Re: [PATCH v6 33/41] x86/shstk: Introduce map_shadow_stack syscall

[Deepak Gupta <debug@rivosinc.com>]

On Sat, Feb 18, 2023 at 01:14:25PM -0800, Rick Edgecombe wrote:
>When operating with shadow stacks enabled, the kernel will automatically
>allocate shadow stacks for new threads, however in some cases userspace
>will need additional shadow stacks. The main example of this is the
>ucontext family of functions, which require userspace allocating and
>pivoting to userspace managed stacks.
>
>Unlike most other user memory permissions, shadow stacks need to be
>provisioned with special data in order to be useful. They need to be setup
>with a restore token so that userspace can pivot to them via the RSTORSSP
>instruction. But, the security design of shadow stack's is that they
>should not be written to except in limited circumstances. This presents a
>problem for userspace, as to how userspace can provision this special
>data, without allowing for the shadow stack to be generally writable.
>
>Previously, a new PROT_SHADOW_STACK was attempted, which could be
>mprotect()ed from RW permissions after the data was provisioned. This was
>found to not be secure enough, as other thread's could write to the
>shadow stack during the writable window.
>
>The kernel can use a special instruction, WRUSS, to write directly to
>userspace shadow stacks. So the solution can be that memory can be mapped
>as shadow stack permissions from the beginning (never generally writable
>in userspace), and the kernel itself can write the restore token.
>
>First, a new madvise() flag was explored, which could operate on the
>PROT_SHADOW_STACK memory. This had a couple downsides:
>1. Extra checks were needed in mprotect() to prevent writable memory from
>   ever becoming PROT_SHADOW_STACK.
>2. Extra checks/vma state were needed in the new madvise() to prevent
>   restore tokens being written into the middle of pre-used shadow stacks.
>   It is ideal to prevent restore tokens being added at arbitrary
>   locations, so the check was to make sure the shadow stack had never been
>   written to.
>3. It stood out from the rest of the madvise flags, as more of direct
>   action than a hint at future desired behavior.
>
>So rather than repurpose two existing syscalls (mmap, madvise) that don't
>quite fit, just implement a new map_shadow_stack syscall to allow
>userspace to map and setup new shadow stacks in one step. While ucontext
>is the primary motivator, userspace may have other unforeseen reasons to
>setup it's own shadow stacks using the WRSS instruction. Towards this
>provide a flag so that stacks can be optionally setup securely for the
>common case of ucontext without enabling WRSS. Or potentially have the
>kernel set up the shadow stack in some new way.

Was following ever attempted?

void *shstk = mmap(0, size, PROT_SHADOWSTACK, ...);
- limit PROT_SHADOWSTACK protection flag to only mmap (and thus mprotect can't
   convert memory from shadow stack to non-shadow stack type or vice versa)
- limit PROT_SHADOWSTACK protection flag to anonymous memory only.
- top level mmap handler to put a token at the base using WRUSS if prot == PROT_SHADOWSTACK

You essentially would get shadow stack manufacturing with existing (single) syscall.
Acting a bit selfish here, this allows other architectures as well to re-use this and 
do their own implementation of mapping and placing the token at the base.

