[PATCH v4 3/3] m68k: track syscalls being traced with shallow user context stack

Michael Schmitz
2021-06-23T00:21:48Z

Add 'status' field to thread_info struct to hold syscall trace
status info.

Set flag bit in thread_info->status at syscall trace entry, clear
flag bit on trace exit.

Set another flag bit on entering syscall where the full stack
frame has been saved. These flags can be checked whenever a
syscall calls ptrace_stop().

Check flag bits in get_reg()/put_reg() and prevent access to
registers that are saved on the switch stack, in case the
syscall did not actually save these registers on the switch
stack.

Tested on ARAnyM only - boots and survives running strace on a
binary, nothing fancy.

CC: Eric W. Biederman <ebiederm@xmission.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>

--

Changes from v3:

- complete flag bit handling for all syscalls that use a m68k
wrapper
- add flag checking code to get_reg()/put_reg() in m68k ptrace.c
---
arch/m68k/include/asm/entry.h | 10 +++++++
arch/m68k/include/asm/thread_info.h | 1 +
arch/m68k/kernel/asm-offsets.c | 1 +
arch/m68k/kernel/entry.S | 54 +++++++++++++++++++++++++++++++++++++
arch/m68k/kernel/ptrace.c | 44 +++++++++++++++++++++++++-----
5 files changed, 104 insertions(+), 6 deletions(-)

diff --git a/arch/m68k/include/asm/entry.h b/arch/m68k/include/asm/entry.h
index 9b52b06..37ba65b 100644
--- a/arch/m68k/include/asm/entry.h
+++ b/arch/m68k/include/asm/entry.h
@@ -41,6 +41,16 @@
#define ALLOWINT (~0x700)
#endif /* machine compilation types */

+#define TIS_TRACING 0
+#define TIS_ALLREGS_SAVED 1
+#define _TIS_TRACING (1<<TIS_TRACING)
+#define _TIS_ALLREGS_SAVED (1<<TIS_ALLREGS_SAVED)
+
+#define TIS_TRACE_ON _TIS_TRACING
+#define TIS_TRACE_OFF (~(_TIS_TRACING))
+#define TIS_SWITCH_STACK _TIS_ALLREGS_SAVED
+#define TIS_NO_SWITCH_STACK (~(_TIS_ALLREGS_SAVED))
+
#ifdef __ASSEMBLY__
/*
* This defines the normal kernel pt-regs layout.
diff --git a/arch/m68k/include/asm/thread_info.h b/arch/m68k/include/asm/thread_info.h
index 15a7570..a88b48b 100644
--- a/arch/m68k/include/asm/thread_info.h
+++ b/arch/m68k/include/asm/thread_info.h
@@ -29,6 +29,7 @@ struct thread_info {
unsigned long flags;
mm_segment_t addr_limit; /* thread address space */
int preempt_count; /* 0 => preemptable, <0 => BUG */
+ unsigned int status; /* thread-synchronous flags */
__u32 cpu; /* should always be 0 on m68k */
unsigned long tp_value; /* thread pointer */
};
diff --git a/arch/m68k/kernel/asm-offsets.c b/arch/m68k/kernel/asm-offsets.c
index ccea355..ac1ec8f 100644
--- a/arch/m68k/kernel/asm-offsets.c
+++ b/arch/m68k/kernel/asm-offsets.c
@@ -41,6 +41,7 @@ int main(void)
/* offsets into the thread_info struct */
DEFINE(TINFO_PREEMPT, offsetof(struct thread_info, preempt_count));
DEFINE(TINFO_FLAGS, offsetof(struct thread_info, flags));
+ DEFINE(TINFO_STATUS, offsetof(struct thread_info, status));

/* offsets into the pt_regs */
DEFINE(PT_OFF_D0, offsetof(struct pt_regs, d0));
diff --git a/arch/m68k/kernel/entry.S b/arch/m68k/kernel/entry.S
index 0c25038..4cc24d5 100644
--- a/arch/m68k/kernel/entry.S
+++ b/arch/m68k/kernel/entry.S
@@ -51,75 +51,115 @@

.text
ENTRY(__sys_fork)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
jbsr sys_fork
lea %sp@(24),%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(__sys_clone)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
pea %sp@(SWITCH_STACK_SIZE)
jbsr m68k_clone
lea %sp@(28),%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(__sys_vfork)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
jbsr sys_vfork
lea %sp@(24),%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(__sys_clone3)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
pea %sp@(SWITCH_STACK_SIZE)
jbsr m68k_clone3
lea %sp@(28),%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(__sys_exit)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
pea %sp@(SWITCH_STACK_SIZE)
jbsr m68k_exit
lea %sp@(28),%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(__sys_exit_group)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
pea %sp@(SWITCH_STACK_SIZE)
jbsr m68k_exit_group
lea %sp@(28),%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(__sys_execve)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
pea %sp@(SWITCH_STACK_SIZE)
jbsr m68k_execve
lea %sp@(28),%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(__sys_execveat)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
pea %sp@(SWITCH_STACK_SIZE)
jbsr m68k_execveat
lea %sp@(28),%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(sys_sigreturn)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
movel %sp,%sp@- | switch_stack pointer
pea %sp@(SWITCH_STACK_SIZE+4) | pt_regs pointer
jbsr do_sigreturn
addql #8,%sp
RESTORE_SWITCH_STACK
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(sys_rt_sigreturn)
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
movel %sp,%sp@- | switch_stack pointer
pea %sp@(SWITCH_STACK_SIZE+4) | pt_regs pointer
jbsr do_rt_sigreturn
addql #8,%sp
RESTORE_SWITCH_STACK
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
rts

