Messages in this thread | | | Date | Mon, 23 Nov 2020 11:08:12 +0000 | From | Will Deacon <> | Subject | Re: [PATCH 1/2] scs: switch to vmapped shadow stacks |
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Hi Sami,
On Fri, Nov 20, 2020 at 09:00:17AM -0800, Sami Tolvanen wrote: > On Thu, Nov 19, 2020 at 5:00 AM Will Deacon <will@kernel.org> wrote: > > On Thu, Oct 22, 2020 at 01:23:54PM -0700, Sami Tolvanen wrote: > > > The kernel currently uses kmem_cache to allocate shadow call stacks, > > > which means an overflow may not be immediately detected and can > > > potentially result in another task's shadow stack to be overwritten. > > > > > > This change switches SCS to use virtually mapped shadow stacks, > > > which increases shadow stack size to a full page and provides more > > > robust overflow detection similarly to VMAP_STACK. > > > > > > Signed-off-by: Sami Tolvanen <samitolvanen@google.com> > > > --- > > > include/linux/scs.h | 7 +---- > > > kernel/scs.c | 63 ++++++++++++++++++++++++++++++++++++++------- > > > 2 files changed, 55 insertions(+), 15 deletions(-) > > > > Cheers for posting this. I _much_ prefer handling the SCS this way, but I > > have some comments on the implementation below. > > > > > diff --git a/include/linux/scs.h b/include/linux/scs.h > > > index 6dec390cf154..86e3c4b7b714 100644 > > > --- a/include/linux/scs.h > > > +++ b/include/linux/scs.h > > > @@ -15,12 +15,7 @@ > > > > > > #ifdef CONFIG_SHADOW_CALL_STACK > > > > > > -/* > > > - * In testing, 1 KiB shadow stack size (i.e. 128 stack frames on a 64-bit > > > - * architecture) provided ~40% safety margin on stack usage while keeping > > > - * memory allocation overhead reasonable. > > > - */ > > > -#define SCS_SIZE SZ_1K > > > +#define SCS_SIZE PAGE_SIZE > > > > We could make this SCS_ORDER and then forget about alignment etc. > > It's still convenient to have SCS_SIZE defined, I think. I can > certainly define SCS_ORDER and use that to define SCS_SIZE, but do you > think we'll need an order >0 here at some point in future?
I'm not daft enough to comment on SCS size again ;) Let's define SCS_ORDER 0 and then SCS_SIZE in terms of that.
> > > > #define GFP_SCS (GFP_KERNEL | __GFP_ZERO) > > > > > > /* An illegal pointer value to mark the end of the shadow stack. */ > > > diff --git a/kernel/scs.c b/kernel/scs.c > > > index 4ff4a7ba0094..2136edba548d 100644 > > > --- a/kernel/scs.c > > > +++ b/kernel/scs.c > > > @@ -5,50 +5,95 @@ > > > * Copyright (C) 2019 Google LLC > > > */ > > > > > > +#include <linux/cpuhotplug.h> > > > #include <linux/kasan.h> > > > #include <linux/mm.h> > > > #include <linux/scs.h> > > > -#include <linux/slab.h> > > > +#include <linux/vmalloc.h> > > > #include <linux/vmstat.h> > > > > > > -static struct kmem_cache *scs_cache; > > > - > > > static void __scs_account(void *s, int account) > > > { > > > - struct page *scs_page = virt_to_page(s); > > > + struct page *scs_page = vmalloc_to_page(s); > > > > > > mod_node_page_state(page_pgdat(scs_page), NR_KERNEL_SCS_KB, > > > account * (SCS_SIZE / SZ_1K)); > > > } > > > > > > +/* Matches NR_CACHED_STACKS for VMAP_STACK */ > > > +#define NR_CACHED_SCS 2 > > > +static DEFINE_PER_CPU(void *, scs_cache[NR_CACHED_SCS]); > > > + > > > static void *scs_alloc(int node) > > > { > > > - void *s = kmem_cache_alloc_node(scs_cache, GFP_SCS, node); > > > + int i; > > > + void *s; > > > + > > > + for (i = 0; i < NR_CACHED_SCS; i++) { > > > + s = this_cpu_xchg(scs_cache[i], NULL); > > > + if (s) { > > > + memset(s, 0, SCS_SIZE); > > > + goto out; > > > + } > > > + } > > > + > > > + /* > > > + * We allocate a full page for the shadow stack, which should be > > > + * more than we need. Check the assumption nevertheless. > > > + */ > > > + BUILD_BUG_ON(SCS_SIZE > PAGE_SIZE);i > > > > With SCS_ORDER, you can drop this. > > > > > + > > > + s = __vmalloc_node_range(PAGE_SIZE, SCS_SIZE, > > > + VMALLOC_START, VMALLOC_END, > > > + GFP_SCS, PAGE_KERNEL, 0, > > > + node, __builtin_return_address(0)); > > > > Do we actually need vmalloc here? If we used alloc_pages() + vmap() > > Does it matter that vmap() always uses NUMA_NO_NODE? We'll also lose > the ability to use vfree_atomic() in scs_release() unless we use > VM_MAP_PUT_PAGES and allocate the page array passed to vmap() with > kvmalloc(), which I think we need to do to avoid sleeping in > scs_free().
Huh, I didn't realise we didn't have vunmap_atomic(). In which case, I take that back -- let's stick with vmalloc() for now.
Cheers,
Will
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