>
>The following example demonstrates how to create a new shadow stack with
>map_shadow_stack:
>void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
>
>Tested-by: Pengfei Xu <pengfei.xu@intel.com>
>Tested-by: John Allen <john.allen@amd.com>
>Reviewed-by: Kees Cook <keescook@chromium.org>
>Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>
>---
>v5:
> - Fix addr/mapped_addr (Kees)
> - Switch to EOPNOTSUPP (Kees suggested ENOTSUPP, but checkpatch
>   suggests this)
> - Return error for addresses below 4G
>
>v3:
> - Change syscall common -> 64 (Kees)
> - Use bit shift notation instead of 0x1 for uapi header (Kees)
> - Call do_mmap() with MAP_FIXED_NOREPLACE (Kees)
> - Block unsupported flags (Kees)
> - Require size >= 8 to set token (Kees)
>
>v2:
> - Change syscall to take address like mmap() for CRIU's usage
>
>v1:
> - New patch (replaces PROT_SHADOW_STACK).
>---
> arch/x86/entry/syscalls/syscall_64.tbl |  1 +
> arch/x86/include/uapi/asm/mman.h       |  3 ++
> arch/x86/kernel/shstk.c                | 59 ++++++++++++++++++++++----
> include/linux/syscalls.h               |  1 +
> include/uapi/asm-generic/unistd.h      |  2 +-
> kernel/sys_ni.c                        |  1 +
> 6 files changed, 58 insertions(+), 9 deletions(-)
>
>diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
>index c84d12608cd2..f65c671ce3b1 100644
>--- a/arch/x86/entry/syscalls/syscall_64.tbl
>+++ b/arch/x86/entry/syscalls/syscall_64.tbl
>@@ -372,6 +372,7 @@
> 448	common	process_mrelease	sys_process_mrelease
> 449	common	futex_waitv		sys_futex_waitv
> 450	common	set_mempolicy_home_node	sys_set_mempolicy_home_node
>+451	64	map_shadow_stack	sys_map_shadow_stack
>
> #
> # Due to a historical design error, certain syscalls are numbered differently
>diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
>index 5a0256e73f1e..8148bdddbd2c 100644
>--- a/arch/x86/include/uapi/asm/mman.h
>+++ b/arch/x86/include/uapi/asm/mman.h
>@@ -13,6 +13,9 @@
> 		((key) & 0x8 ? VM_PKEY_BIT3 : 0))
> #endif
>
>+/* Flags for map_shadow_stack(2) */
>+#define SHADOW_STACK_SET_TOKEN	(1ULL << 0)	/* Set up a restore token in the shadow stack */
>+
> #include <asm-generic/mman.h>
>
> #endif /* _ASM_X86_MMAN_H */
>diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
>index 40f0a55762a9..0a3decab70ee 100644
>--- a/arch/x86/kernel/shstk.c
>+++ b/arch/x86/kernel/shstk.c
>@@ -17,6 +17,7 @@
> #include <linux/compat.h>
> #include <linux/sizes.h>
> #include <linux/user.h>
>+#include <linux/syscalls.h>
> #include <asm/msr.h>
> #include <asm/fpu/xstate.h>
> #include <asm/fpu/types.h>
>@@ -71,19 +72,31 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr)
> 	return 0;
> }
>
>-static unsigned long alloc_shstk(unsigned long size)
>+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 addr, unused;
>+	unsigned long mapped_addr, unused;
>
>-	mmap_write_lock(mm);
>-	addr = do_mmap(NULL, 0, size, PROT_READ, flags,
>-		       VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL);
>+	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);
>
>-	return addr;
>+	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)
>@@ -134,7 +147,7 @@ static int shstk_setup(void)
> 		return -EOPNOTSUPP;
>
> 	size = adjust_shstk_size(0);
>-	addr = alloc_shstk(size);
>+	addr = alloc_shstk(0, size, 0, false);
> 	if (IS_ERR_VALUE(addr))
> 		return PTR_ERR((void *)addr);
>
>@@ -178,7 +191,7 @@ int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
> 		return 0;
>
> 	size = adjust_shstk_size(stack_size);
>-	addr = alloc_shstk(size);
>+	addr = alloc_shstk(0, size, 0, false);
> 	if (IS_ERR_VALUE(addr))
> 		return PTR_ERR((void *)addr);
>
>@@ -371,6 +384,36 @@ 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 -EINVAL;
>+
>+	if (addr && addr <= 0xFFFFFFFF)
>+		return -EINVAL;
>+
>+	/*
>+	 * 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 features)
> {
> 	if (option == ARCH_SHSTK_LOCK) {
>diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>index 33a0ee3bcb2e..392dc11e3556 100644
>--- a/include/linux/syscalls.h
>+++ b/include/linux/syscalls.h
>@@ -1058,6 +1058,7 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
> asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
> 					    unsigned long home_node,
> 					    unsigned long flags);
>+asmlinkage long sys_map_shadow_stack(unsigned long addr, unsigned long size, unsigned int flags);
>
> /*
>  * Architecture-specific system calls
>diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
>index 45fa180cc56a..b12940ec5926 100644
>--- a/include/uapi/asm-generic/unistd.h
>+++ b/include/uapi/asm-generic/unistd.h
>@@ -887,7 +887,7 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
> __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
>
> #undef __NR_syscalls
>-#define __NR_syscalls 451
>+#define __NR_syscalls 452
>
> /*
>  * 32 bit systems traditionally used different
>diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
>index 860b2dcf3ac4..cb9aebd34646 100644
>--- a/kernel/sys_ni.c
>+++ b/kernel/sys_ni.c
>@@ -381,6 +381,7 @@ COND_SYSCALL(vm86old);
> COND_SYSCALL(modify_ldt);
> COND_SYSCALL(vm86);
> COND_SYSCALL(kexec_file_load);
>+COND_SYSCALL(map_shadow_stack);
>
> /* s390 */
> COND_SYSCALL(s390_pci_mmio_read);
>-- 
>2.17.1
>