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

Commit-ID: e34dbbc85d64af59176fe59fad7b4122f4330fe2 Gitweb: https://git.kernel.org/tip/e34dbbc85d64af59176fe59fad7b4122f4330fe2 Author: Xin Li (Intel) xin@zytor.com AuthorDate: Mon, 09 Jun 2025 01:40:53 -07:00 Committer: Dave Hansen dave.hansen@linux.intel.com CommitterDate: Mon, 09 Jun 2025 08:50:58 -07:00

x86/fred/signal: Prevent immediate repeat of single step trap on return from SIGTRAP handler

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.

Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code") Suggested-by: H. Peter Anvin (Intel) hpa@zytor.com Signed-off-by: Xin Li (Intel) xin@zytor.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Tested-by: Sohil Mehta sohil.mehta@intel.com Cc:stable@vger.kernel.org Link: https://lore.kernel.org/all/20250609084054.2083189-2-xin%40zytor.com --- 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 e770c4f..8727c7e 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 98123ff..42bbc42 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 ee94538..d483b58 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)))