linux-kernel - Re: [PATCH v9 13/29] x86/insn-eval: Add utility functions to get segment selector

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Date:   Thu, 12 Oct 2017 11:48:33 +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 v9 13/29] x86/insn-eval: Add utility functions to get
 segment selector

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. :-\

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().

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.

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.

Thx.

---
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index e7e82b343bd0..3c65fa9178bc 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -71,9 +71,6 @@ static int get_seg_reg_override_idx(struct insn *insn)
 	int idx = INAT_SEG_REG_DEFAULT;
 	int num_overrides = 0, i;
 
-	if (!insn)
-		return -EINVAL;
-
 	insn_get_prefixes(insn);
 
 	/* Look for any segment override prefixes. */
@@ -128,28 +125,16 @@ static int get_seg_reg_override_idx(struct insn *insn)
  *
  * Returns:
  *
- * 1 if segment override prefixes can be used with the register indicated
- * in regoff. 0 if otherwise.
+ * True if segment override prefixes can be used with the register indicated
+ * in regoff. False if otherwise.
  *
- * -EINVAL in case of error.
  */
-static int check_seg_overrides(struct insn *insn, int regoff)
+static bool check_seg_overrides(struct insn *insn, int regoff)
 {
-	/*
-	 * Segment override prefixes should not be used for rIP. It is not
-	 * necessary to inspect the instruction structure.
-	 */
-	if (regoff == offsetof(struct pt_regs, ip))
-		return 0;
-
-	/* Subsequent checks require a valid insn. */
-	if (!insn)
-		return -EINVAL;
-
 	if (regoff == offsetof(struct pt_regs, di) && is_string_insn(insn))
-		return 0;
+		return false;
 
-	return 1;
+	return true;
 }
 
 static int resolve_default_seg(struct insn *insn, struct pt_regs *regs, int off)
@@ -247,18 +232,21 @@ static int resolve_default_seg(struct insn *insn, struct pt_regs *regs, int off)
  */
 static int resolve_seg_reg(struct insn *insn, struct pt_regs *regs, int regoff)
 {
-	int ret, idx;
+	int idx;
 
-	ret = check_seg_overrides(insn, regoff);
-	if (ret < 0)
-		return ret;
-
-	if (!ret)
-		return resolve_default_seg(insn, regs, regoff);
+	/*
+	 * Segment override prefixes should not be used for rIP. It is not
+	 * necessary to inspect the instruction.
+	 */
+	if (regoff == offsetof(struct pt_regs, ip))
+		return INAT_SEG_REG_CS;
 
 	if (!insn)
 		return -EINVAL;
 
+	if (!check_seg_overrides(insn, regoff))
+		return resolve_default_seg(insn, regs, regoff);
+
 	idx = get_seg_reg_override_idx(insn);
 	if (idx < 0)
 		return idx;

> Could this check be removed? insn is not used for anything but passed to other
> functions that do perform this check.

Yap, eventually. :)

-- 
Regards/Gruss,
    Boris.

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