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Peter Anvin" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Lorenzo Stoakes , Arnd Bergmann Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Rick Edgecombe , Mark Brown , Deepak Gupta X-Mailer: b4 0.14.0 CPU assisted shadow stack are supported by x86, arm64 and risc-v. In terms of enabling shadow stack feature for usermode code in kernel, they have following commonalities - Expose a user ABI (via a prctl) to allow user mode to explicitly ask for enabling shadow stack instead of by default enabling it. x86 series pre-dates arm64 or risc-v announcment of support, so it ended up doing a arch specific prctl instead of generic one. arm64 and risc-v have converged on using generic prctl and each of them can handle it appropriatley. - On fork or clone, shadow stack has to be COWed or not COWed depending on CLONE_VM was passed or not. Additionally if CLONE_VFORK was passed then same (parent one) shadow stack should be used. - To create shadow stack mappings, implement `map_shadow_stack` system call. This patch picks up Mark Brown's `ARCH_HAS_USER_SHADOW_STACK` config introduction and incorproate most of the common flows between different architectures. On a high level, shadow stack allocation and shadow stack de-allocation are base operations on virtual memory and common between architectures. Similarly shadow stack setup on prctl (arch specific or otherwise) is a common flow. Treatment of shadow stack virtual memory on `clone/fork` and implementaiton of `map_shadow_stack` is also converged into common flow. To implement these common flows, each architecture have arch-specific enabling mechanism as well as arch-specific data structures in task/ thread struct. So additionally this patch tries to abstract certain operation/helpers and allowing each architecture to have their arch_* implementation to implement the abstractions. Signed-off-by: Deepak Gupta --- arch/x86/include/asm/shstk.h | 7 + arch/x86/include/uapi/asm/mman.h | 3 - arch/x86/kernel/shstk.c | 223 +++++--------------------------- include/linux/usershstk.h | 22 ++++ include/uapi/asm-generic/mman-common.h | 5 + kernel/Makefile | 2 + kernel/usershstk.c | 230 +++++++++++++++++++++++++++++++++ 7 files changed, 296 insertions(+), 196 deletions(-) diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h index 4cb77e004615..b40c3d91538b 100644 --- a/arch/x86/include/asm/shstk.h +++ b/arch/x86/include/asm/shstk.h @@ -37,6 +37,13 @@ static inline int shstk_update_last_frame(unsigned long val) { return 0; } static inline bool shstk_is_enabled(void) { return false; } #endif /* CONFIG_X86_USER_SHADOW_STACK */ +int arch_create_shstk_token(unsigned long ssp, unsigned long *token_addr); +bool arch_user_shstk_supported(void); +bool arch_is_shstk_enabled(struct task_struct *task); +void arch_set_shstk_base_size(struct task_struct *task, unsigned long base, + unsigned long size); +void arch_get_shstk_base_size(struct task_struct *task, unsigned long *base, + unsigned long *size); #endif /* __ASSEMBLY__ */ #endif /* _ASM_X86_SHSTK_H */ diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h index 46cdc941f958..ac1e6277212b 100644 --- a/arch/x86/include/uapi/asm/mman.h +++ b/arch/x86/include/uapi/asm/mman.h @@ -5,9 +5,6 @@ #define MAP_32BIT 0x40 /* only give out 32bit addresses */ #define MAP_ABOVE4G 0x80 /* only map above 4GB */ -/* Flags for map_shadow_stack(2) */ -#define SHADOW_STACK_SET_TOKEN (1ULL << 0) /* Set up a restore token in the shadow stack */ - #include #endif /* _ASM_X86_MMAN_H */ diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index 059685612362..d53a7efd70b5 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -25,6 +25,7 @@ #include #include #include +#include #define SS_FRAME_SIZE 8 @@ -43,11 +44,39 @@ static void features_clr(unsigned long features) current->thread.features &= ~features; } +bool arch_user_shstk_supported(void) +{ + return cpu_feature_enabled(X86_FEATURE_USER_SHSTK); +} + +bool arch_is_shstk_enabled(struct task_struct *task) +{ + return features_enabled(ARCH_SHSTK_SHSTK); +} + +void arch_set_shstk_base_size(struct task_struct *task, unsigned long base, + unsigned long size) +{ + struct thread_shstk *shstk = &task->thread.shstk; + + shstk->base = base; + shstk->size = size; +} + +void arch_get_shstk_base_size(struct task_struct *task, unsigned long *base, + unsigned long *size) +{ + struct thread_shstk *shstk = &task->thread.shstk; + + *base = shstk->base; + *size = shstk->size; +} + /* * Create a restore token on the shadow stack. A token is always 8-byte * and aligned to 8. */ -static int create_rstor_token(unsigned long ssp, unsigned long *token_addr) +int arch_create_shstk_token(unsigned long ssp, unsigned long *token_addr) { unsigned long addr; @@ -72,88 +101,6 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr) return 0; } -/* - * VM_SHADOW_STACK will have a guard page. This helps userspace protect - * itself from attacks. The reasoning is as follows: - * - * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The - * INCSSP instruction can increment the shadow stack pointer. It is the - * shadow stack analog of an instruction like: - * - * addq $0x80, %rsp - * - * However, there is one important difference between an ADD on %rsp - * and INCSSP. In addition to modifying SSP, INCSSP also reads from the - * memory of the first and last elements that were "popped". It can be - * thought of as acting like this: - * - * READ_ONCE(ssp); // read+discard top element on stack - * ssp += nr_to_pop * 8; // move the shadow stack - * READ_ONCE(ssp-8); // read+discard last popped stack element - * - * The maximum distance INCSSP can move the SSP is 2040 bytes, before - * it would read the memory. Therefore a single page gap will be enough - * to prevent any operation from shifting the SSP to an adjacent stack, - * since it would have to land in the gap at least once, causing a - * fault. - */ -static unsigned long alloc_shstk(unsigned long addr, unsigned long size, - unsigned long token_offset, bool set_res_tok) -{ - int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_ABOVE4G; - struct mm_struct *mm = current->mm; - unsigned long mapped_addr, unused; - - if (addr) - flags |= MAP_FIXED_NOREPLACE; - - mmap_write_lock(mm); - mapped_addr = do_mmap(NULL, addr, size, PROT_READ, flags, - VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL); - mmap_write_unlock(mm); - - if (!set_res_tok || IS_ERR_VALUE(mapped_addr)) - goto out; - - if (create_rstor_token(mapped_addr + token_offset, NULL)) { - vm_munmap(mapped_addr, size); - return -EINVAL; - } - -out: - return mapped_addr; -} - -static unsigned long adjust_shstk_size(unsigned long size) -{ - if (size) - return PAGE_ALIGN(size); - - return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G)); -} - -static void unmap_shadow_stack(u64 base, u64 size) -{ - int r; - - r = vm_munmap(base, size); - - /* - * mmap_write_lock_killable() failed with -EINTR. This means - * the process is about to die and have it's MM cleaned up. - * This task shouldn't ever make it back to userspace. In this - * case it is ok to leak a shadow stack, so just exit out. - */ - if (r == -EINTR) - return; - - /* - * For all other types of vm_munmap() failure, either the - * system is out of memory or there is bug. - */ - WARN_ON_ONCE(r); -} - static int shstk_setup(void) { struct thread_shstk *shstk = ¤t->thread.shstk; @@ -191,48 +138,6 @@ void reset_thread_features(void) current->thread.features_locked = 0; } -unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags, - unsigned long stack_size) -{ - struct thread_shstk *shstk = &tsk->thread.shstk; - unsigned long addr, size; - - /* - * If shadow stack is not enabled on the new thread, skip any - * switch to a new shadow stack. - */ - if (!features_enabled(ARCH_SHSTK_SHSTK)) - return 0; - - /* - * For CLONE_VFORK the child will share the parents shadow stack. - * Make sure to clear the internal tracking of the thread shadow - * stack so the freeing logic run for child knows to leave it alone. - */ - if (clone_flags & CLONE_VFORK) { - shstk->base = 0; - shstk->size = 0; - return 0; - } - - /* - * For !CLONE_VM the child will use a copy of the parents shadow - * stack. - */ - if (!