[PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack

From: Yu-cheng Yu
Date: Tue Apr 27 2021 - 16:45:59 EST

When shadow stack is enabled, a task's shadow stack states must be saved
along with the signal context and later restored in sigreturn. However,
currently there is no systematic facility for extending a signal context.
There is some space left in the ucontext, but changing ucontext is likely
to create compatibility issues and there is not enough space for further
extensions.

Introduce a signal context extension struct 'sc_ext', which is used to save
shadow stack restore token address. The extension is located above the fpu
states, plus alignment. The struct can be extended (such as the ibt's
wait_endbr status to be introduced later), and sc_ext.total_size field
keeps track of total size.

Introduce routines for the allocation, save, and restore for sc_ext:
- fpu__alloc_sigcontext_ext(),
- save_extra_state_to_sigframe(),
- get_extra_state_from_sigframe(),
- restore_extra_state_to_xregs().

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
---
v25:
- Update commit log/comments for the sc_ext struct.
- Use restorer address already calculated.
- Change CONFIG_X86_CET to CONFIG_X86_SHADOW_STACK.
- Change X86_FEATURE_CET to X86_FEATURE_SHSTK.
- Eliminate writing to MSR_IA32_U_CET for shadow stack.
- Change wrmsrl() to wrmsrl_safe() and handle error.

v24:
- Split out shadow stack token routines to a separate patch.
- Put signal frame save/restore routines to fpu/signal.c and re-name accordingly.

arch/x86/ia32/ia32_signal.c | 24 +++--
arch/x86/include/asm/cet.h | 2 +
arch/x86/include/asm/fpu/internal.h | 2 +
arch/x86/include/uapi/asm/sigcontext.h | 9 ++
arch/x86/kernel/fpu/signal.c | 137 ++++++++++++++++++++++++-
arch/x86/kernel/signal.c | 9 ++
6 files changed, 172 insertions(+), 11 deletions(-)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 5e3d9b7fd5fb..423abcd181f2 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -202,7 +202,8 @@ do { \
*/
static void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,
size_t frame_size,
- void __user **fpstate)
+ void __user **fpstate,
+ void __user *restorer)
{
unsigned long sp, fx_aligned, math_size;

@@ -220,6 +221,10 @@ static void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,

sp = fpu__alloc_mathframe(sp, 1, &fx_aligned, &math_size);
*fpstate = (struct _fpstate_32 __user *) sp;
+
+ if (save_extra_state_to_sigframe(1, *fpstate, restorer))
+ return (void __user *)-1L;
+
if (copy_fpstate_to_sigframe(*fpstate, (void __user *)fx_aligned,
math_size) < 0)
return (void __user *) -1L;
@@ -249,8 +254,6 @@ int ia32_setup_frame(int sig, struct ksignal *ksig,
0x80cd, /* int $0x80 */
};

- frame = get_sigframe(ksig, regs, sizeof(*frame), &fp);
-
if (ksig->ka.sa.sa_flags & SA_RESTORER) {
restorer = ksig->ka.sa.sa_restorer;
} else {
@@ -262,6 +265,8 @@ int ia32_setup_frame(int sig, struct ksignal *ksig,
restorer = &frame->retcode;
}

+ frame = get_sigframe(ksig, regs, sizeof(*frame), &fp, restorer);
+
if (!user_access_begin(frame, sizeof(*frame)))
return -EFAULT;

@@ -317,7 +322,13 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig,
0,
};

- frame = get_sigframe(ksig, regs, sizeof(*frame), &fp);
+ if (ksig->ka.sa.sa_flags & SA_RESTORER)
+ restorer = ksig->ka.sa.sa_restorer;
+ else
+ restorer = current->mm->context.vdso +
+ vdso_image_32.sym___kernel_rt_sigreturn;
+
+ frame = get_sigframe(ksig, regs, sizeof(*frame), &fp, restorer);

if (!user_access_begin(frame, sizeof(*frame)))
return -EFAULT;
@@ -334,11 +345,6 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig,
unsafe_put_user(0, &frame->uc.uc_link, Efault);
unsafe_compat_save_altstack(&frame->uc.uc_stack, regs->sp, Efault);

