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

[Borislav Petkov]

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 
> Cc: Adam Buchbinder 
> Cc: Colin Ian King 
> Cc: Lorenzo Stoakes 
> Cc: Qiaowei Ren 
> Cc: Arnaldo Carvalho de Melo 
> Cc: Masami Hiramatsu 
> Cc: Adrian Hunter 
> Cc: Kees Cook 
> Cc: Thomas Garnier 
> Cc: Peter Zijlstra 
> Cc: Borislav Petkov 
> Cc: Dmitry Vyukov 
> Cc: Ravi V. Shankar 
> Cc: [e-mail]
> Signed-off-by: Ricardo Neri
>
> 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?