Re: KASAN & the vmalloc area

[Dmitry Vyukov]

On Tue, Nov 8, 2016 at 11:03 AM, Mark Rutland <mark.r...@arm.com> wrote:
> Hi,
>
> I see a while back [1] there was a discussion of what to do about KASAN
> and vmapped stacks, but it doesn't look like that was solved, judging by
> the vmapped stacks pull [2] for v4.9.
>
> I wondered whether anyone had looked at that since?
>
> I have an additional reason to want to dynamically allocate the vmalloc
> area shadow: it turns out that KASAN currently interacts rather poorly
> with the arm64 ptdump code.
>
> When KASAN is selected, we allocate shadow for the whole vmalloc area,
> using common zero pte, pmd, pud tables. Walking over these in the ptdump
> code takes a *very* long time (I've seen up to 15 minutes with
> KASAN_OUTLINE enabled). For DEBUG_WX [3], this means boot hangs for that
> long, too.
>
> If I don't allocate vmalloc shadow (and remove the apparently pointlesss
> shadow of the shadow area), and only allocate shadow for the image,
> fixmap, vmemmap and so on, that delay gets cut to a few seconds, which
> is tolerable for a debug configuration...
>
> ... however, things blow up when the kernel touches vmalloc'd memory for
> the first time, as we don't install shadow for that dynamically.


I've seen the same iteration slowness problem on x86 with
CONFIG_DEBUG_RODATA which walks all pages. The is about 1 minute, but
it is enough to trigger rcu stall warning.

The zero pud and vmalloc-ed stacks looks like different problems.
To overcome the slowness we could map zero shadow for vmalloc area lazily.
However for vmalloc-ed stacks we need to map actual memory, because
stack instrumentation will read/write into the shadow. One downside
here is that vmalloc shadow can be as large as 1:1 (if we allocate 1
page in vmalloc area we need to allocate 1 page for shadow).

Re slowness: could we just skip the KASAN zero puds (the top level)
while walking? Can they be interesting for anybody? We can just
pretend that they are not there. Looks like a trivial solution for the
problem at hand.

[Mark Rutland]

On Tue, Nov 08, 2016 at 02:09:27PM -0800, Dmitry Vyukov wrote:
> On Tue, Nov 8, 2016 at 11:03 AM, Mark Rutland <mark.r...@arm.com> wrote:
> > When KASAN is selected, we allocate shadow for the whole vmalloc area,
> > using common zero pte, pmd, pud tables. Walking over these in the ptdump
> > code takes a *very* long time (I've seen up to 15 minutes with
> > KASAN_OUTLINE enabled). For DEBUG_WX [3], this means boot hangs for that
> > long, too.

[...]

> I've seen the same iteration slowness problem on x86 with
> CONFIG_DEBUG_RODATA which walks all pages. The is about 1 minute, but
> it is enough to trigger rcu stall warning.

Interesting; do you know where that happens? I can't spot any obvious
case where we'd have to walk all the page tables for DEBUG_RODATA.

> The zero pud and vmalloc-ed stacks looks like different problems.
> To overcome the slowness we could map zero shadow for vmalloc area lazily.
> However for vmalloc-ed stacks we need to map actual memory, because
> stack instrumentation will read/write into the shadow.

Sure. The point I was trying to make is that there' be fewer page tables
to walk (unless the vmalloc area was exhausted), assuming we also lazily
mapped the common zero shadow for the vmalloc area.

> One downside here is that vmalloc shadow can be as large as 1:1 (if we
> allocate 1 page in vmalloc area we need to allocate 1 page for
> shadow).

I thought per prior discussion we'd only need to allocate new pages for
the stacks in the vmalloc region, and we could re-use the zero pages?

... or are you trying to quantify the cost of the page tables?

> Re slowness: could we just skip the KASAN zero puds (the top level)
> while walking? Can they be interesting for anybody?

They're interesting for the ptdump case (which allows privileged users
to dump the tables via /sys/kernel/debug/kernel_page_tables). I've seen
25+ minute hangs there.

> We can just pretend that they are not there. Looks like a trivial
> solution for the problem at hand.

For the boot time hang it's option. Though I'd prefer that the sanity
checks applied to all of tables, shadow regions included.

Thanks,
Mark.

[Dmitry Vyukov]

On Wed, Nov 9, 2016 at 2:56 AM, Mark Rutland <mark.r...@arm.com> wrote:
> On Tue, Nov 08, 2016 at 02:09:27PM -0800, Dmitry Vyukov wrote:
>> On Tue, Nov 8, 2016 at 11:03 AM, Mark Rutland <mark.r...@arm.com> wrote:
>> > When KASAN is selected, we allocate shadow for the whole vmalloc area,
>> > using common zero pte, pmd, pud tables. Walking over these in the ptdump
>> > code takes a *very* long time (I've seen up to 15 minutes with
>> > KASAN_OUTLINE enabled). For DEBUG_WX [3], this means boot hangs for that
>> > long, too.
>
> [...]
>
>> I've seen the same iteration slowness problem on x86 with
>> CONFIG_DEBUG_RODATA which walks all pages. The is about 1 minute, but
>> it is enough to trigger rcu stall warning.
>
> Interesting; do you know where that happens? I can't spot any obvious
> case where we'd have to walk all the page tables for DEBUG_RODATA.