- if (ksig->ka.sa.sa_flags & SA_RESTORER)
- restorer = ksig->ka.sa.sa_restorer;
- else
- restorer = current->mm->context.vdso +
- vdso_image_32.sym___kernel_rt_sigreturn;
unsafe_put_user(ptr_to_compat(restorer), &frame->pretcode, Efault);

/*
diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index ef6155213b7e..5e66919bd2fe 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -6,6 +6,8 @@
#include <linux/types.h>

struct task_struct;
+struct sc_ext;
+
/*
* Per-thread CET status
*/
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 8d33ad80704f..2a3c6a160dd6 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -443,6 +443,8 @@ static inline void copy_kernel_to_fpregs(union fpregs_state *fpstate)
__copy_kernel_to_fpregs(fpstate, -1);
}

+extern int save_extra_state_to_sigframe(int ia32, void __user *fp,
+ void __user *restorer);
extern int copy_fpstate_to_sigframe(void __user *buf, void __user *fp, int size);

/*
diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index 844d60eb1882..10d7fa192d48 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -196,6 +196,15 @@ struct _xstate {
/* New processor state extensions go here: */
};

+/*
+ * Located at the end of sigcontext->fpstate, aligned to 8.
+ * total_size keeps track of further extension of the struct.
+ */
+struct sc_ext {
+ unsigned long total_size;
+ unsigned long ssp;
+};
+
/*
* The 32-bit signal frame:
*/
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index a4ec65317a7f..0488407bec81 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -52,6 +52,107 @@ static inline int check_for_xstate(struct fxregs_state __user *buf,
return 0;
}

+int save_extra_state_to_sigframe(int ia32, void __user *fp, void __user *restorer)
+{
+ int err = 0;
+
+#ifdef CONFIG_X86_SHADOW_STACK
+ struct cet_status *cet = &current->thread.cet;
+ unsigned long token_addr = 0, new_ssp = 0;
+ struct sc_ext ext = {};
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return 0;
+
+ if (cet->shstk_size) {
+ err = shstk_setup_rstor_token(ia32, (unsigned long)restorer,
+ &token_addr, &new_ssp);
+ if (err)
+ return err;
+
+ ext.ssp = token_addr;
+
+ fpregs_lock();
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ __fpregs_load_activate();
+ if (new_ssp)
+ err = wrmsrl_safe(MSR_IA32_PL3_SSP, new_ssp);
+ fpregs_unlock();
+ }
+
+ if (!err && ext.ssp) {
+ void __user *p = fp;
+
+ ext.total_size = sizeof(ext);
+
+ p = fp;
+ if (ia32)
+ p += sizeof(struct fregs_state);
+
+ p += fpu_user_xstate_size + FP_XSTATE_MAGIC2_SIZE;
+ p = (void __user *)ALIGN((unsigned long)p, 8);
+
+ if (copy_to_user(p, &ext, sizeof(ext)))
+ return -EFAULT;
+ }
+#endif
+ return err;
+}
+
+static int get_extra_state_from_sigframe(int ia32, void __user *fp, struct sc_ext *ext)
+{
+ int err = 0;
+
+#ifdef CONFIG_X86_SHADOW_STACK
+ struct cet_status *cet = &current->thread.cet;
+ void __user *p;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return 0;
+
+ if (!cet->shstk_size)
+ return 0;
+
+ memset(ext, 0, sizeof(*ext));
+
+ p = fp;
+ if (ia32)
+ p += sizeof(struct fregs_state);
+
+ p += fpu_user_xstate_size + FP_XSTATE_MAGIC2_SIZE;
+ p = (void __user *)ALIGN((unsigned long)p, 8);
+
+ if (copy_from_user(ext, p, sizeof(*ext)))
+ return -EFAULT;
+
+ if (ext->total_size != sizeof(*ext))
+ return -EFAULT;
+
+ if (cet->shstk_size)
+ err = shstk_check_rstor_token(ia32, ext->ssp, &ext->ssp);
+#endif
+ return err;
+}
+
+/*
+ * Called from __fpu__restore_sig() and protected by fpregs_lock().
+ */
+static int restore_extra_state_to_xregs(struct sc_ext *sc_ext)
+{
+ int err = 0;
+
+#ifdef CONFIG_X86_SHADOW_STACK
+ struct cet_status *cet = &current->thread.cet;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return 0;
+
+ if (cet->shstk_size)
+ err = wrmsrl_safe(MSR_IA32_PL3_SSP, sc_ext->ssp);
+#endif
+ return err;
+}
+
/*
* Signal frame handlers.
*/
@@ -295,6 +396,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
struct task_struct *tsk = current;
struct fpu *fpu = &tsk->thread.fpu;
struct user_i387_ia32_struct env;
+ struct sc_ext sc_ext;
u64 user_xfeatures = 0;
int fx_only = 0;
int ret = 0;
@@ -335,6 +437,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
if ((unsigned long)buf_fx % 64)
fx_only = 1;

