Clear the software event flag in the augmented SS to prevent infinite SIGTRAP handler loop 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 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 (H. Peter Anvin). --- 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..b8481d33ba8e 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 infinite SIGTRAP handler loop 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)))

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 ---

Changes in this version: *) Address review comments from Sohil. --- tools/testing/selftests/x86/Makefile | 2 +- tools/testing/selftests/x86/sigtrap_loop.c | 97 ++++++++++++++++++++++ 2 files changed, 98 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..dfb05769551d --- /dev/null +++ b/tools/testing/selftests/x86/sigtrap_loop.c @@ -0,0 +1,97 @@ +// 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__ + "add $128, %rsp\n\t" +#endif + ); + + printf("[OK]\tNo sigtrap infinite loop detected\n"); + return 0; +}