linux/arch/x86/kernel/step.c
Oleg Nesterov 9899d11f65 ptrace: ensure arch_ptrace/ptrace_request can never race with SIGKILL
putreg() assumes that the tracee is not running and pt_regs_access() can
safely play with its stack.  However a killed tracee can return from
ptrace_stop() to the low-level asm code and do RESTORE_REST, this means
that debugger can actually read/modify the kernel stack until the tracee
does SAVE_REST again.

set_task_blockstep() can race with SIGKILL too and in some sense this
race is even worse, the very fact the tracee can be woken up breaks the
logic.

As Linus suggested we can clear TASK_WAKEKILL around the arch_ptrace()
call, this ensures that nobody can ever wakeup the tracee while the
debugger looks at it.  Not only this fixes the mentioned problems, we
can do some cleanups/simplifications in arch_ptrace() paths.

Probably ptrace_unfreeze_traced() needs more callers, for example it
makes sense to make the tracee killable for oom-killer before
access_process_vm().

While at it, add the comment into may_ptrace_stop() to explain why
ptrace_stop() still can't rely on SIGKILL and signal_pending_state().

Reported-by: Salman Qazi <sqazi@google.com>
Reported-by: Suleiman Souhlal <suleiman@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-01-22 10:08:00 -08:00

231 lines
5.9 KiB
C

/*
* x86 single-step support code, common to 32-bit and 64-bit.
*/
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/ptrace.h>
#include <asm/desc.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 & 0xffff;
if (v8086_mode(regs)) {
addr = (addr & 0xffff) + (seg << 4);
return addr;
}
/*
* 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 &= ~7UL;
mutex_lock(&child->mm->context.lock);
if (unlikely((seg >> 3) >= child->mm->context.size))
addr = -1L; /* bogus selector, access would fault */
else {
desc = child->mm->context.ldt + seg;
base = get_desc_base(desc);
/* 16-bit code segment? */
if (!desc->d)
addr &= 0xffff;
addr += base;
}
mutex_unlock(&child->mm->context.lock);
}
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), 0);
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() but
* PTRACE_KILL is not safe.
*/
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;
}