[tip: x86/shstk] x86/shstk: Add user control-protection fault handler

From: tip-bot2 for Rick Edgecombe
Date: Mon Mar 20 2023 - 12:51:34 EST

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The following commit has been merged into the x86/shstk branch of tip:

Commit-ID: ac591488436ec62d5d43327719ee9ee5c12ab4ba
Gitweb: https://git.kernel.org/tip/ac591488436ec62d5d43327719ee9ee5c12ab4ba
Author: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
AuthorDate: Sat, 18 Mar 2023 17:15:03 -07:00
Committer: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Mon, 20 Mar 2023 09:01:08 -07:00

x86/shstk: Add user control-protection fault handler

A control-protection fault is triggered when a control-flow transfer
attempt violates Shadow Stack or Indirect Branch Tracking constraints.
For example, the return address for a RET instruction differs from the copy
on the shadow stack.

There already exists a control-protection fault handler for handling kernel
IBT faults. Refactor this fault handler into separate user and kernel
handlers, like the page fault handler. Add a control-protection handler
for usermode. To avoid ifdeffery, put them both in a new file cet.c, which
is compiled in the case of either of the two CET features supported in the
kernel: kernel IBT or user mode shadow stack. Move some static inline
functions from traps.c into a header so they can be used in cet.c.

Opportunistically fix a comment in the kernel IBT part of the fault
handler that is on the end of the line instead of preceding it.

Keep the same behavior for the kernel side of the fault handler, except for
converting a BUG to a WARN in the case of a #CP happening when the feature
is missing. This unifies the behavior with the new shadow stack code, and
also prevents the kernel from crashing under this situation which is
potentially recoverable.

The control-protection fault handler works in a similar way as the general
protection fault handler. It provides the si_code SEGV_CPERR to the signal
handler.

Co-developed-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20230319001535.23210-9-rick.p.edgecombe%40intel.com
---
arch/arm/kernel/signal.c | 2 +-
arch/arm64/kernel/signal.c | 2 +-
arch/arm64/kernel/signal32.c | 2 +-
arch/sparc/kernel/signal32.c | 2 +-
arch/sparc/kernel/signal_64.c | 2 +-
arch/x86/include/asm/disabled-features.h | 8 +-
arch/x86/include/asm/idtentry.h | 2 +-
arch/x86/include/asm/traps.h | 12 +++-
arch/x86/kernel/cet.c | 94 ++++++++++++++++++++---
arch/x86/kernel/idt.c | 2 +-
arch/x86/kernel/signal_32.c | 2 +-
arch/x86/kernel/signal_64.c | 2 +-
arch/x86/kernel/traps.c | 12 +---
arch/x86/xen/enlighten_pv.c | 2 +-
arch/x86/xen/xen-asm.S | 2 +-
include/uapi/asm-generic/siginfo.h | 3 +-
16 files changed, 117 insertions(+), 34 deletions(-)

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index e07f359..9a3c9de 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -681,7 +681,7 @@ asmlinkage void do_rseq_syscall(struct pt_regs *regs)
*/
static_assert(NSIGILL == 11);
static_assert(NSIGFPE == 15);
-static_assert(NSIGSEGV == 9);
+static_assert(NSIGSEGV == 10);
static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 06a0270..19b6b29 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -1341,7 +1341,7 @@ void __init minsigstksz_setup(void)
*/
static_assert(NSIGILL == 11);
static_assert(NSIGFPE == 15);
-static_assert(NSIGSEGV == 9);
+static_assert(NSIGSEGV == 10);
static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
index 4700f85..bbd5427 100644
--- a/arch/arm64/kernel/signal32.c
+++ b/arch/arm64/kernel/signal32.c
@@ -460,7 +460,7 @@ void compat_setup_restart_syscall(struct pt_regs *regs)
*/
static_assert(NSIGILL == 11);
static_assert(NSIGFPE == 15);
-static_assert(NSIGSEGV == 9);
+static_assert(NSIGSEGV == 10);
static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
diff --git a/arch/sparc/kernel/signal32.c b/arch/sparc/kernel/signal32.c
index dad3896..82da8a2 100644
--- a/arch/sparc/kernel/signal32.c
+++ b/arch/sparc/kernel/signal32.c
@@ -751,7 +751,7 @@ out:
*/
static_assert(NSIGILL == 11);
static_assert(NSIGFPE == 15);
-static_assert(NSIGSEGV == 9);
+static_assert(NSIGSEGV == 10);
static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
diff --git a/arch/sparc/kernel/signal_64.c b/arch/sparc/kernel/signal_64.c
index 570e43e..b4e4109 100644
--- a/arch/sparc/kernel/signal_64.c
+++ b/arch/sparc/kernel/signal_64.c
@@ -562,7 +562,7 @@ void do_notify_resume(struct pt_regs *regs, unsigned long orig_i0, unsigned long
*/
static_assert(NSIGILL == 11);
static_assert(NSIGFPE == 15);
-static_assert(NSIGSEGV == 9);
+static_assert(NSIGSEGV == 10);
static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
diff --git a/arch/x86/include/asm/disabled-features.h b/arch/x86/include/asm/disabled-features.h
index 505f78d..652e366 100644
--- a/arch/x86/include/asm/disabled-features.h
+++ b/arch/x86/include/asm/disabled-features.h
@@ -105,6 +105,12 @@
#define DISABLE_USER_SHSTK (1 << (X86_FEATURE_USER_SHSTK & 31))
#endif

