linux-kernel - Re: [PATCH v8 12/28] x86/insn-eval: Add utility functions to get segment selector

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Date:   Thu, 28 Sep 2017 11:36:39 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Huang Rui <ray.huang@....com>, Jiri Slaby <jslaby@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Chen Yucong <slaoub@...il.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Adam Buchbinder <adam.buchbinder@...il.com>,
        Colin Ian King <colin.king@...onical.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Qiaowei Ren <qiaowei.ren@...el.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v8 12/28] x86/insn-eval: Add utility functions to get
 segment selector

On Wed, Sep 27, 2017 at 03:32:26PM -0700, Ricardo Neri wrote:
> The idea is that get_overridden_seg_reg() would implement the logic you
> just described. It would return return INAT_SEG_REG_DEFAULT/IGNORE when
> segment override prefixes are not allowed (i.e., valid insn with
> operand rDI and string instruction; and rIP) or needed (i.e., long
> mode, except if there are override prefixes for FS or GS); or
> INAT_SEG_REG_[CSDEFG]S otherwise.

Ok, lemme see if we're talking the same thing. Your diff is linewrapped
so parsing that is hard.

Do this

        if (regoff == offsetof(struct pt_regs, ip)) {
                if (user_64bit_mode(regs))
                        return INAT_SEG_REG_IGNORE;
                else
                        return INAT_SEG_REG_DEFAULT;
        }

and all the other checking *before* you do insn_init(). Because you have
crazy stuff like:

        if (seg_reg == INAT_SEG_REG_IGNORE)
                return seg_reg;

which shortcuts those functions and is simply clumsy and complicates
following the code. The mere fact that you have to call the function
"get_overridden_seg_reg_if_any_or_needed()" already tells you that that
function is doing too many things at once.

When the function is called get_segment_register() then it should do
only that. And all the checking is done before or in wrappers.

IOW, all the rIP checking and early return down the
insn_get_seg_base() -> resolve_seg_register() -> .. should be done
separately.

*Then* you do insn_init() and hand it down to insn_get_seg_base() and
from now on you have a proper insn pointer which you hand around and
check for NULL only once, on function entry.

Then your code flow is much simpler: first you take care of the case
where rIP doesn't do segment overrides and all the other cases are
handled by the normal path, with a proper struct insn.

Makes more sense?

-- 
Regards/Gruss,
    Boris.

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