Messages in this thread | | | From | Sami Tolvanen <> | Date | Fri, 20 Nov 2020 09:00:17 -0800 | Subject | Re: [PATCH 1/2] scs: switch to vmapped shadow stacks |
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On Thu, Nov 19, 2020 at 5:00 AM Will Deacon <will@kernel.org> wrote: > > Hi Sami, > > 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?
> > #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().
> instead, then we could avoid the expensive call to vmalloc_to_page() > in __scs_account().
We still need vmalloc_to_page() in scs_release(). I suppose we could alternatively follow the example in kernel/fork.c and cache the vm_struct from find_vm_area() and use vm->pages[0] for the accounting. Thoughts?
> > > > > if (!s) > > return NULL; > > > > +out: > > *__scs_magic(s) = SCS_END_MAGIC; > > > > /* > > * Poison the allocation to catch unintentional accesses to > > * the shadow stack when KASAN is enabled. > > */ > > - kasan_poison_object_data(scs_cache, s); > > + kasan_poison_vmalloc(s, SCS_SIZE); > > __scs_account(s, 1); > > return s; > > } > > > > static void scs_free(void *s) > > { > > + int i; > > + > > __scs_account(s, -1); > > - kasan_unpoison_object_data(scs_cache, s); > > - kmem_cache_free(scs_cache, s); > > + kasan_unpoison_vmalloc(s, SCS_SIZE); > > I don't see the point in unpoisoning here tbh; vfree_atomic() re-poisons > almost immediately, so we should probably defer this to scs_alloc() and > only when picking the stack out of the cache.
Sure, I'll change this in v2.
> > > + > > + for (i = 0; i < NR_CACHED_SCS; i++) > > Can you add a comment about the re-entrancy here and why we're using > this_cpu_cmpxchg() please?
I'll add a comment.
Sami
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