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 <samitolva...@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 <w...@kernel.org> Will