linux-kernel - Re: [PATCH v9 14/29] x86/insn-eval: Add utility function to get segment descriptor

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Message-ID: <20171011145729.j7rm3jduvczdhbek@pd.tnic>
Date:   Wed, 11 Oct 2017 16:57:29 +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, ricardo.neri@...el.com,
        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 14/29] x86/insn-eval: Add utility function to get
 segment descriptor

On Tue, Oct 03, 2017 at 08:54:17PM -0700, Ricardo Neri wrote:
> The segment descriptor contains information that is relevant to how linear
> addresses need to be computed. It contains the default size of addresses
> as well as the base address of the segment. Thus, given a segment
> selector, we ought to look at segment descriptor to correctly calculate
> the linear address.
> 
> In protected mode, the segment selector might indicate a segment
> descriptor from either the global descriptor table or a local descriptor
> table. Both cases are considered in this function.

...

> +static struct desc_struct *get_desc(unsigned short sel)
> +{
> +	struct desc_ptr gdt_desc = {0, 0};
> +	unsigned long desc_base;
> +
> +#ifdef CONFIG_MODIFY_LDT_SYSCALL
> +	struct desc_struct *desc = NULL;
> +	struct ldt_struct *ldt;

You moved those out of the if-statement even though they're needed only
in that scope. Why?

Here's a diff that moves them back there and improves the function comment.

---
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 62975f825556..c4e82bb4c4d3 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -456,11 +456,11 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
 }
 
 /**
- * get_desc() - Obtain address of segment descriptor
+ * get_desc() - Obtain pointer to a segment descriptor
  * @sel:	Segment selector
  *
- * Given a segment selector, obtain a pointer to the segment descriptor.
- * Both global and local descriptor tables are supported.
+ * Given a segment selector, obtain a pointer to the corresponding segment
+ * descriptor. Both global and local descriptor tables are supported.
  *
  * Returns:
  *
@@ -474,10 +474,10 @@ static struct desc_struct *get_desc(unsigned short sel)
 	unsigned long desc_base;
 
 #ifdef CONFIG_MODIFY_LDT_SYSCALL
-	struct desc_struct *desc = NULL;
-	struct ldt_struct *ldt;
-
 	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT) {
+		struct desc_struct *desc = NULL;
+		struct ldt_struct *ldt;
+
 		/* Bits [15:3] contain the index of the desired entry. */
 		sel >>= 3;
 
-- 
Regards/Gruss,
    Boris.

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