Messages in this thread Patch in this message | | | Subject | Re: [PATCH v9 13/29] x86/insn-eval: Add utility functions to get segment selector | From | Ricardo Neri <> | Date | Thu, 12 Oct 2017 18:08:17 -0700 |
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On Thu, 2017-10-12 at 11:48 +0200, Borislav Petkov wrote: > On Wed, Oct 11, 2017 at 06:12:30PM -0700, Ricardo Neri wrote: > > > > Shouldn't this function check for a null insn since it is used here? > I have to say, this whole codepath from insn_get_seg_base() with > insn==NULL is nasty but I don't see a way around it as we need to know > how many bytes to copy and from where. Can't think of a better solution > without duplicating a lot of code. :-\
I have looked at your two proposals. I think I prefer the first one plus a couple of tweaks.
> > So how about this? > > If the patch is hard to read, you can apply it and look at the code. But > here's the gist: > > * You pull up the rIP check and do that directly in resolve_seg_reg() > and return INAT_SEG_REG_CS there immediately so you don't have to call > resolve_default_seg().
In my opinion it would be better to have all the checks in a single place. This makes the code easier to read that having this special case directly in resolve_default_seg(). Also, strictly speaking we would need to return INAT_SEG_REG_IGNORE in long mode. Indeed, insn_get_seg_base() would return base 0 in such a case, but I feel it is better if this logic is explicit in resolve_default_seg(). > > This way, you get the only case out of the way where insn can be NULL. > > Then you can do the if (!insn) check once and now you have a valid insn.
Rather than checking for null insn in resolve_seg_reg(), which does not use it, let the functions it calls do the check if they need to. > > check_seg_overrides() can then return simply bool and you can get rid of > the remaining if (!insn) checks down the road. > > But please double-check me if I missed a case - the flow is not trivial.
This is a diff based on your first proposal (I hope text does not wrap). I feel this makes it clear how resolve_seg_reg() handles errors as well it uses overridden or default segment register indices. Plus, insn is only checked when used.
@@ -155,6 +155,16 @@ static int resolve_default_seg(struct insn *insn, struct pt_regs *regs, int off) { if (user_64bit_mode(regs)) return INAT_SEG_REG_IGNORE; + + /* + * insn may be null as we may be about to copy the instruction. + * However is not needed at all. + */ + if (off == offsetof(struct pt_regs, ip)) + INAT_SEG_REG_CS; + + if(!insn) + return -EINVAL; /* * If we are here, we use the default segment register as described * in the Intel documentation: @@ -191,9 +201,6 @@ static int resolve_default_seg(struct insn *insn, struct pt_regs *regs, int off) case offsetof(struct pt_regs, sp): return INAT_SEG_REG_SS; - case offsetof(struct pt_regs, ip): - return INAT_SEG_REG_CS; - default: return -EINVAL; } @@ -254,9 +261,6 @@ static int resolve_seg_reg(struct insn *insn, struct pt_regs *regs, int regoff) if (!ret) return resolve_default_seg(insn, regs, regoff); - if (!insn) - return -EINVAL; - idx = get_seg_reg_override_idx(insn); if (idx < 0) return idx;
Thanks and BR, Ricardo
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