[PATCH v5 0/2] x86/fred: Prevent immediate repeat of single step trap on return from SIGTRAP handler

[PATCH v5 0/2] x86/fred: Prevent immediate repeat of single step trap on return from SIGTRAP handler

[Xin Li (Intel)]

IDT event delivery has a debug hole in which it does not generate #DB
upon returning to userspace before the first userspace instruction is
executed if the Trap Flag (TF) is set.

FRED closes this hole by introducing a software event flag, i.e., bit
17 of the augmented SS: if the bit is set and ERETU would result in
RFLAGS.TF = 1, a single-step trap will be pending upon completion of
ERETU.

However I overlooked properly setting and clearing the bit in different
situations.  Thus when FRED is enabled, if the Trap Flag (TF) is set
without an external debugger attached, it can lead to an infinite loop
in the SIGTRAP handler.  To avoid this, the software event flag in the
augmented SS must be cleared, ensuring that no single-step trap remains
pending when ERETU completes.

This patch set combines the fix [1] and its corresponding selftest [2]
(requested by Dave Hansen) into one patch set.

[1] https://lore.kernel.org/lkml/20250523050153.3308237-1-xin@zytor.com/
[2] https://lore.kernel.org/lkml/20250530230707.2528916-1-xin@zytor.com/


This patch set is based on tip/x86/urgent branch as of today.


Link to v4 of this patch set:
https://lore.kernel.org/lkml/20250605181020.590459-1-xin@zytor.com/

Changes in v5:
*) Accurately rephrase the shortlog (hpa).
*) Do "sub $-128, %rsp" rather than "add $128, %rsp", which is more
   efficient in code size (hpa).
*) Add TB from Sohil.
*) Add Cc: stable@vger.kernel.org to all patches.


Xin Li (Intel) (2):
  x86/fred/signal: Prevent immediate repeat of single step trap on
    return from SIGTRAP handler
  selftests/x86: Add a test to detect infinite sigtrap handler loop

 arch/x86/include/asm/sighandling.h         | 22 +++++
 arch/x86/kernel/signal_32.c                |  4 +
 arch/x86/kernel/signal_64.c                |  4 +
 tools/testing/selftests/x86/Makefile       |  2 +-
 tools/testing/selftests/x86/sigtrap_loop.c | 98 ++++++++++++++++++++++
 5 files changed, 129 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/x86/sigtrap_loop.c


base-commit: dd2922dcfaa3296846265e113309e5f7f138839f
-- 
2.49.0

[PATCH v5 1/2] x86/fred/signal: Prevent immediate repeat of single step trap on return from SIGTRAP handler

[Xin Li (Intel)]

Clear the software event flag in the augmented SS to prevent immediate
repeat of single step trap on return from SIGTRAP handler if the trap
flag (TF) is set without an external debugger attached.

Following is a typical single-stepping flow for a user process:

1) The user process is prepared for single-stepping by setting
   RFLAGS.TF = 1.
2) When any instruction in user space completes, a #DB is triggered.
3) The kernel handles the #DB and returns to user space, invoking the
   SIGTRAP handler with RFLAGS.TF = 0.
4) After the SIGTRAP handler finishes, the user process performs a
   sigreturn syscall, restoring the original state, including
   RFLAGS.TF = 1.
5) Goto step 2.

According to the FRED specification:

A) Bit 17 in the augmented SS is designated as the software event
   flag, which is set to 1 for FRED event delivery of SYSCALL,
   SYSENTER, or INT n.
B) If bit 17 of the augmented SS is 1 and ERETU would result in
   RFLAGS.TF = 1, a single-step trap will be pending upon completion
   of ERETU.

In step 4) above, the software event flag is set upon the sigreturn
syscall, and its corresponding ERETU would restore RFLAGS.TF = 1.
This combination causes a pending single-step trap upon completion of
ERETU.  Therefore, another #DB is triggered before any user space
instruction is executed, which leads to an infinite loop in which the
SIGTRAP handler keeps being invoked on the same user space IP.

Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Cc: stable@vger.kernel.org
---

Change in v5:
*) Accurately rephrase the shortlog (hpa).

Change in v4:
*) Add a selftest to the patch set (Dave Hansen).

Change in v3:
*) Use "#ifdef CONFIG_X86_FRED" instead of IS_ENABLED(CONFIG_X86_FRED)
   (Intel LKP).

