linux-kernel - Re: [v6 PATCH 07/21] x86/insn-eval: Add utility function to get segment descriptor

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Message-ID: <20170419102636.chx5fwofysehy3yx@pd.tnic>
Date:   Wed, 19 Apr 2017 12:26:36 +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>,
        Alexandre Julliard <julliard@...ehq.org>,
        Stas Sergeev <stsp@...t.ru>, Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-msdos@...r.kernel.org, wine-devel@...ehq.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: [v6 PATCH 07/21] x86/insn-eval: Add utility function to get
 segment descriptor

On Tue, Mar 07, 2017 at 04:32:40PM -0800, Ricardo Neri wrote:
> The segment descriptor contains information that is relevant to how linear
> address 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 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.
> 
> This function is the initial implementation for subsequent functions that
> will obtain the aforementioned attributes of the segment descriptor.
> 
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Adam Buchbinder <adam.buchbinder@...il.com>
> Cc: Colin Ian King <colin.king@...onical.com>
> Cc: Lorenzo Stoakes <lstoakes@...il.com>
> Cc: Qiaowei Ren <qiaowei.ren@...el.com>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Adrian Hunter <adrian.hunter@...el.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Thomas Garnier <thgarnie@...gle.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Ravi V. Shankar <ravi.v.shankar@...el.com>
> Cc: x86@...nel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> ---
>  arch/x86/lib/insn-eval.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 61 insertions(+)
> 
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 8d45df8..8608adf 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -5,9 +5,13 @@
>   */
>  #include <linux/kernel.h>
>  #include <linux/string.h>
> +#include <asm/desc_defs.h>
> +#include <asm/desc.h>
>  #include <asm/inat.h>
>  #include <asm/insn.h>
>  #include <asm/insn-eval.h>
> +#include <asm/ldt.h>
> +#include <linux/mmu_context.h>
>  #include <asm/vm86.h>
>  
>  enum reg_type {
> @@ -294,6 +298,63 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
>  }
>  
>  /**
> + * get_desc() - Obtain address of segment descriptor
> + * @seg:	Segment selector

Maybe that should be

@sel

if it is a sel-ector. :)

And using "sel" makes more sense then when you look at:

	desc_base = sel & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK);

for example:

> + * @desc:	Pointer to the selected segment descriptor
> + *
> + * Given a segment selector, obtain a memory pointer to the segment

s/memory //

> + * descriptor. Both global and local descriptor tables are supported.
> + * desc will contain the address of the descriptor.
> + *
> + * Return: 0 if success, -EINVAL if failure

Why isn't this function returning the pointer or NULL on error? Maybe
the later patches have an answer and I'll discover it if I continue
reviewing :)

> + */
> +static int get_desc(unsigned short seg, struct desc_struct **desc)
> +{
> +	struct desc_ptr gdt_desc = {0, 0};
> +	unsigned long desc_base;
> +
> +	if (!desc)
> +		return -EINVAL;
> +
> +	desc_base = seg & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK);

That looks useless as you're doing it below again.

> +
> +#ifdef CONFIG_MODIFY_LDT_SYSCALL
> +	if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
> +		seg >>= 3;
> +
> +		mutex_lock(&current->active_mm->context.lock);
> +		if (unlikely(!current->active_mm->context.ldt ||

Is that really a fast path to complicate the if-test with an unlikely()?
If not, you don't really need it.

> +			     seg >= current->active_mm->context.ldt->size)) {

ldt->size is the size of the descriptor table but you've shifted seg by
3. That selector index is shifted by 3 (to the left) to form an offset
into the descriptor table because the entries there are 8 bytes.

So I *think* you wanna use the "useless" desc_base above... :)

> +			*desc = NULL;
> +			mutex_unlock(&current->active_mm->context.lock);
> +			return -EINVAL;
> +		}
> +
> +		*desc = &current->active_mm->context.ldt->entries[seg];

... and seg here as it is an index into the table.

> +		mutex_unlock(&current->active_mm->context.lock);
> +		return 0;
> +	}
> +#endif
> +	native_store_gdt(&gdt_desc);
> +
> +	/*
> +	 * Bits [15:3] of the segment selector contain the index. Such
> +	 * index needs to be multiplied by 8.

... because <insert reason I typed in above>.

-- 
Regards/Gruss,
    Boris.

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