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SubjectRe: [PATCH v8 13/28] x86/insn-eval: Add utility function to get segment descriptor
On Fri, Aug 18, 2017 at 05:27:54PM -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 look at segment descriptor to correctly calculate the
^
to

> 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 a prerequisite for functions in subsequent commits that
> will obtain the aforementioned attributes of the segment descriptor.
>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
> Cc: Colin Ian King <colin.king@canonical.com>
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: Qiaowei Ren <qiaowei.ren@intel.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Thomas Garnier <thgarnie@google.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
> Cc: x86@kernel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> arch/x86/lib/insn-eval.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 55 insertions(+)
>
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 86f58ce6c302..9cf2c49afc15 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -6,9 +6,13 @@
> #include <linux/kernel.h>
> #include <linux/string.h>
> #include <linux/ratelimit.h>
> +#include <linux/mmu_context.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 <asm/vm86.h>
>
> enum reg_type {
> @@ -402,6 +406,57 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> }
>
> /**
> + * get_desc() - Obtain address of segment descriptor

Get segment descriptor.

> + * @sel: Segment selector
> + *
> + * Given a segment selector, obtain a pointer to the segment descriptor.
> + * Both global and local descriptor tables are supported.
> + *
> + * Return: pointer to segment descriptor on success. NULL on error.
> + */
> +static struct desc_struct *get_desc(unsigned short sel)

I've simplified this function to be more readable, here's a diff ontop.

More specifically, if you flip the logic and move @desc inside the if,
you don't need to have mutex_unlock() twice in there.

And having a local @ldt ptr makes the selector check more readable.

---
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 9cf2c49afc15..48af787cb160 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -417,24 +417,24 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
static struct desc_struct *get_desc(unsigned short sel)
{
struct desc_ptr gdt_desc = {0, 0};
- struct desc_struct *desc = NULL;
unsigned long desc_base;

#ifdef CONFIG_MODIFY_LDT_SYSCALL
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;

mutex_lock(&current->active_mm->context.lock);
- /* The size of the LDT refers to the number of entries. */
- if (!current->active_mm->context.ldt ||
- sel >= current->active_mm->context.ldt->nr_entries) {
- mutex_unlock(&current->active_mm->context.lock);
- return NULL;
- }

- desc = &current->active_mm->context.ldt->entries[sel];
+ ldt = current->active_mm->context.ldt;
+ if (ldt && sel < ldt->nr_entries)
+ desc = &ldt->entries[sel];
+
mutex_unlock(&current->active_mm->context.lock);
+
return desc;
}
#endif
@@ -452,8 +452,7 @@ static struct desc_struct *get_desc(unsigned short sel)
if (desc_base > gdt_desc.size)
return NULL;

- desc = (struct desc_struct *)(gdt_desc.address + desc_base);
- return desc;
+ return (struct desc_struct *)(gdt_desc.address + desc_base);
}

/**
--
Regards/Gruss,
Boris.

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

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