Change in v2:
*) Remove the check cpu_feature_enabled(X86_FEATURE_FRED), because
   regs->fred_ss.swevent will always be 0 otherwise (hpa).
---
 arch/x86/include/asm/sighandling.h | 22 ++++++++++++++++++++++
 arch/x86/kernel/signal_32.c        |  4 ++++
 arch/x86/kernel/signal_64.c        |  4 ++++
 3 files changed, 30 insertions(+)

diff --git a/arch/x86/include/asm/sighandling.h b/arch/x86/include/asm/sighandling.h
index e770c4fc47f4..8727c7e21dd1 100644
--- a/arch/x86/include/asm/sighandling.h
+++ b/arch/x86/include/asm/sighandling.h
@@ -24,4 +24,26 @@ int ia32_setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs);
 int x64_setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs);
 int x32_setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs);
 
+/*
+ * To prevent immediate repeat of single step trap on return from SIGTRAP
+ * handler if the trap flag (TF) is set without an external debugger attached,
+ * clear the software event flag in the augmented SS, ensuring no single-step
+ * trap is pending upon ERETU completion.
+ *
+ * Note, this function should be called in sigreturn() before the original
+ * state is restored to make sure the TF is read from the entry frame.
+ */
+static __always_inline void prevent_single_step_upon_eretu(struct pt_regs *regs)
+{
+	/*
+	 * If the trap flag (TF) is set, i.e., the sigreturn() SYSCALL instruction
+	 * is being single-stepped, do not clear the software event flag in the
+	 * augmented SS, thus a debugger won't skip over the following instruction.
+	 */
+#ifdef CONFIG_X86_FRED
+	if (!(regs->flags & X86_EFLAGS_TF))
+		regs->fred_ss.swevent = 0;
+#endif
+}
+
 #endif /* _ASM_X86_SIGHANDLING_H */
diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
index 98123ff10506..42bbc42bd350 100644
--- a/arch/x86/kernel/signal_32.c
+++ b/arch/x86/kernel/signal_32.c
@@ -152,6 +152,8 @@ SYSCALL32_DEFINE0(sigreturn)
 	struct sigframe_ia32 __user *frame = (struct sigframe_ia32 __user *)(regs->sp-8);
 	sigset_t set;
 
+	prevent_single_step_upon_eretu(regs);
+
 	if (!access_ok(frame, sizeof(*frame)))
 		goto badframe;
 	if (__get_user(set.sig[0], &frame->sc.oldmask)
@@ -175,6 +177,8 @@ SYSCALL32_DEFINE0(rt_sigreturn)
 	struct rt_sigframe_ia32 __user *frame;
 	sigset_t set;
 
+	prevent_single_step_upon_eretu(regs);
+
 	frame = (struct rt_sigframe_ia32 __user *)(regs->sp - 4);
 
 	if (!access_ok(frame, sizeof(*frame)))
diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index ee9453891901..d483b585c6c6 100644
--- a/arch/x86/kernel/signal_64.c
+++ b/arch/x86/kernel/signal_64.c
@@ -250,6 +250,8 @@ SYSCALL_DEFINE0(rt_sigreturn)
 	sigset_t set;
 	unsigned long uc_flags;
 
+	prevent_single_step_upon_eretu(regs);
+
 	frame = (struct rt_sigframe __user *)(regs->sp - sizeof(long));
 	if (!access_ok(frame, sizeof(*frame)))
 		goto badframe;
@@ -366,6 +368,8 @@ COMPAT_SYSCALL_DEFINE0(x32_rt_sigreturn)
 	sigset_t set;
 	unsigned long uc_flags;
 
+	prevent_single_step_upon_eretu(regs);
+
 	frame = (struct rt_sigframe_x32 __user *)(regs->sp - 8);
 
 	if (!access_ok(frame, sizeof(*frame)))
-- 
2.49.0

[PATCH v5 2/2] selftests/x86: Add a test to detect infinite sigtrap handler loop

[Xin Li (Intel)]

When FRED is enabled, if the Trap Flag (TF) is set without an external
debugger attached, it can lead to an infinite loop in the SIGTRAP
handler.  To avoid this, the software event flag in the augmented SS
must be cleared, ensuring that no single-step trap remains pending when
ERETU completes.

This test checks for that specific scenario—verifying whether the kernel
correctly prevents an infinite SIGTRAP loop in this edge case when FRED
is enabled.

The test should _always_ pass with IDT event delivery, thus no need to
disable the test even when FRED is not enabled.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Cc: stable@vger.kernel.org
---

Changes in v5:
*) Do "sub $-128, %rsp" rather than "add $128, %rsp", which is more
   efficient in code size (hpa).
*) Add TB from Sohil.
*) Add Cc: stable@vger.kernel.org.

