KASAN & the vmalloc area

[Andrey Ryabinin]

On 11/08/2016 10:03 PM, Mark Rutland 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?
> 

Sadly, but I didn't have much time for this yet, so it's in an initial state.

> 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.
> 

I didn't look at any code, but we probably could can remember last visited pgd
and skip next pgd if it's the same as previous.
Alternatively - just skip kasan_zero_p*d in ptdump walker.

> 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.
> 
> Thanks,
> Mark.
> 
> [1] https://lkml.kernel.org/r/CALCETrWucrYp+yq8RHSDqf93xtg793duByirurzJbLRhrz=tcA@mail.gmail.com
> [2] https://lkml.kernel.org/r/20161003092940.GA691@gmail.com
> [3] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-October/464191.html
>