[PATCH v11 24/25] x86/cet/shstk: Handle thread shadow stack
From: Yu-cheng Yu <hidden>
Date: 2020-08-25 00:30:19
Also in:
linux-arch, linux-doc, linux-mm, lkml
Subsystem:
the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
The kernel allocates (and frees on thread exit) a new shadow stack for a
pthread child.
It is possible for the kernel to complete the clone syscall and set the
child's shadow stack pointer to NULL and let the child thread allocate
a shadow stack for itself. There are two issues in this approach: It
is not compatible with existing code that does inline syscall and it
cannot handle signals before the child can successfully allocate a
shadow stack.
A 64-bit shadow stack has a size of min(RLIMIT_STACK, 4 GB). A compat-mode
thread shadow stack has a size of 1/4 min(RLIMIT_STACK, 4 GB). This allows
more threads to run in a 32-bit address space.
Signed-off-by: Yu-cheng Yu <redacted>
---
v10:
- Limit shadow stack size to 4 GB.
arch/x86/include/asm/cet.h | 2 ++
arch/x86/include/asm/mmu_context.h | 3 +++
arch/x86/kernel/cet.c | 41 ++++++++++++++++++++++++++++++
arch/x86/kernel/process.c | 7 +++++
4 files changed, 53 insertions(+)
diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index 56fe08eebae6..71dc92acd2f2 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h@@ -18,11 +18,13 @@ struct cet_status { #ifdef CONFIG_X86_INTEL_CET int cet_setup_shstk(void); +int cet_setup_thread_shstk(struct task_struct *p); void cet_disable_free_shstk(struct task_struct *p); int cet_verify_rstor_token(bool ia32, unsigned long ssp, unsigned long *new_ssp); void cet_restore_signal(struct sc_ext *sc); int cet_setup_signal(bool ia32, unsigned long rstor, struct sc_ext *sc); #else +static inline int cet_setup_thread_shstk(struct task_struct *p) { return 0; } static inline void cet_disable_free_shstk(struct task_struct *p) {} static inline void cet_restore_signal(struct sc_ext *sc) { return; } static inline int cet_setup_signal(bool ia32, unsigned long rstor,
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index d98016b83755..e3ca4397258b 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h@@ -11,6 +11,7 @@ #include <asm/tlbflush.h> #include <asm/paravirt.h> +#include <asm/cet.h> #include <asm/debugreg.h> extern atomic64_t last_mm_ctx_id;
@@ -142,6 +143,8 @@ do { \ #else #define deactivate_mm(tsk, mm) \ do { \ + if (!tsk->vfork_done) \ + cet_disable_free_shstk(tsk); \ load_gs_index(0); \ loadsegment(fs, 0); \ } while (0)
diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index a08a956ac3aa..b30c61a66c8e 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c@@ -172,6 +172,47 @@ int cet_setup_shstk(void) return 0; } +int cet_setup_thread_shstk(struct task_struct *tsk) +{ + unsigned long addr, size; + struct cet_user_state *state; + struct cet_status *cet = &tsk->thread.cet; + + if (!cet->shstk_size) + return 0; + + state = get_xsave_addr(&tsk->thread.fpu.state.xsave, + XFEATURE_CET_USER); + + if (!state) + return -EINVAL; + + /* Cap shadow stack size to 4 GB */ + size = min(rlimit(RLIMIT_STACK), 1UL << 32); + + /* + * Compat-mode pthreads share a limited address space. + * If each function call takes an average of four slots + * stack space, we need 1/4 of stack size for shadow stack. + */ + if (in_compat_syscall()) + size /= 4; + size = round_up(size, PAGE_SIZE); + addr = alloc_shstk(size, 0); + + if (IS_ERR_VALUE(addr)) { + cet->shstk_base = 0; + cet->shstk_size = 0; + return PTR_ERR((void *)addr); + } + + fpu__prepare_write(&tsk->thread.fpu); + state->user_ssp = (u64)(addr + size); + cet->shstk_base = addr; + cet->shstk_size = size; + return 0; +} + void cet_disable_free_shstk(struct task_struct *tsk) { struct cet_status *cet = &tsk->thread.cet;
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index e41f1a468ee3..b6fe5b061841 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c@@ -109,6 +109,7 @@ void exit_thread(struct task_struct *tsk) free_vm86(t); + cet_disable_free_shstk(tsk); fpu__drop(fpu); }
@@ -181,6 +182,12 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg, if (clone_flags & CLONE_SETTLS) ret = set_new_tls(p, tls); +#ifdef CONFIG_X86_64 + /* Allocate a new shadow stack for pthread */ + if (!ret && (clone_flags & (CLONE_VFORK | CLONE_VM)) == CLONE_VM) + ret = cet_setup_thread_shstk(p); +#endif + if (!ret && unlikely(test_tsk_thread_flag(current, TIF_IO_BITMAP))) io_bitmap_share(p);
--
2.21.0