As far as I remember it was this path:

mark_readonly in main.c -> mark_rodata_ro -> debug_checkwx ->
ptdump_walk_pgd_level_checkwx -> ptdump_walk_pgd_level_core.

>> The zero pud and vmalloc-ed stacks looks like different problems.
>> To overcome the slowness we could map zero shadow for vmalloc area lazily.
>> However for vmalloc-ed stacks we need to map actual memory, because
>> stack instrumentation will read/write into the shadow.
>
> Sure. The point I was trying to make is that there' be fewer page tables
> to walk (unless the vmalloc area was exhausted), assuming we also lazily
> mapped the common zero shadow for the vmalloc area.
>
>> One downside here is that vmalloc shadow can be as large as 1:1 (if we
>> allocate 1 page in vmalloc area we need to allocate 1 page for
>> shadow).
>
> I thought per prior discussion we'd only need to allocate new pages for
> the stacks in the vmalloc region, and we could re-use the zero pages?

We can't reuse zero ro pages for stacks, because stack instrumentation
writes to stack shadow.
When we have a large continuous range of memory, shadow for it is
1/8th. However, if we have a separate page, we will need to map whole
page of shadow for it, i.e. 1:1 shadow overhead.

> ... or are you trying to quantify the cost of the page tables?
>
>> Re slowness: could we just skip the KASAN zero puds (the top level)
>> while walking? Can they be interesting for anybody?
>
> They're interesting for the ptdump case (which allows privileged users
> to dump the tables via /sys/kernel/debug/kernel_page_tables). I've seen
> 25+ minute hangs there.
>
>> We can just pretend that they are not there. Looks like a trivial
>> solution for the problem at hand.
>
> For the boot time hang it's option. Though I'd prefer that the sanity
> checks applied to all of tables, shadow regions included.
>
> Thanks,
> Mark.
>

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[Mark Rutland]

On Wed, Nov 09, 2016 at 10:16:03AM -0800, Dmitry Vyukov wrote:
> On Wed, Nov 9, 2016 at 2:56 AM, Mark Rutland <mark.r...@arm.com> wrote:
> > On Tue, Nov 08, 2016 at 02:09:27PM -0800, Dmitry Vyukov wrote:
> >> On Tue, Nov 8, 2016 at 11:03 AM, Mark Rutland <mark.r...@arm.com> wrote:
> >> I've seen the same iteration slowness problem on x86 with
> >> CONFIG_DEBUG_RODATA which walks all pages. The is about 1 minute, but
> >> it is enough to trigger rcu stall warning.
> >
> > Interesting; do you know where that happens? I can't spot any obvious
> > case where we'd have to walk all the page tables for DEBUG_RODATA.
>
> As far as I remember it was this path:
>
> mark_readonly in main.c -> mark_rodata_ro -> debug_checkwx ->
> ptdump_walk_pgd_level_checkwx -> ptdump_walk_pgd_level_core.

Ah, that's x86's equivalent DEBUG_WX checks.

> >> The zero pud and vmalloc-ed stacks looks like different problems.
> >> To overcome the slowness we could map zero shadow for vmalloc area lazily.
> >> However for vmalloc-ed stacks we need to map actual memory, because
> >> stack instrumentation will read/write into the shadow.
> >
> > Sure. The point I was trying to make is that there' be fewer page tables
> > to walk (unless the vmalloc area was exhausted), assuming we also lazily
> > mapped the common zero shadow for the vmalloc area.
> >
> >> One downside here is that vmalloc shadow can be as large as 1:1 (if we
> >> allocate 1 page in vmalloc area we need to allocate 1 page for
> >> shadow).
> >
> > I thought per prior discussion we'd only need to allocate new pages for
> > the stacks in the vmalloc region, and we could re-use the zero pages?
>
> We can't reuse zero ro pages for stacks, because stack instrumentation
> writes to stack shadow.

Sorry, I'd meant we'd use the zero pages for everything else but stacks.
I understand we'd have to allocate real shadow for the stacks.

> When we have a large continuous range of memory, shadow for it is
> 1/8th. However, if we have a separate page, we will need to map whole
> page of shadow for it, i.e. 1:1 shadow overhead.

Sure, but for everything but stacks we can re-use the same zero pages,
no?

For everything else, the cost would be dominated by the page tables for
the shadow.

Thanks,
Mark.

[Dmitry Vyukov]

Can we estimate the memory overhead?