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

Borislav Petkov
2017-04-25T13:51:50Z

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?

> +#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.

> 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?

--
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--