Re: [v6 PATCH 12/21] x86/insn: Support both signed 32-bit and 64-bit effective addresses

Ricardo Neri
2017-04-27T03:33:46Z

On Tue, 2017-04-25 at 15:51 +0200, Borislav Petkov wrote:
> On Tue, Mar 07, 2017 at 04:32:45PM -0800, Ricardo Neri wrote:
> > The 32-bit and 64-bit address encodings are identical. This means that we
> > can use the same function in both cases. In order to reuse the function for
> > 32-bit address encodings, we must sign-extend our 32-bit signed operands to
> > 64-bit signed variables (only for 64-bit builds). To decide on whether sign
> > extension is needed, we rely on the address size as given by the
> > instruction structure.
> >
> > Lastly, before computing the linear address, we must truncate our signed
> > 64-bit signed effective address if the address size is 32-bit.
> >
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
> > Cc: Colin Ian King <colin.king@canonical.com>
> > Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> > Cc: Qiaowei Ren <qiaowei.ren@intel.com>
> > Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> > Cc: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: Adrian Hunter <adrian.hunter@intel.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Thomas Garnier <thgarnie@google.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Borislav Petkov <bp@suse.de>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> > Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
> > Cc: x86@kernel.org
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > ---
> > arch/x86/lib/insn-eval.c | 44 ++++++++++++++++++++++++++++++++------------
> > 1 file changed, 32 insertions(+), 12 deletions(-)
> >
> > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > index edb360f..a9a1704 100644
> > --- a/arch/x86/lib/insn-eval.c
> > +++ b/arch/x86/lib/insn-eval.c
> > @@ -559,6 +559,15 @@ int insn_get_reg_offset_sib_index(struct insn *insn, struct pt_regs *regs)
> > return get_reg_offset(insn, regs, REG_TYPE_INDEX);
> > }
> >
> > +static inline long __to_signed_long(unsigned long val, int long_bytes)
> > +{
> > +#ifdef CONFIG_X86_64
> > + return long_bytes == 4 ? (long)((int)((val) & 0xffffffff)) : (long)val;
>
> I don't think this always works as expected:
>
> ---
> typedef unsigned int u32;
> typedef unsigned long u64;
>
> int main()
> {
> u64 v = 0x1ffffffff;
>
> printf("v: %ld, 0x%lx, %ld\n", v, v, (long)((int)((v) & 0xffffffff)));
>
> return 0;
> }
> --
> ...
>
> v: 8589934591, 0x1ffffffff, -1
>
> Now, this should not happen on 32-bit because unsigned long is 32-bit
> there but can that happen on 64-bit?

This is the reason I check the value of long_bytes. If long_bytes is not
4, being the only other possible value 8 (perhaps I need to issue an
error when the value is not any of these values), the cast is simply
(long)val. I modified your test program with:

printf("v: %ld, 0x%lx, %ld, %ld\n", v, v, (long)((int)((v) &
0xffffffff)), (long)v);

and I get:

v: 8589934591, 0x1ffffffff, -1, 8589934591.

Am I missing something?

>
> > +#else
> > + return (long)val;
> > +#endif
> > +}
> > +
> > /*
> > * return the address being referenced be instruction
> > * for rm=3 returning the content of the rm reg
> > @@ -567,19 +576,21 @@ int insn_get_reg_offset_sib_index(struct insn *insn, struct pt_regs *regs)
> > void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> > {
> > unsigned long linear_addr, seg_base_addr;
> > - long eff_addr, base, indx;
> > - int addr_offset, base_offset, indx_offset;
> > + long eff_addr, base, indx, tmp;
> > + int addr_offset, base_offset, indx_offset, addr_bytes;
> > insn_byte_t sib;
> >
> > insn_get_modrm(insn);
> > insn_get_sib(insn);
> > sib = insn->sib.value;
> > + addr_bytes = insn->addr_bytes;
> >
> > if (X86_MODRM_MOD(insn->modrm.value) == 3) {
> > addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
> > if (addr_offset < 0)
> > goto out_err;
> > - eff_addr = regs_get_register(regs, addr_offset);
> > + tmp = regs_get_register(regs, addr_offset);
> > + eff_addr = __to_signed_long(tmp, addr_bytes);
>
> This repeats throughout the function so it begs to be a separate:
>
> get_mem_addr()
>
> or so.

Yes, the same pattern is used in all places except when using register
operands (ModRM.rm == 11b). I will look into putting it in a function.
>
> > seg_base_addr = insn_get_seg_base(regs, insn, addr_offset,
> > false);
> > } else {
> > @@ -591,20 +602,24 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> > * in the address computation.
> > */
> > base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE);
> > - if (unlikely(base_offset == -EDOM))
> > + if (unlikely(base_offset == -EDOM)) {
> > base = 0;
> > - else if (unlikely(base_offset < 0))
> > + } else if (unlikely(base_offset < 0)) {
> > goto out_err;
> > - else
> > - base = regs_get_register(regs, base_offset);
> > + } else {
> > + tmp = regs_get_register(regs, base_offset);
> > + base = __to_signed_long(tmp, addr_bytes);
> > + }
> >
> > indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX);
> > - if (unlikely(indx_offset == -EDOM))
> > + if (unlikely(indx_offset == -EDOM)) {
> > indx = 0;
> > - else if (unlikely(indx_offset < 0))
> > + } else if (unlikely(indx_offset < 0)) {
> > goto out_err;
> > - else
> > - indx = regs_get_register(regs, indx_offset);
> > + } else {
> > + tmp = regs_get_register(regs, indx_offset);
> > + indx = __to_signed_long(tmp, addr_bytes);
> > + }
> >
> > eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
> > seg_base_addr = insn_get_seg_base(regs, insn,
> > @@ -625,13 +640,18 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> > } else if (addr_offset < 0) {
> > goto out_err;
> > } else {
> > - eff_addr = regs_get_register(regs, addr_offset);
> > + tmp = regs_get_register(regs, addr_offset);
> > + eff_addr = __to_signed_long(tmp, addr_bytes);
> > }
> > seg_base_addr = insn_get_seg_base(regs, insn,
> > addr_offset, false);
> > }
> > eff_addr += insn->displacement.value;
> > }
> > + /* truncate to 4 bytes for 32-bit effective addresses */
> > + if (addr_bytes == 4)
> > + eff_addr &= 0xffffffff;
>
> Why again?

eff_addr is a long variable, which in x86_64 has 64-bit. However, in
32-bit segments the effective address is 32-bit. Thus, I discard the 32
most significant bytes.

Thanks and BR,
Ricardo