The following commit has been merged into the core/urgent branch of tip: Commit-ID: 6342adcaa683c2b705c24ed201dc11b35854c88d Gitweb: https://git.kernel.org/tip/6342adcaa683c2b705c24ed201dc11b35854c88d Author: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx> AuthorDate: Wed, 03 Feb 2021 13:00:48 -05:00 Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CommitterDate: Sat, 06 Feb 2021 00:21:42 +01:00 entry: Ensure trap after single-step on system call return Commit 299155244770 ("entry: Drop usage of TIF flags in the generic syscall code") introduced a bug on architectures using the generic syscall entry code, in which processes stopped by PTRACE_SYSCALL do not trap on syscall return after receiving a TIF_SINGLESTEP. The reason is that the meaning of TIF_SINGLESTEP flag is overloaded to cause the trap after a system call is executed, but since the above commit, the syscall call handler only checks for the SYSCALL_WORK flags on the exit work. Split the meaning of TIF_SINGLESTEP such that it only means single-step mode, and create a new type of SYSCALL_WORK to request a trap immediately after a syscall in single-step mode. In the current implementation, the SYSCALL_WORK flag shadows the TIF_SINGLESTEP flag for simplicity. Update x86 to flip this bit when a tracer enables single stepping. Fixes: 299155244770 ("entry: Drop usage of TIF flags in the generic syscall code") Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Tested-by: Kyle Huey <me@xxxxxxxxxxxx> Link: https://lore.kernel.org/r/87h7mtc9pr.fsf_-_@xxxxxxxxxxxxx --- arch/x86/include/asm/entry-common.h | 2 -- arch/x86/kernel/step.c | 10 ++++++++-- include/linux/entry-common.h | 1 + include/linux/thread_info.h | 2 ++ kernel/entry/common.c | 12 ++---------- 5 files changed, 13 insertions(+), 14 deletions(-) diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h index 6fe54b2..2b87b19 100644 --- a/arch/x86/include/asm/entry-common.h +++ b/arch/x86/include/asm/entry-common.h @@ -43,8 +43,6 @@ static __always_inline void arch_check_user_regs(struct pt_regs *regs) } #define arch_check_user_regs arch_check_user_regs -#define ARCH_SYSCALL_EXIT_WORK (_TIF_SINGLESTEP) - static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs, unsigned long ti_work) { diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c index 60d2c37..0f3c307 100644 --- a/arch/x86/kernel/step.c +++ b/arch/x86/kernel/step.c @@ -127,12 +127,17 @@ static int enable_single_step(struct task_struct *child) regs->flags |= X86_EFLAGS_TF; /* - * Always set TIF_SINGLESTEP - this guarantees that - * we single-step system calls etc.. This will also + * Always set TIF_SINGLESTEP. This will also * cause us to set TF when returning to user mode. */ set_tsk_thread_flag(child, TIF_SINGLESTEP); + /* + * Ensure that a trap is triggered once stepping out of a system + * call prior to executing any user instruction. + */ + set_task_syscall_work(child, SYSCALL_EXIT_TRAP); + oflags = regs->flags; /* Set TF on the kernel stack.. */ @@ -230,6 +235,7 @@ void user_disable_single_step(struct task_struct *child) /* Always clear TIF_SINGLESTEP... */ clear_tsk_thread_flag(child, TIF_SINGLESTEP); + clear_task_syscall_work(child, SYSCALL_EXIT_TRAP); /* But touch TF only if it was set by us.. */ if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF)) diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h index ca86a00..a104b29 100644 --- a/include/linux/entry-common.h +++ b/include/linux/entry-common.h @@ -46,6 +46,7 @@ SYSCALL_WORK_SYSCALL_TRACE | \ SYSCALL_WORK_SYSCALL_AUDIT | \ SYSCALL_WORK_SYSCALL_USER_DISPATCH | \ + SYSCALL_WORK_SYSCALL_EXIT_TRAP | \ ARCH_SYSCALL_WORK_EXIT) /* diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h index c8a974c..9b2158c 100644 --- a/include/linux/thread_info.h +++ b/include/linux/thread_info.h @@ -43,6 +43,7 @@ enum syscall_work_bit { SYSCALL_WORK_BIT_SYSCALL_EMU, SYSCALL_WORK_BIT_SYSCALL_AUDIT, SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH, + SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP, }; #define SYSCALL_WORK_SECCOMP BIT(SYSCALL_WORK_BIT_SECCOMP) @@ -51,6 +52,7 @@ enum syscall_work_bit { #define SYSCALL_WORK_SYSCALL_EMU BIT(SYSCALL_WORK_BIT_SYSCALL_EMU) #define SYSCALL_WORK_SYSCALL_AUDIT BIT(SYSCALL_WORK_BIT_SYSCALL_AUDIT) #define SYSCALL_WORK_SYSCALL_USER_DISPATCH BIT(SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH) +#define SYSCALL_WORK_SYSCALL_EXIT_TRAP BIT(SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP) #endif #include <asm/thread_info.h> diff --git a/kernel/entry/common.c b/kernel/entry/common.c index 6dd82be..f9d491b 100644 --- a/kernel/entry/common.c +++ b/kernel/entry/common.c @@ -209,15 +209,9 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs) lockdep_sys_exit(); } -#ifndef _TIF_SINGLESTEP -static inline bool report_single_step(unsigned long work) -{ - return false; -} -#else /* * If SYSCALL_EMU is set, then the only reason to report is when - * TIF_SINGLESTEP is set (i.e. PTRACE_SYSEMU_SINGLESTEP). This syscall + * SINGLESTEP is set (i.e. PTRACE_SYSEMU_SINGLESTEP). This syscall * instruction has been already reported in syscall_enter_from_user_mode(). */ static inline bool report_single_step(unsigned long work) @@ -225,10 +219,8 @@ static inline bool report_single_step(unsigned long work) if (work & SYSCALL_WORK_SYSCALL_EMU) return false; - return !!(current_thread_info()->flags & _TIF_SINGLESTEP); + return work & SYSCALL_WORK_SYSCALL_EXIT_TRAP; } -#endif - static void syscall_exit_work(struct pt_regs *regs, unsigned long work) {
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