+#ifdef CONFIG_X86_KERNEL_IBT
+#define DISABLE_IBT 0
+#else
+#define DISABLE_IBT (1 << (X86_FEATURE_IBT & 31))
+#endif
+
/*
* Make sure to add features to the correct mask
*/
@@ -128,7 +134,7 @@
#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP| \
DISABLE_ENQCMD)
#define DISABLED_MASK17 0
-#define DISABLED_MASK18 0
+#define DISABLED_MASK18 (DISABLE_IBT)
#define DISABLED_MASK19 0
#define DISABLED_MASK20 0
#define DISABLED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 21)
diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index b241af4..61e0e63 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -614,7 +614,7 @@ DECLARE_IDTENTRY_RAW_ERRORCODE(X86_TRAP_DF, xenpv_exc_double_fault);
#endif

/* #CP */
-#ifdef CONFIG_X86_KERNEL_IBT
+#ifdef CONFIG_X86_CET
DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_CP, exc_control_protection);
#endif

diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index 47ecfff..75e0dab 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -47,4 +47,16 @@ void __noreturn handle_stack_overflow(struct pt_regs *regs,
struct stack_info *info);
#endif

+static inline void cond_local_irq_enable(struct pt_regs *regs)
+{
+ if (regs->flags & X86_EFLAGS_IF)
+ local_irq_enable();
+}
+
+static inline void cond_local_irq_disable(struct pt_regs *regs)
+{
+ if (regs->flags & X86_EFLAGS_IF)
+ local_irq_disable();
+}
+
#endif /* _ASM_X86_TRAPS_H */
diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index 7ad22b7..cc10d8b 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c
@@ -4,10 +4,6 @@
#include <asm/bugs.h>
#include <asm/traps.h>

-static __ro_after_init bool ibt_fatal = true;
-
-extern void ibt_selftest_ip(void); /* code label defined in asm below */
-
enum cp_error_code {
CP_EC = (1 << 15) - 1,

@@ -20,15 +16,80 @@ enum cp_error_code {
CP_ENCL = 1 << 15,
};

-DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
+static const char cp_err[][10] = {
+ [0] = "unknown",
+ [1] = "near ret",
+ [2] = "far/iret",
+ [3] = "endbranch",
+ [4] = "rstorssp",
+ [5] = "setssbsy",
+};
+
+static const char *cp_err_string(unsigned long error_code)
+{
+ unsigned int cpec = error_code & CP_EC;
+
+ if (cpec >= ARRAY_SIZE(cp_err))
+ cpec = 0;
+ return cp_err[cpec];
+}
+
+static void do_unexpected_cp(struct pt_regs *regs, unsigned long error_code)
+{
+ WARN_ONCE(1, "Unexpected %s #CP, error_code: %s\n",
+ user_mode(regs) ? "user mode" : "kernel mode",
+ cp_err_string(error_code));
+}
+
+static DEFINE_RATELIMIT_STATE(cpf_rate, DEFAULT_RATELIMIT_INTERVAL,
+ DEFAULT_RATELIMIT_BURST);
+
+static void do_user_cp_fault(struct pt_regs *regs, unsigned long error_code)
{
- if (!cpu_feature_enabled(X86_FEATURE_IBT)) {
- pr_err("Unexpected #CP\n");
- BUG();
+ struct task_struct *tsk;
+ unsigned long ssp;
+
+ /*
+ * An exception was just taken from userspace. Since interrupts are disabled
+ * here, no scheduling should have messed with the registers yet and they
+ * will be whatever is live in userspace. So read the SSP before enabling
+ * interrupts so locking the fpregs to do it later is not required.
+ */
+ rdmsrl(MSR_IA32_PL3_SSP, ssp);
+
+ cond_local_irq_enable(regs);
+
+ tsk = current;
+ tsk->thread.error_code = error_code;
+ tsk->thread.trap_nr = X86_TRAP_CP;
+
+ /* Ratelimit to prevent log spamming. */
+ if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
+ __ratelimit(&cpf_rate)) {
+ pr_emerg("%s[%d] control protection ip:%lx sp:%lx ssp:%lx error:%lx(%s)%s",
+ tsk->comm, task_pid_nr(tsk),
+ regs->ip, regs->sp, ssp, error_code,
+ cp_err_string(error_code),
+ error_code & CP_ENCL ? " in enclave" : "");
+ print_vma_addr(KERN_CONT " in ", regs->ip);
+ pr_cont("\n");
}

