Re: [RFC] security: replace indirect calls with static calls

[Kees Cook]

On Thu, Aug 20, 2020 at 06:47:53PM +0200, Brendan Jackman wrote:
> From: Paul Renauld <renauld@xxxxxxxxxx>
> 
> LSMs have high overhead due to indirect function calls through
> retpolines. This RPC proposes to replace these with static calls [1]

typo: RFC

> instead.

Yay! :)

> [...]
> This overhead prevents the adoption of bpf LSM on performance critical
> systems, and also, in general, slows down all LSMs.

I'd be curious to see other workloads too. (Your measurements are a bit
synthetic, mostly showing "worst case": one short syscall in a tight
loop. I'm curious how much performance gain can be had -- we should
still do it, it'll be a direct performance improvement, but I'm curious
about "real world" impact too.)

> [...]
> Previously, the code for this hook would have looked like this:
> 
> 	ret = DEFAULT_RET;
> 
>         for each cb in [A, B, C]:
>                 ret = cb(args); <--- costly indirect call here
>                 if ret != 0:
>                         break;
> 
>         return ret;
> 
> Static calls are defined at build time and are initially empty (NOP
> instructions). When the LSMs are initialized, the slots are filled as
> follows:
> 
>  slot idx     content
>            |-----------|
>     0      |           |
>            |-----------|
>     1      |           |
>            |-----------|
>     2      |   call A  | <-- base_slot_idx = 2
>            |-----------|
>     3      |   call B  |
>            |-----------|
>     4      |   call C  |
>            |-----------|
> 
> The generated code will unroll the foreach loop to have a static call for
> each possible LSM:
> 
>         ret = DEFAULT_RET;
>         switch(base_slot_idx):
> 
>                 case 0:
>                         NOP
>                         if ret != 0:
>                                 break;
>                         // fallthrough
>                 case 1:
>                         NOP
>                         if ret != 0:
>                                 break;
>                         // fallthrough
>                 case 2:
>                         ret = A(args); <--- direct call, no retpoline
>                         if ret != 0:
>                                 break;
>                         // fallthrough
>                 case 3:
>                         ret = B(args); <--- direct call, no retpoline
>                         if ret != 0:
>                                 break;
>                         // fallthrough
> 
>                 [...]
> 
>                 default:
>                         break;
> 
>         return ret;
> 
> A similar logic is applied for void hooks.
> 
> Why this trick with a switch statement? The table of static call is defined
> at compile time. The number of hook callbacks that will be defined is
> unknown at that time, and the table cannot be resized at runtime.  Static
> calls do not define a conditional execution for a non-void function, so the
> executed slots must be non-empty.  With this use of the table and the
> switch, it is possible to jump directly to the first used slot and execute
> all of the slots after. This essentially makes the entry point of the table
> dynamic. Instead, it would also be possible to start from 0 and break after
> the final populated slot, but that would require an additional conditional
> after each slot.

Instead of just "NOP", having the static branches perform a jump would
solve this pretty cleanly, yes? Something like:

	ret = DEFAULT_RET;

	ret = A(args); <--- direct call, no retpoline
	if ret != 0:
		goto out;

	ret = B(args); <--- direct call, no retpoline
	if ret != 0:
		goto out;

	goto out;
	if ret != 0:
		goto out;

out:
	return ret;


> [...]
> The number of available slots for each LSM hook is currently fixed at
> 11 (the number of LSMs in the kernel). Ideally, it should automatically
> adapt to the number of LSMs compiled into the kernel.

Seems like a reasonable thing to do and could be a separate patch.

> If there’s no practical way to implement such automatic adaptation, an
> option instead would be to remove the panic call by falling-back to the old
> linked-list mechanism, which is still present anyway (see below).
> 
> A few special cases of LSM don't use the macro call_[int/void]_hook but
> have their own calling logic. The linked-lists are kept as a possible slow
> path fallback for them.

I assume you mean the integrity subsystem? That just needs to be fixed
correctly. If we switch to this, let's ditch the linked list entirely.
Fixing integrity's stacking can be a separate patch too.

> [...]
> Signed-off-by: Paul Renauld <renauld@xxxxxxxxxx>
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>

This implies a maintainership chain, with Paul as the sole author. If
you mean all of you worked on the patch, include Co-developed-by: as
needed[1].

-Kees

[1] https://www.kernel.org/doc/html/latest/process/submitting-patches.html#when-to-use-acked-by-cc-and-co-developed-by

-- 
Kees Cook