trans_ssamoswap_w(DisasContext *ctx, arg_amoswap_w *a)
>>  {
>>      REQUIRE_A_OR_ZAAMO(ctx);
>> -    if (!ctx->bcfi_enabled) {
>> +    REQUIRE_ZICFISS(ctx);
>> +    if ((ctx->priv != PRV_M) && !ctx->bcfi_enabled) {
>>          return false;
>>      }
>>
>> @@ -97,7 +105,8 @@ static bool trans_ssamoswap_d(DisasContext *ctx, arg_amoswap_w *a)
>>  {
>>      REQUIRE_64BIT(ctx);
>>      REQUIRE_A_OR_ZAAMO(ctx);
>> -    if (!ctx->bcfi_enabled) {
>> +    REQUIRE_ZICFISS(ctx);
>> +    if ((ctx->priv != PRV_M) && !ctx->bcfi_enabled) {
>>          return false;
>>      }
>>
>> --
>> 2.34.1
>>
>>