Re: [RFC PATCH 2/6] x86/mm: temporary mm struct

On Wed, 29 Aug 2018 18:59:52 -0700
Andy Lutomirski [off-list ref] wrote:

quoted

On Aug 29, 2018, at 6:38 PM, Masami Hiramatsu [off-list ref] wrote:

On Wed, 29 Aug 2018 08:41:00 -0700
Andy Lutomirski [off-list ref] wrote:

quoted

quoted

On Wed, Aug 29, 2018 at 2:49 AM, Masami Hiramatsu [off-list ref] wrote:
On Wed, 29 Aug 2018 01:11:43 -0700
Nadav Amit [off-list ref] wrote:

quoted

From: Andy Lutomirski <luto@kernel.org>

Sometimes we want to set a temporary page-table entries (PTEs) in one of
the cores, without allowing other cores to use - even speculatively -
these mappings. There are two benefits for doing so:

(1) Security: if sensitive PTEs are set, temporary mm prevents their use
in other cores. This hardens the security as it prevents exploding a
dangling pointer to overwrite sensitive data using the sensitive PTE.

(2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in
remote page-tables.

To do so a temporary mm_struct can be used. Mappings which are private
for this mm can be set in the userspace part of the address-space.
During the whole time in which the temporary mm is loaded, interrupts
must be disabled.

The first use-case for temporary PTEs, which will follow, is for poking
the kernel text.

[ Commit message was written by Nadav ]

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Kees Cook <redacted>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nadav Amit <redacted>
---
arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index eeeb9289c764..96afc8c0cf15 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h

@@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void)
     return cr3;

}

+typedef struct {
+     struct mm_struct *prev;
+} temporary_mm_state_t;
+
+static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm)
+{
+     temporary_mm_state_t state;
+
+     lockdep_assert_irqs_disabled();
+     state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
+     switch_mm_irqs_off(NULL, mm, current);
+     return state;
+}

Hmm, why don't we return mm_struct *prev directly?

I did it this way to make it easier to add future debugging stuff
later. Also, when I first wrote this, I stashed the old CR3 instead
of the old mm_struct, and it seemed like callers should be insulated
from details like this.

Hmm, I see. But in that case, we should call it "struct temporary_mm"
and explicitly allocate (and pass) it, since we can not return the
data structure from stack.

Why not?

Ah, ok as far as it returns a data structure as immediate value.
(I don't recommend it because it hides a copy..)

quoted

If we can combine it with new mm, it will
be more encapsulated e.g.

struct temporary_mm {
   struct mm_struct *mm;
   struct mm_struct *prev;
};

static struct temporary_mm poking_tmp_mm;

poking_init()
{
   if (init_temporary_mm(&tmp_mm, &init_mm))
       goto error;
   ...
}

text_poke_safe()
{
   ...
   use_temporary_mm(&tmp_mm);
   ...
   unuse_temporary_mm(&tmp_mm);
}

Any thought?

That seems more complicated for not very much gain.

Hmm, OK. anyway that is just a style note. The code itself looks good for me.

Thank you,

-- 
Masami Hiramatsu [off-list ref]