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

[Borislav Petkov]

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@xxxxxxxxxxxxxxx>
> Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
> Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
>  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 ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
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