Commit 6342adca authored by Gabriel Krisman Bertazi's avatar Gabriel Krisman Bertazi Committed by Thomas Gleixner
Browse files

entry: Ensure trap after single-step on system call return 



Commit 29915524 ("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: 29915524 ("entry: Drop usage of TIF flags in the generic syscall code")
Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarGabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarKyle Huey <me@kylehuey.com>
Link: https://lore.kernel.org/r/87h7mtc9pr.fsf_-_@collabora.com
parent 1048ba83

arch/x86/include/asm/entry-common.h

+0 −2
Original line number Diff line number Diff line
@@ -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)

{

arch/x86/kernel/step.c

+8 −2
Original line number Diff line number Diff line
@@ -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))

include/linux/entry-common.h

+1 −0
Original line number Diff line number Diff line
@@ -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)



/*

include/linux/thread_info.h

+2 −0
Original line number Diff line number Diff line
@@ -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>

kernel/entry/common.c

+2 −10
Original line number Diff line number Diff line
@@ -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)

{