ENTRY(buserr)
@@ -200,25 +240,33 @@ ENTRY(ret_from_user_rt_signal)
#else

do_trace_entry:
+ orb #TIS_TRACE_ON, %a1@(TINFO_STATUS+3)
movel #-ENOSYS,%sp@(PT_OFF_D0)| needed for strace
subql #4,%sp
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
jbsr syscall_trace
RESTORE_SWITCH_STACK
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
addql #4,%sp
movel %sp@(PT_OFF_ORIG_D0),%d0
cmpl #NR_syscalls,%d0
jcs syscall
badsys:
+ andb #TIS_TRACE_OFF, %a1@(TINFO_STATUS+3)
movel #-ENOSYS,%sp@(PT_OFF_D0)
jra ret_from_syscall

do_trace_exit:
subql #4,%sp
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
jbsr syscall_trace
RESTORE_SWITCH_STACK
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
addql #4,%sp
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_TRACE_OFF, %a1@(TINFO_STATUS+3)
jra .Lret_from_exception

ENTRY(ret_from_signal)
@@ -227,6 +275,8 @@ ENTRY(ret_from_signal)
jge 1f
jbsr syscall_trace
1: RESTORE_SWITCH_STACK
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_TRACE_OFF, %a1@(TINFO_STATUS+3)
addql #4,%sp
/* on 68040 complete pending writebacks if any */
#ifdef CONFIG_M68040
@@ -303,11 +353,15 @@ exit_work:
do_signal_return:
|andw #ALLOWINT,%sr
subql #4,%sp | dummy return address
+ movel %curptr@(TASK_STACK),%a1
+ orb #TIS_SWITCH_STACK, %a1@(TINFO_STATUS+3)
SAVE_SWITCH_STACK
pea %sp@(SWITCH_STACK_SIZE)
bsrl do_notify_resume
addql #4,%sp
RESTORE_SWITCH_STACK
+ movel %curptr@(TASK_STACK),%a1
+ andb #TIS_NO_SWITCH_STACK, %a1@(TINFO_STATUS+3)
addql #4,%sp
jbra resume_userspace

diff --git a/arch/m68k/kernel/ptrace.c b/arch/m68k/kernel/ptrace.c
index 94b3b27..ae4ef61 100644
--- a/arch/m68k/kernel/ptrace.c
+++ b/arch/m68k/kernel/ptrace.c
@@ -68,6 +68,12 @@ static const int regoff[] = {
[18] = PT_REG(pc),
};

+static inline int test_ti_thread_status(struct thread_info *ti, int flag)
+{
+ return test_bit(flag, (unsigned long *)&ti->status);
+}
+
+
/*
* Get contents of register REGNO in task TASK.
*/
@@ -77,9 +83,22 @@ static inline long get_reg(struct task_struct *task, int regno)

if (regno == PT_USP)
addr = &task->thread.usp;
- else if (regno < ARRAY_SIZE(regoff))
- addr = (unsigned long *)(task->thread.esp0 + regoff[regno]);
- else
+ else if (regno < ARRAY_SIZE(regoff)) {
+ int off =regoff[regno];
+
+ if (WARN_ON_ONCE((off < PT_REG(d1)) &&
+ test_ti_thread_status(task_thread_info(task), TIS_TRACING) &&
+ !test_ti_thread_status(task_thread_info(task),
+ TIS_ALLREGS_SAVED))) {
+ unsigned long *addr_d0;
+
+ addr_d0 = (unsigned long *)(task->thread.esp0 + regoff[16]);
+ pr_err("register read from incomplete stack, regno %d offs %d orig_d0 %lx\n",
+ regno, off, *addr_d0);
+ return 0;
+ }
+ addr = (unsigned long *)(task->thread.esp0 + off);
+ } else
return 0;
/* Need to take stkadj into account. */
if (regno == PT_SR || regno == PT_PC) {
@@ -102,9 +121,22 @@ static inline int put_reg(struct task_struct *task, int regno,

if (regno == PT_USP)
addr = &task->thread.usp;
- else if (regno < ARRAY_SIZE(regoff))
- addr = (unsigned long *)(task->thread.esp0 + regoff[regno]);
- else
+ else if (regno < ARRAY_SIZE(regoff)) {
+ int off = regoff[regno];
+
+ if (WARN_ON_ONCE((off < PT_REG(d1)) &&
+ test_ti_thread_status(task_thread_info(task), TIS_TRACING) &&
+ !test_ti_thread_status(task_thread_info(task),
+ TIS_ALLREGS_SAVED))) {
+ unsigned long *addr_d0;
+
+ addr_d0 = (unsigned long *)(task->thread.esp0 + regoff[16]);
+ pr_err("register write to incomplete stack, regno %d offs %d orig_d0 %lx\n",
+ regno, off, *addr_d0);
+ return -1;
+ }
+ addr = (unsigned long *)(task->thread.esp0 + off);
+ } else
return -1;
/* Need to take stkadj into account. */
if (regno == PT_SR || regno == PT_PC) {
--
2.7.4