(clone_flags & CLONE_VM)) - return 0; - - size = adjust_shstk_size(stack_size); - addr = alloc_shstk(0, size, 0, false); - if (IS_ERR_VALUE(addr)) - return addr; - - shstk->base = addr; - shstk->size = size; - - return addr + size; -} - static unsigned long get_user_shstk_addr(void) { unsigned long long ssp; @@ -402,44 +307,6 @@ int restore_signal_shadow_stack(void) return 0; } -void shstk_free(struct task_struct *tsk) -{ - struct thread_shstk *shstk = &tsk->thread.shstk; - - if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) || - !features_enabled(ARCH_SHSTK_SHSTK)) - return; - - /* - * When fork() with CLONE_VM fails, the child (tsk) already has a - * shadow stack allocated, and exit_thread() calls this function to - * free it. In this case the parent (current) and the child share - * the same mm struct. - */ - if (!tsk->mm || tsk->mm != current->mm) - return; - - /* - * If shstk->base is NULL, then this task is not managing its - * own shadow stack (CLONE_VFORK). So skip freeing it. - */ - if (!shstk->base) - return; - - /* - * shstk->base is NULL for CLONE_VFORK child tasks, and so is - * normal. But size = 0 on a shstk->base is not normal and - * indicated an attempt to free the thread shadow stack twice. - * Warn about it. - */ - if (WARN_ON(!shstk->size)) - return; - - unmap_shadow_stack(shstk->base, shstk->size); - - shstk->size = 0; -} - static int wrss_control(bool enable) { u64 msrval; @@ -502,36 +369,6 @@ static int shstk_disable(void) return 0; } -SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags) -{ - bool set_tok = flags & SHADOW_STACK_SET_TOKEN; - unsigned long aligned_size; - - if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) - return -EOPNOTSUPP; - - if (flags & ~SHADOW_STACK_SET_TOKEN) - return -EINVAL; - - /* If there isn't space for a token */ - if (set_tok && size < 8) - return -ENOSPC; - - if (addr && addr < SZ_4G) - return -ERANGE; - - /* - * An overflow would result in attempting to write the restore token - * to the wrong location. Not catastrophic, but just return the right - * error code and block it. - */ - aligned_size = PAGE_ALIGN(size); - if (aligned_size < size) - return -EOVERFLOW; - - return alloc_shstk(addr, aligned_size, size, set_tok); -} - long shstk_prctl(struct task_struct *task, int option, unsigned long arg2) { unsigned long features = arg2; diff --git a/include/linux/usershstk.h b/include/linux/usershstk.h new file mode 100644 index 000000000000..4ab27a1ab3f8 --- /dev/null +++ b/include/linux/usershstk.h @@ -0,0 +1,22 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _SHSTK_H +#define _SHSTK_H + +#ifndef __ASSEMBLY__ +#include + +unsigned long alloc_shstk(unsigned long addr, unsigned long size, + unsigned long token_offset, bool set_res_tok); +int create_shstk_token(unsigned long ssp, unsigned long *token_addr); +bool user_shstk_supported(void); +bool is_shstk_enabled(struct task_struct *task); +void set_shstk_base_size(struct task_struct *task, unsigned long base, + unsigned long size); +void get_shstk_base_size(struct task_struct *task, unsigned long *base, + unsigned long *size); +unsigned long adjust_shstk_size(unsigned long size); +void unmap_shadow_stack(u64 base, u64 size); + +#endif /* __ASSEMBLY__ */ + +#endif /* _SHSTK_H */ diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h index 6ce1f1ceb432..5d6fb32fda95 100644 --- a/include/uapi/asm-generic/mman-common.h +++ b/include/uapi/asm-generic/mman-common.h @@ -87,4 +87,9 @@ #define PKEY_ACCESS_MASK (PKEY_DISABLE_ACCESS |\ PKEY_DISABLE_WRITE) +/* Flags for map_shadow_stack(2) */ +#define SHADOW_STACK_SET_TOKEN (1ULL << 0) /* Set up a restore token in the shadow stack */ +#define SHADOW_STACK_SET_MARKER (1ULL << 1) /* Set up a top of stack marker in the shadow stack */ + +#define SHADOW_STACK_SET_MASK (SHADOW_STACK_SET_TOKEN | SHADOW_STACK_SET_MARKER) #endif /* __ASM_GENERIC_MMAN_COMMON_H */ diff --git a/kernel/Makefile b/kernel/Makefile index 87866b037fbe..1922c456b954 100644 --- a/kernel/Makefile +++ b/kernel/Makefile @@ -140,6 +140,8 @@ KCOV_INSTRUMENT_stackleak.o := n obj-$(CONFIG_SCF_TORTURE_TEST) += scftorture.o +obj-$(CONFIG_ARCH_HAS_USER_SHADOW_STACK) += usershstk.o + $(obj)/configs.o: $(obj)/config_data.gz targets += config_data config_data.gz diff --git a/kernel/usershstk.c b/kernel/usershstk.c new file mode 100644 index 000000000000..1ebce6b768aa --- /dev/null +++ b/kernel/usershstk.c @@ -0,0 +1,230 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * shstk.c - Intel shadow stack support + * + * Copyright (c) 2021, Intel Corporation. + * Yu-cheng Yu + */ + +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include + +#define SHSTK_ENTRY_SIZE sizeof(void *) + +bool user_shstk_supported(void) +{ + return arch_user_shstk_supported(); +} + +bool is_shstk_enabled(struct task_struct *task) +{ + return arch_is_shstk_enabled(task); +} + +void set_shstk_base_size(struct task_struct *task, unsigned long base, + unsigned long size) +{ + arch_set_shstk_base_size(task, base, size); +} + +void get_shstk_base_size(struct task_struct *task, unsigned long *base, + unsigned long *size) +{ + arch_get_shstk_base_size(task, base, size); +} + +int create_shstk_token(unsigned long ssp, unsigned long *token_addr) +{ + return arch_create_shstk_token(ssp, token_addr); +} + +unsigned long adjust_shstk_size(unsigned long size) +{ + if (size) + return PAGE_ALIGN(size); + + return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G)); +} + +void unmap_shadow_stack(u64 base, u64 size) +{ + int r; + + r = vm_munmap(base, size); + + /* + * mmap_write_lock_killable() failed with -EINTR. This means + * the process is about to die and have it's MM cleaned up. + * This task shouldn't ever make it back to userspace. In this + * case it is ok to leak a shadow stack, so just exit out. + */ + if (r == -EINTR) + return; + + /* + * For all other types of vm_munmap() failure, either the + * system is out of memory or there is bug. + */ + WARN_ON_ONCE(r); +} + +/* + * allocates a fresh shadow stack mapping and if required place a shadow + * stack token at base + */ +unsigned long alloc_shstk(unsigned long addr, unsigned long size, + unsigned long token_offset, bool set_res_tok) +{ + int flags = MAP_ANONYMOUS | MAP_PRIVATE; + + flags |= IS_ENABLED(CONFIG_X86_64) ? MAP_ABOVE4G : 0; + + struct mm_struct *mm = current->mm; + unsigned long mapped_addr, unused; + + if (addr) + flags |= MAP_FIXED_NOREPLACE; + + mmap_write_lock(mm); + mapped_addr = do_mmap(NULL, addr, size, PROT_READ, flags, + VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL); + mmap_write_unlock(mm); + + if (!set_res_tok || IS_ERR_VALUE(mapped_addr)) + goto out; + + if (create_shstk_token(mapped_addr + token_offset, NULL)) { + vm_munmap(mapped_addr, size); + return -EINVAL; + } + +out: + return mapped_addr; +} + +void shstk_free(struct task_struct *tsk) +{ + unsigned long base, size; + + if (!user_shstk_supported() || + !is_shstk_enabled(current)) + return; + + /* + * When fork() with CLONE_VM fails, the child (tsk) already has a + * shadow stack allocated, and exit_thread() calls this function to + * free it. In this case the parent (current) and the child share + * the same mm struct. + */ + if (!tsk->mm || tsk->mm != current->mm) + return; + + get_shstk_base_size(tsk, &base, &size); + /* + * If shstk->base is NULL, then this task is not managing its + * own shadow stack (CLONE_VFORK). So skip freeing it. + */ + if (!base) + return; + + /* + * shstk->base is NULL for CLONE_VFORK child tasks, and so is + * normal. But size = 0 on a shstk->base is not normal and + * indicated an attempt to free the thread shadow stack twice. + * Warn about it. + */ + if (WARN_ON(!size)) + return; + + unmap_shadow_stack(base, size); + + set_shstk_base_size(tsk, 0, 0); +} + +SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags) +{ + bool set_tok = flags & SHADOW_STACK_SET_TOKEN; + unsigned long aligned_size; + + if (!user_shstk_supported()) + return -EOPNOTSUPP; + + if (flags & ~SHADOW_STACK_SET_MASK) + return -EINVAL; + + /* If there isn't space for a token */ + if (set_tok && size < SHSTK_ENTRY_SIZE) + return -ENOSPC; + + if (addr && (addr & (PAGE_SIZE - 1))) + return -EINVAL; + + if (IS_ENABLED(CONFIG_X86_64) && + addr && addr < SZ_4G) + return -ERANGE; + + /* + * An overflow would result in attempting to write the restore token + * to the wrong location. Not catastrophic, but just return the right + * error code and block it. + */ + aligned_size = PAGE_ALIGN(size); + if (aligned_size < size) + return -EOVERFLOW; + + return alloc_shstk(addr, aligned_size, size, set_tok); +} + +unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags, + unsigned long stack_size) +{ + unsigned long addr, size; + + if (!user_shstk_supported()) + return -EOPNOTSUPP; + + /* + * If shadow stack is not enabled on the new thread, skip any + * switch to a new shadow stack. + */ + if (!is_shstk_enabled(tsk)) + return 0; + + /* + * For CLONE_VFORK the child will share the parents shadow stack. + * Make sure to clear the internal tracking of the thread shadow + * stack so the freeing logic run for child knows to leave it alone. + */ + if (clone_flags & CLONE_VFORK) { + set_shstk_base_size(tsk, 0, 0); + return 0; + } + + /* + * For !CLONE_VM the child will use a copy of the parents shadow + * stack. + */ + if (!(clone_flags & CLONE_VM)) + return 0; + + size = adjust_shstk_size(stack_size); + addr = alloc_shstk(0, size, 0, false); + if (IS_ERR_VALUE(addr)) + return addr; + + set_shstk_base_size(tsk, addr, size); + + return addr + size; +}