IBT state machine tracks CALL/JMP instructions. When a such instruction is executed and before arriving at an ENDBR, it is in WAIT_FOR_ENDBR state, which can be read from CET_WAIT_ENDBR bit of MSR_IA32_U_CET. Further details are described in Intel SDM Vol. 1, Sec. 18.3. In handling signals, WAIT_FOR_ENDBR state is saved/restored with a new UC_WAIT_ENDBR flag being introduced. A legacy IA32 signal frame does not have ucontext, and cannot be supported with a uc flag. Thus, IBT feature is not supported for ia32 app's, which is handled in a separate patch. Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxx> Cc: Florian Weimer <fweimer@xxxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Link: https://lore.kernel.org/linux-api/f6e61dae-9805-c855-8873-7481ceb7ea79@xxxxxxxxx/ --- arch/x86/ia32/ia32_signal.c | 15 ++++++++-- arch/x86/include/asm/cet.h | 4 +++ arch/x86/include/uapi/asm/ucontext.h | 5 ++++ arch/x86/kernel/ibt.c | 41 ++++++++++++++++++++++++++++ arch/x86/kernel/signal.c | 6 ++++ 5 files changed, 68 insertions(+), 3 deletions(-) diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c index d7a30bc98e66..77d0fa90cc19 100644 --- a/arch/x86/ia32/ia32_signal.c +++ b/arch/x86/ia32/ia32_signal.c @@ -129,6 +129,7 @@ COMPAT_SYSCALL_DEFINE0(rt_sigreturn) { struct pt_regs *regs = current_pt_regs(); struct rt_sigframe_ia32 __user *frame; + unsigned int uc_flags; sigset_t set; frame = (struct rt_sigframe_ia32 __user *)(regs->sp - 4); @@ -137,6 +138,11 @@ COMPAT_SYSCALL_DEFINE0(rt_sigreturn) goto badframe; if (__get_user(set.sig[0], (__u64 __user *)&frame->uc.uc_sigmask)) goto badframe; + if (__get_user(uc_flags, &frame->uc.uc_flags)) + goto badframe; + + if (uc_flags & UC_WAIT_ENDBR) + ibt_set_wait_endbr(); set_current_blocked(&set); @@ -312,6 +318,7 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig, compat_sigset_t *set, struct pt_regs *regs) { struct rt_sigframe_ia32 __user *frame; + unsigned int uc_flags = 0; void __user *restorer; void __user *fp = NULL; @@ -339,6 +346,9 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig, if (setup_signal_shadow_stack(1, restorer)) return -EFAULT; + if (ibt_get_clear_wait_endbr()) + uc_flags |= UC_WAIT_ENDBR; + if (!user_access_begin(frame, sizeof(*frame))) return -EFAULT; @@ -348,9 +358,8 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig, /* Create the ucontext. */ if (static_cpu_has(X86_FEATURE_XSAVE)) - unsafe_put_user(UC_FP_XSTATE, &frame->uc.uc_flags, Efault); - else - unsafe_put_user(0, &frame->uc.uc_flags, Efault); + uc_flags |= UC_FP_XSTATE; + unsafe_put_user(uc_flags, &frame->uc.uc_flags, Efault); unsafe_put_user(0, &frame->uc.uc_link, Efault); unsafe_compat_save_altstack(&frame->uc.uc_stack, regs->sp, Efault); diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h index bc04b2d3487d..152c717c9cdb 100644 --- a/arch/x86/include/asm/cet.h +++ b/arch/x86/include/asm/cet.h @@ -43,9 +43,13 @@ static inline int restore_signal_shadow_stack(void) { return 0; } #ifdef CONFIG_X86_IBT int ibt_setup(void); void ibt_disable(void); +int ibt_get_clear_wait_endbr(void); +int ibt_set_wait_endbr(void); #else static inline int ibt_setup(void) { return 0; } static inline void ibt_disable(void) {} +static inline int ibt_get_clear_wait_endbr(void) { return 0; } +static inline int ibt_set_wait_endbr(void) { return 0; } #endif #ifdef CONFIG_X86_SHADOW_STACK diff --git a/arch/x86/include/uapi/asm/ucontext.h b/arch/x86/include/uapi/asm/ucontext.h index 5657b7a49f03..905419de2cc7 100644 --- a/arch/x86/include/uapi/asm/ucontext.h +++ b/arch/x86/include/uapi/asm/ucontext.h @@ -51,6 +51,11 @@ #define UC_STRICT_RESTORE_SS 0x4 #endif +/* + * Indicates IBT WAIT-ENDBR status. + */ +#define UC_WAIT_ENDBR 0x08 + #include <asm-generic/ucontext.h> #endif /* _ASM_X86_UCONTEXT_H */ diff --git a/arch/x86/kernel/ibt.c b/arch/x86/kernel/ibt.c index 4ab7af33b274..5ab8632a1f7e 100644 --- a/arch/x86/kernel/ibt.c +++ b/arch/x86/kernel/ibt.c @@ -56,3 +56,44 @@ void ibt_disable(void) ibt_set_clear_msr_bits(0, CET_ENDBR_EN); current->thread.shstk.ibt = 0; } + +int ibt_get_clear_wait_endbr(void) +{ + u64 msr_val = 0; + + if (!current->thread.shstk.ibt) + return 0; + + fpregs_lock(); + + if (!test_thread_flag(TIF_NEED_FPU_LOAD)) { + if (!rdmsrl_safe(MSR_IA32_U_CET, &msr_val)) + wrmsrl(MSR_IA32_U_CET, msr_val & ~CET_WAIT_ENDBR); + } else { + struct cet_user_state *cet; + + /* + * If !TIF_NEED_FPU_LOAD and get_xsave_addr() returns zero, + * XFEATURE_CET_USER is in init state (cet is not active). + * Return zero status. + */ + cet = get_xsave_addr(¤t->thread.fpu.state.xsave, + XFEATURE_CET_USER); + if (cet) { + msr_val = cet->user_cet; + cet->user_cet = msr_val & ~CET_WAIT_ENDBR; + } + } + + fpregs_unlock(); + + return msr_val & CET_WAIT_ENDBR; +} + +int ibt_set_wait_endbr(void) +{ + if (!current->thread.shstk.ibt) + return 0; + + return ibt_set_clear_msr_bits(CET_WAIT_ENDBR, 0); +} diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c index 661e46803b84..a1285650852e 100644 --- a/arch/x86/kernel/signal.c +++ b/arch/x86/kernel/signal.c @@ -135,6 +135,9 @@ static int restore_sigcontext(struct pt_regs *regs, */ if (unlikely(!(uc_flags & UC_STRICT_RESTORE_SS) && user_64bit_mode(regs))) force_valid_ss(regs); + + if (uc_flags & UC_WAIT_ENDBR) + ibt_set_wait_endbr(); #endif return fpu__restore_sig((void __user *)sc.fpstate, @@ -455,6 +458,9 @@ static unsigned long frame_uc_flags(struct pt_regs *regs) if (likely(user_64bit_mode(regs))) flags |= UC_STRICT_RESTORE_SS; + if (ibt_get_clear_wait_endbr()) + flags |= UC_WAIT_ENDBR; + return flags; } -- 2.21.0
