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

Ricardo Neri
2017-04-26T21:51:56Z

On Wed, 2017-04-19 at 12:26 +0200, Borislav Petkov wrote:
> 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@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 | 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. :)

It makes sense. I will rename it.
>
> 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 //

Will update it.
>
> > + * 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 :)

After revisiting the code, I don't see why the function cannot return
NULL.
>
> > + */
> > +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.

Yes, it is useless. Please see my comment below.
>
> > +
> > +#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.

I will remove 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.

I double-checked the ldt code and it seems to me that size refers to the
number of entries in the table; it is always multiplied by
LDT_ENTRY_SIZE [1], [2]. Am I missing something?

>
> 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>.

I will elaborate on the reason for this.

Thanks and BR,
Ricardo

[1].
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/ldt.c?id=refs/tags/v4.11-rc8#n260
[2].