1 files changed, 236 insertions, 0 deletions
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
new file mode 100644
index 000000000..2f97d1a10
--- /dev/null
+++ b/arch/x86/kernel/step.c
@@ -0,0 +1,236 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * x86 single-step support code, common to 32-bit and 64-bit.
+ */
+#include <linux/sched.h>
+#include <linux/sched/task_stack.h>
+#include <linux/mm.h>
+#include <linux/ptrace.h>
+#include <asm/desc.h>
+#include <asm/mmu_context.h>
+
+unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
+{
+	unsigned long addr, seg;
+
+	addr = regs->ip;
+	seg = regs->cs;
+	if (v8086_mode(regs)) {
+		addr = (addr & 0xffff) + (seg << 4);
+		return addr;
+	}
+
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+	/*
+	 * We'll assume that the code segments in the GDT
+	 * are all zero-based. That is largely true: the
+	 * TLS segments are used for data, and the PNPBIOS
+	 * and APM bios ones we just ignore here.
+	 */
+	if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
+		struct desc_struct *desc;
+		unsigned long base;
+
+		seg >>= 3;
+
+		mutex_lock(&child->mm->context.lock);
+		if (unlikely(!child->mm->context.ldt ||
+			     seg >= child->mm->context.ldt->nr_entries))
+			addr = -1L; /* bogus selector, access would fault */
+		else {
+			desc = &child->mm->context.ldt->entries[seg];
+			base = get_desc_base(desc);
+
+			/* 16-bit code segment? */
+			if (!desc->d)
+				addr &= 0xffff;
+			addr += base;
+		}
+		mutex_unlock(&child->mm->context.lock);
+	}
+#endif
+
+	return addr;
+}
+
+static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
+{
+	int i, copied;
+	unsigned char opcode[15];
+	unsigned long addr = convert_ip_to_linear(child, regs);
+
+	copied = access_process_vm(child, addr, opcode, sizeof(opcode),
+			FOLL_FORCE);
+	for (i = 0; i < copied; i++) {
+		switch (opcode[i]) {
+		/* popf and iret */
+		case 0x9d: case 0xcf:
+			return 1;
+
+			/* CHECKME: 64 65 */
+
+		/* opcode and address size prefixes */
+		case 0x66: case 0x67:
+			continue;
+		/* irrelevant prefixes (segment overrides and repeats) */
+		case 0x26: case 0x2e:
+		case 0x36: case 0x3e:
+		case 0x64: case 0x65:
+		case 0xf0: case 0xf2: case 0xf3:
+			continue;
+
+#ifdef CONFIG_X86_64
+		case 0x40 ... 0x4f:
+			if (!user_64bit_mode(regs))
+				/* 32-bit mode: register increment */
+				return 0;
+			/* 64-bit mode: REX prefix */
+			continue;
+#endif
+
+			/* CHECKME: f2, f3 */
+
+		/*
+		 * pushf: NOTE! We should probably not let
+		 * the user see the TF bit being set. But
+		 * it's more pain than it's worth to avoid
+		 * it, and a debugger could emulate this
+		 * all in user space if it _really_ cares.
+		 */
+		case 0x9c:
+		default:
+			return 0;
+		}
+	}
+	return 0;
+}
+
+/*
+ * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
+ */
+static int enable_single_step(struct task_struct *child)
+{
+	struct pt_regs *regs = task_pt_regs(child);
+	unsigned long oflags;
+
+	/*
+	 * If we stepped into a sysenter/syscall insn, it trapped in
+	 * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
+	 * If user-mode had set TF itself, then it's still clear from
+	 * do_debug() and we need to set it again to restore the user
+	 * state so we don't wrongly set TIF_FORCED_TF below.
+	 * If enable_single_step() was used last and that is what
+	 * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
+	 * already set and our bookkeeping is fine.
+	 */
+	if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
+		regs->flags |= X86_EFLAGS_TF;
+
+	/*
+	 * Always set TIF_SINGLESTEP - this guarantees that
+	 * we single-step system calls etc..  This will also
+	 * cause us to set TF when returning to user mode.
+	 */
+	set_tsk_thread_flag(child, TIF_SINGLESTEP);
+
+	oflags = regs->flags;
+
+	/* Set TF on the kernel stack.. */
+	regs->flags |= X86_EFLAGS_TF;
+
+	/*
+	 * ..but if TF is changed by the instruction we will trace,
+	 * don't mark it as being "us" that set it, so that we
+	 * won't clear it by hand later.
+	 *
+	 * Note that if we don't actually execute the popf because
+	 * of a signal arriving right now or suchlike, we will lose
+	 * track of the fact that it really was "us" that set it.
+	 */
+	if (is_setting_trap_flag(child, regs)) {
+		clear_tsk_thread_flag(child, TIF_FORCED_TF);
+		return 0;
+	}
+
+	/*
+	 * If TF was already set, check whether it was us who set it.
+	 * If not, we should never attempt a block step.
+	 */
+	if (oflags & X86_EFLAGS_TF)
+		return test_tsk_thread_flag(child, TIF_FORCED_TF);
+
+	set_tsk_thread_flag(child, TIF_FORCED_TF);
+
+	return 1;
+}
+
+void set_task_blockstep(struct task_struct *task, bool on)
+{
+	unsigned long debugctl;
+
+	/*
+	 * Ensure irq/preemption can't change debugctl in between.
+	 * Note also that both TIF_BLOCKSTEP and debugctl should
+	 * be changed atomically wrt preemption.
+	 *
+	 * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
+	 * task is current or it can't be running, otherwise we can race
+	 * with __switch_to_xtra(). We rely on ptrace_freeze_traced().
+	 */
+	local_irq_disable();
+	debugctl = get_debugctlmsr();
+	if (on) {
+		debugctl |= DEBUGCTLMSR_BTF;
+		set_tsk_thread_flag(task, TIF_BLOCKSTEP);
+	} else {
+		debugctl &= ~DEBUGCTLMSR_BTF;
+		clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
+	}
+	if (task == current)
+		update_debugctlmsr(debugctl);
+	local_irq_enable();
+}
+
+/*
+ * Enable single or block step.
+ */
+static void enable_step(struct task_struct *child, bool block)
+{
+	/*
+	 * Make sure block stepping (BTF) is not enabled unless it should be.
+	 * Note that we don't try to worry about any is_setting_trap_flag()
+	 * instructions after the first when using block stepping.
+	 * So no one should try to use debugger block stepping in a program
+	 * that uses user-mode single stepping itself.
+	 */
+	if (enable_single_step(child) && block)
+		set_task_blockstep(child, true);
+	else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
+		set_task_blockstep(child, false);
+}
+
+void user_enable_single_step(struct task_struct *child)
+{
+	enable_step(child, 0);
+}
+
+void user_enable_block_step(struct task_struct *child)
+{
+	enable_step(child, 1);
+}
+
+void user_disable_single_step(struct task_struct *child)
+{
+	/*
+	 * Make sure block stepping (BTF) is disabled.
+	 */
+	if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
+		set_task_blockstep(child, false);
+
+	/* Always clear TIF_SINGLESTEP... */
+	clear_tsk_thread_flag(child, TIF_SINGLESTEP);
+
+	/* But touch TF only if it was set by us.. */
+	if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
+		task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
+}