Changes in v4:
*) merge this selftest with its bug fix patch set (Dave Hansen).
*) Address review comments from Sohil.
---
 tools/testing/selftests/x86/Makefile       |  2 +-
 tools/testing/selftests/x86/sigtrap_loop.c | 98 ++++++++++++++++++++++
 2 files changed, 99 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/x86/sigtrap_loop.c

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index f703fcfe9f7c..83148875a12c 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -12,7 +12,7 @@ CAN_BUILD_WITH_NOPIE := $(shell ./check_cc.sh "$(CC)" trivial_program.c -no-pie)
 
 TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \
 			check_initial_reg_state sigreturn iopl ioperm \
-			test_vsyscall mov_ss_trap \
+			test_vsyscall mov_ss_trap sigtrap_loop \
 			syscall_arg_fault fsgsbase_restore sigaltstack
 TARGETS_C_BOTHBITS += nx_stack
 TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
diff --git a/tools/testing/selftests/x86/sigtrap_loop.c b/tools/testing/selftests/x86/sigtrap_loop.c
new file mode 100644
index 000000000000..9eecf32c79c2
--- /dev/null
+++ b/tools/testing/selftests/x86/sigtrap_loop.c
@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2025 Intel Corporation
+ */
+#define _GNU_SOURCE
+
+#include <err.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ucontext.h>
+
+#ifdef __x86_64__
+# define REG_IP REG_RIP
+#else
+# define REG_IP REG_EIP
+#endif
+
+static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *), int flags)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_sigaction = handler;
+	sa.sa_flags = SA_SIGINFO | flags;
+	sigemptyset(&sa.sa_mask);
+
+	if (sigaction(sig, &sa, 0))
+		err(1, "sigaction");
+
+	return;
+}
+
+static unsigned int loop_count_on_same_ip;
+
+static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
+{
+	ucontext_t *ctx = (ucontext_t *)ctx_void;
+	static unsigned long last_trap_ip;
+
+	if (last_trap_ip == ctx->uc_mcontext.gregs[REG_IP]) {
+		printf("\tTrapped at %016lx\n", last_trap_ip);
+
+		/*
+		 * If the same IP is hit more than 10 times in a row, it is
+		 * _considered_ an infinite loop.
+		 */
+		if (++loop_count_on_same_ip > 10) {
+			printf("[FAIL]\tDetected sigtrap infinite loop\n");
+			exit(1);
+		}
+
+		return;
+	}
+
+	loop_count_on_same_ip = 0;
+	last_trap_ip = ctx->uc_mcontext.gregs[REG_IP];
+	printf("\tTrapped at %016lx\n", last_trap_ip);
+}
+
+int main(int argc, char *argv[])
+{
+	sethandler(SIGTRAP, sigtrap, 0);
+
+	/*
+	 * Set the Trap Flag (TF) to single-step the test code, therefore to
+	 * trigger a SIGTRAP signal after each instruction until the TF is
+	 * cleared.
+	 *
+	 * Because the arithmetic flags are not significant here, the TF is
+	 * set by pushing 0x302 onto the stack and then popping it into the
+	 * flags register.
+	 *
+	 * Four instructions in the following asm code are executed with the
+	 * TF set, thus the SIGTRAP handler is expected to run four times.
+	 */
+	printf("[RUN]\tsigtrap infinite loop detection\n");
+	asm volatile(
+#ifdef __x86_64__
+		/* Avoid clobbering the redzone */
+		"sub $128, %rsp\n\t"
+#endif
+		"push $0x302\n\t"
+		"popf\n\t"
+		"nop\n\t"
+		"nop\n\t"
+		"push $0x202\n\t"
+		"popf\n\t"
+#ifdef __x86_64__
+		/* Equivalent to "add $128, %rsp" with 3 fewer bytes in encoding */
+		"sub $-128, %rsp\n\t"
+#endif
+	);
+
+	printf("[OK]\tNo sigtrap infinite loop detected\n");
+	return 0;
+}
-- 
2.49.0

Re: [PATCH v5 1/2] x86/fred/signal: Prevent immediate repeat of single step trap on return from SIGTRAP handler

[Sohil Mehta]

On 6/6/2025 10:45 AM, Xin Li (Intel) wrote:
> ---
>  arch/x86/include/asm/sighandling.h | 22 ++++++++++++++++++++++
>  arch/x86/kernel/signal_32.c        |  4 ++++
>  arch/x86/kernel/signal_64.c        |  4 ++++
>  3 files changed, 30 insertions(+)
> 

Tested-by: Sohil Mehta <sohil.mehta@intel.com>

Re: [PATCH v5 1/2] x86/fred/signal: Prevent immediate repeat of single step trap on return from SIGTRAP handler

[Xin Li]

On 6/6/2025 4:34 PM, Sohil Mehta wrote:
> On 6/6/2025 10:45 AM, Xin Li (Intel) wrote:
>> ---
>>   arch/x86/include/asm/sighandling.h | 22 ++++++++++++++++++++++
>>   arch/x86/kernel/signal_32.c        |  4 ++++
>>   arch/x86/kernel/signal_64.c        |  4 ++++
>>   3 files changed, 30 insertions(+)
>>
> 
> Tested-by: Sohil Mehta <sohil.mehta@intel.com>

I should have added your TB.  Shame!