Re: [PATCH v2] target/i386: Use assert() to sanity-check b1 in SSE decode

[Peter Maydell]

Ping^2 !

thanks
-- PMM

On Mon, 13 Sept 2021 at 13:34, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> Ping? (this has been reviewed)
>
> thanks
> -- PMM
>
> On Wed, 1 Sept 2021 at 15:10, Peter Maydell <peter.maydell@linaro.org> wrote:
> >
> > In the SSE decode function gen_sse(), we combine a byte
> > 'b' and a value 'b1' which can be [0..3], and switch on them:
> >    b |= (b1 << 8);
> >    switch (b) {
> >    ...
> >    default:
> >    unknown_op:
> >        gen_unknown_opcode(env, s);
> >        return;
> >    }
> >
> > In three cases inside this switch, we were then also checking for
> >  "if (b1 >= 2) { goto unknown_op; }".
> > However, this can never happen, because the 'case' values in each place
> > are 0x0nn or 0x1nn and the switch will have directed the b1 == (2, 3)
> > cases to the default already.
> >
> > This check was added in commit c045af25a52e9 in 2010; the added code
> > was unnecessary then as well, and was apparently intended only to
> > ensure that we never accidentally ended up indexing off the end
> > of an sse_op_table with only 2 entries as a result of future bugs
> > in the decode logic.
> >
> > Change the checks to assert() instead, and make sure they're always
> > immediately before the array access they are protecting.
> >
> > Fixes: Coverity CID 1460207
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> > v1->v2: use assert() rather than just deleting the if()s
> >
> >  target/i386/tcg/translate.c | 12 +++---------
> >  1 file changed, 3 insertions(+), 9 deletions(-)
> >
> > diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
> > index aacb605eee4..a4fee5e445d 100644
> > --- a/target/i386/tcg/translate.c
> > +++ b/target/i386/tcg/translate.c
> > @@ -3521,9 +3521,6 @@ static void gen_sse(CPUX86State *env, DisasContext 
> > *s, int b,
> >          case 0x171: /* shift xmm, im */
> >          case 0x172:
> >          case 0x173:
> > -            if (b1 >= 2) {
> > -                goto unknown_op;
> > -            }
> >              val = x86_ldub_code(env, s);
> >              if (is_xmm) {
> >                  tcg_gen_movi_tl(s->T0, val);
> > @@ -3542,6 +3539,7 @@ static void gen_sse(CPUX86State *env, DisasContext 
> > *s, int b,
> >                                  offsetof(CPUX86State, mmx_t0.MMX_L(1)));
> >                  op1_offset = offsetof(CPUX86State,mmx_t0);
> >              }
> > +            assert(b1 < 2);
> >              sse_fn_epp = sse_op_table2[((b - 1) & 3) * 8 +
> >                                         (((modrm >> 3)) & 7)][b1];
> >              if (!sse_fn_epp) {
> > @@ -3772,10 +3770,8 @@ static void gen_sse(CPUX86State *env, DisasContext 
> > *s, int b,
> >              rm = modrm & 7;
> >              reg = ((modrm >> 3) & 7) | REX_R(s);
> >              mod = (modrm >> 6) & 3;
> > -            if (b1 >= 2) {
> > -                goto unknown_op;
> > -            }
> >
> > +            assert(b1 < 2);
> >              sse_fn_epp = sse_op_table6[b].op[b1];
> >              if (!sse_fn_epp) {
> >                  goto unknown_op;
> > @@ -4202,10 +4198,8 @@ static void gen_sse(CPUX86State *env, DisasContext 
> > *s, int b,
> >              rm = modrm & 7;
> >              reg = ((modrm >> 3) & 7) | REX_R(s);
> >              mod = (modrm >> 6) & 3;
> > -            if (b1 >= 2) {
> > -                goto unknown_op;
> > -            }
> >
> > +            assert(b1 < 2);
> >              sse_fn_eppi = sse_op_table7[b].op[b1];
> >              if (!sse_fn_eppi) {
> >                  goto unknown_op;
> > --
> > 2.20.1