Re: [PATCH v9 18/29] x86/insn-eval: Incorporate segment base in linear address computation

[Ricardo Neri]

On Fri, Oct 20, 2017 at 06:08:41PM +0200, Borislav Petkov wrote:
> On Tue, Oct 03, 2017 at 08:54:21PM -0700, Ricardo Neri wrote:
> > insn_get_addr_ref() returns the effective address as defined by the
> > section 3.7.5.1 Vol 1 of the Intel 64 and IA-32 Architectures Software
> > Developer's Manual. In order to compute the linear address, we must add
> > to the effective address the segment base address as set in the segment
> > descriptor. The segment descriptor to use depends on the register used as
> > operand and segment override prefixes, if any.
> > 
> > In most cases, the segment base address will be 0 if the USER_DS/USER32_DS
> > segment is used or if segmentation is not used. However, the base address
> > is not necessarily zero if a user programs defines its own segments. This
> > is possible by using a local descriptor table.
> > 
> > Since the effective address is a signed quantity, the unsigned segment
> > base address is saved in a separate variable and added to the final,
> > unsigned, effective address.
> > 
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
> > Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> > Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/lib/insn-eval.c | 30 +++++++++++++++++++++++++++---
> >  1 file changed, 27 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > index dd84819..b3aa891 100644
> > --- a/arch/x86/lib/insn-eval.c
> > +++ b/arch/x86/lib/insn-eval.c
> > @@ -719,8 +719,8 @@ int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs)
> >   */
> >  void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> >  {
> > -	int addr_offset, base_offset, indx_offset;
> > -	unsigned long linear_addr = -1L;
> > +	int addr_offset, base_offset, indx_offset, seg_reg_indx;
> > +	unsigned long linear_addr = -1L, seg_base_addr;
> >  	long eff_addr, base, indx;
> >  	insn_byte_t sib;
> >  
> > @@ -734,6 +734,14 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> >  			goto out;
> >  
> >  		eff_addr = regs_get_register(regs, addr_offset);
> > +
> > +		seg_reg_indx = resolve_seg_reg(insn, regs, addr_offset);
> > +		if (seg_reg_indx < 0)
> > +			goto out;
> > +
> > +		seg_base_addr = insn_get_seg_base(regs, seg_reg_indx);
> > +		if (seg_base_addr == -1L)
> > +			goto out;
> 
> Instead of replicating the same calls three times, add a
> get_seg_base_addr() helper and call it where needed.

I will add this function.

Thanks and BR,
Ricardo
> --