+ ret = get_extra_state_from_sigframe(ia32_fxstate, buf, &sc_ext);
+ if (ret)
+ return ret;
+
if (!ia32_fxstate) {
/*
* Attempt to restore the FPU registers directly from user
@@ -365,9 +471,17 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
xfeatures_mask_supervisor())
copy_kernel_to_xregs(&fpu->state.xsave,
xfeatures_mask_supervisor());
- fpregs_mark_activate();
+
+ ret = restore_extra_state_to_xregs(&sc_ext);
+ if (!ret) {
+ fpregs_mark_activate();
+ } else {
+ fpregs_deactivate(fpu);
+ fpu__clear_user_states(fpu);
+ }
+
fpregs_unlock();
- return 0;
+ return ret;
}
fpregs_unlock();
} else {
@@ -430,6 +544,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
ret = copy_kernel_to_xregs_err(&fpu->state.xsave,
user_xfeatures | xfeatures_mask_supervisor());

+ if (!ret)
+ ret = restore_extra_state_to_xregs(&sc_ext);
} else if (use_fxsr()) {
ret = __copy_from_user(&fpu->state.fxsave, buf_fx, state_size);
if (ret) {
@@ -491,12 +607,29 @@ int fpu__restore_sig(void __user *buf, int ia32_frame)
return __fpu__restore_sig(buf, buf_fx, size);
}

+static unsigned long fpu__alloc_sigcontext_ext(unsigned long sp)
+{
+#ifdef CONFIG_X86_SHADOW_STACK
+ struct cet_status *cet = &current->thread.cet;
+
+ /*
+ * sigcontext_ext is at: fpu + fpu_user_xstate_size +
+ * FP_XSTATE_MAGIC2_SIZE, then aligned to 8.
+ */
+ if (cet->shstk_size)
+ sp -= (sizeof(struct sc_ext) + 8);
+#endif
+ return sp;
+}
+
unsigned long
fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
unsigned long *buf_fx, unsigned long *size)
{
unsigned long frame_size = xstate_sigframe_size();

+ sp = fpu__alloc_sigcontext_ext(sp);
+
*buf_fx = sp = round_down(sp - frame_size, 64);
if (ia32_frame && use_fxsr()) {
frame_size += sizeof(struct fregs_state);
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index f306e85a08a6..56d2412174c8 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -239,6 +239,9 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
unsigned long buf_fx = 0;
int onsigstack = on_sig_stack(sp);
int ret;
+#ifdef CONFIG_X86_64
+ void __user *restorer = NULL;
+#endif

/* redzone */
if (IS_ENABLED(CONFIG_X86_64))
@@ -270,6 +273,12 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
if (onsigstack && !likely(on_sig_stack(sp)))
return (void __user *)-1L;

+#ifdef CONFIG_X86_64
+ if (ka->sa.sa_flags & SA_RESTORER)
+ restorer = ka->sa.sa_restorer;
+ ret = save_extra_state_to_sigframe(0, *fpstate, restorer);
+#endif
+
/* save i387 and extended state */
ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size);
if (ret < 0)
--
2.21.0