- if (WARN_ON_ONCE(user_mode(regs) || (error_code & CP_EC) != CP_ENDBR))
+ force_sig_fault(SIGSEGV, SEGV_CPERR, (void __user *)0);
+ cond_local_irq_disable(regs);
+}
+
+static __ro_after_init bool ibt_fatal = true;
+
+/* code label defined in asm below */
+extern void ibt_selftest_ip(void);
+
+static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
+{
+ if ((error_code & CP_EC) != CP_ENDBR) {
+ do_unexpected_cp(regs, error_code);
return;
+ }

if (unlikely(regs->ip == (unsigned long)&ibt_selftest_ip)) {
regs->ax = 0;
@@ -74,3 +135,18 @@ static int __init ibt_setup(char *str)
}

__setup("ibt=", ibt_setup);
+
+DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
+{
+ if (user_mode(regs)) {
+ if (cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+ do_user_cp_fault(regs, error_code);
+ else
+ do_unexpected_cp(regs, error_code);
+ } else {
+ if (cpu_feature_enabled(X86_FEATURE_IBT))
+ do_kernel_cp_fault(regs, error_code);
+ else
+ do_unexpected_cp(regs, error_code);
+ }
+}
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index a58c6bc..5074b84 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -107,7 +107,7 @@ static const __initconst struct idt_data def_idts[] = {
ISTG(X86_TRAP_MC, asm_exc_machine_check, IST_INDEX_MCE),
#endif

-#ifdef CONFIG_X86_KERNEL_IBT
+#ifdef CONFIG_X86_CET
INTG(X86_TRAP_CP, asm_exc_control_protection),
#endif

diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
index 9027fc0..c12624b 100644
--- a/arch/x86/kernel/signal_32.c
+++ b/arch/x86/kernel/signal_32.c
@@ -402,7 +402,7 @@ Efault:
*/
static_assert(NSIGILL == 11);
static_assert(NSIGFPE == 15);
-static_assert(NSIGSEGV == 9);
+static_assert(NSIGSEGV == 10);
static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index 13a1e60..0e808c7 100644
--- a/arch/x86/kernel/signal_64.c
+++ b/arch/x86/kernel/signal_64.c
@@ -403,7 +403,7 @@ void sigaction_compat_abi(struct k_sigaction *act, struct k_sigaction *oact)
*/
static_assert(NSIGILL == 11);
static_assert(NSIGFPE == 15);
-static_assert(NSIGSEGV == 9);
+static_assert(NSIGSEGV == 10);
static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index cc223e6..18fb9d6 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -77,18 +77,6 @@

DECLARE_BITMAP(system_vectors, NR_VECTORS);

-static inline void cond_local_irq_enable(struct pt_regs *regs)
-{
- if (regs->flags & X86_EFLAGS_IF)
- local_irq_enable();
-}
-
-static inline void cond_local_irq_disable(struct pt_regs *regs)
-{
- if (regs->flags & X86_EFLAGS_IF)
- local_irq_disable();
-}
-
__always_inline int is_valid_bugaddr(unsigned long addr)
{
if (addr < TASK_SIZE_MAX)
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index bb59cc6..9c29cd5 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -640,7 +640,7 @@ static struct trap_array_entry trap_array[] = {
TRAP_ENTRY(exc_coprocessor_error, false ),
TRAP_ENTRY(exc_alignment_check, false ),
TRAP_ENTRY(exc_simd_coprocessor_error, false ),
-#ifdef CONFIG_X86_KERNEL_IBT
+#ifdef CONFIG_X86_CET
TRAP_ENTRY(exc_control_protection, false ),
#endif
};
diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
index 4a184f6..7cdcb4c 100644
--- a/arch/x86/xen/xen-asm.S
+++ b/arch/x86/xen/xen-asm.S
@@ -148,7 +148,7 @@ xen_pv_trap asm_exc_page_fault
xen_pv_trap asm_exc_spurious_interrupt_bug
xen_pv_trap asm_exc_coprocessor_error
xen_pv_trap asm_exc_alignment_check
-#ifdef CONFIG_X86_KERNEL_IBT
+#ifdef CONFIG_X86_CET
xen_pv_trap asm_exc_control_protection
#endif
#ifdef CONFIG_X86_MCE
diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index ffbe4ce..0f52d0a 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -242,7 +242,8 @@ typedef struct siginfo {
#define SEGV_ADIPERR 7 /* Precise MCD exception */
#define SEGV_MTEAERR 8 /* Asynchronous ARM MTE error */
#define SEGV_MTESERR 9 /* Synchronous ARM MTE exception */
-#define NSIGSEGV 9
+#define SEGV_CPERR 10 /* Control protection fault */
+#define NSIGSEGV 10

/*
* SIGBUS si_codes

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