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)))