lkml.org 
[lkml]   [2020]   [Nov]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 1/2] scs: switch to vmapped shadow stacks
On Tue, Nov 24, 2020 at 11:59:39AM -0800, Sami Tolvanen wrote:
> The kernel currently uses kmem_cache to allocate shadow call stacks,
> which means an overflows 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 for
> tasks, 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 | 12 ++++-----
> kernel/scs.c | 66 +++++++++++++++++++++++++++++++++++++--------
> 2 files changed, 61 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/scs.h b/include/linux/scs.h
> index 6dec390cf154..2a506c2a16f4 100644
> --- a/include/linux/scs.h
> +++ b/include/linux/scs.h
> @@ -15,12 +15,8 @@
>
> #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_ORDER 0
> +#define SCS_SIZE (PAGE_SIZE << SCS_ORDER)
> #define GFP_SCS (GFP_KERNEL | __GFP_ZERO)
>
> /* An illegal pointer value to mark the end of the shadow stack. */
> @@ -33,6 +29,8 @@
> #define task_scs(tsk) (task_thread_info(tsk)->scs_base)
> #define task_scs_sp(tsk) (task_thread_info(tsk)->scs_sp)
>
> +void *scs_alloc(int node);
> +void scs_free(void *s);
> void scs_init(void);
> int scs_prepare(struct task_struct *tsk, int node);
> void scs_release(struct task_struct *tsk);
> @@ -61,6 +59,8 @@ static inline bool task_scs_end_corrupted(struct task_struct *tsk)
>
> #else /* CONFIG_SHADOW_CALL_STACK */
>
> +static inline void *scs_alloc(int node) { return NULL; }
> +static inline void scs_free(void *s) {}
> static inline void scs_init(void) {}
> static inline void scs_task_reset(struct task_struct *tsk) {}
> static inline int scs_prepare(struct task_struct *tsk, int node) { return 0; }
> diff --git a/kernel/scs.c b/kernel/scs.c
> index 4ff4a7ba0094..25b0dd5aa0e2 100644
> --- a/kernel/scs.c
> +++ b/kernel/scs.c
> @@ -5,50 +5,94 @@
> * 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));
> }
>
> -static void *scs_alloc(int node)
> +/* Matches NR_CACHED_STACKS for VMAP_STACK */
> +#define NR_CACHED_SCS 2
> +static DEFINE_PER_CPU(void *, scs_cache[NR_CACHED_SCS]);
> +
> +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) {
> + kasan_unpoison_vmalloc(s, SCS_SIZE);
> + memset(s, 0, SCS_SIZE);
> + goto out;
> + }
> + }
> +
> + s = __vmalloc_node_range(SCS_SIZE, 1, VMALLOC_START, VMALLOC_END,
> + GFP_SCS, PAGE_KERNEL, 0, node,
> + __builtin_return_address(0));
>
> if (!s)
> return NULL;

Sorry I didn't spot this before, but if you put the xchg/vmalloc code
into a new __scs_alloc() function then you can drop the label and this
becomes:

s = __scs_alloc(...);
if (!s)
return NULL;

*__scs_maghic(s) = SCS_ENG_MAGIC;
...

With that:

Acked-by: Will Deacon <will@kernel.org>

Will

\
 
 \ /
  Last update: 2020-11-30 12:46    [W:0.113 / U:0.392 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site