[PATCH 4.19 38/43] x86: Introduce TS_COMPAT_RESTART to fix get_nr_restart_syscall()

From: Greg Kroah-Hartman
Date: Mon Mar 22 2021 - 09:04:27 EST

From: Oleg Nesterov <oleg@xxxxxxxxxx>

commit 8c150ba2fb5995c84a7a43848250d444a3329a7d upstream.

The comment in get_nr_restart_syscall() says:

* The problem is that we can get here when ptrace pokes
* syscall-like values into regs even if we're not in a syscall
* at all.

Yes, but if not in a syscall then the


check below can't really help:

- TS_COMPAT can't be set

- TS_I386_REGS_POKED is only set if regs->orig_ax was changed by
32bit debugger; and even in this case get_nr_restart_syscall()
is only correct if the tracee is 32bit too.

Suppose that a 64bit debugger plays with a 32bit tracee and

* Tracee calls sleep(2) // TS_COMPAT is set
* User interrupts the tracee by CTRL-C after 1 sec and does
"(gdb) call func()"
* gdb saves the regs by PTRACE_GETREGS
* does PTRACE_SETREGS to set %rip='func' and %orig_rax=-1
* PTRACE_CONT // TS_COMPAT is cleared
* func() hits int3.
* Debugger catches SIGTRAP.
* Restore original regs by PTRACE_SETREGS.

get_nr_restart_syscall() wrongly returns __NR_restart_syscall==219, the
tracee calls ia32_sys_call_table[219] == sys_madvise.

Add the sticky TS_COMPAT_RESTART flag which survives after return to user
mode. It's going to be removed in the next step again by storing the
information in the restart block. As a further cleanup it might be possible
to remove also TS_I386_REGS_POKED with that.


$ cvs -d :pserver:anoncvs:anoncvs@xxxxxxxxxxxxxx:/cvs/systemtap co ptrace-tests
$ gcc -o erestartsys-trap-debuggee ptrace-tests/tests/erestartsys-trap-debuggee.c --m32
$ gcc -o erestartsys-trap-debugger ptrace-tests/tests/erestartsys-trap-debugger.c -lutil
$ ./erestartsys-trap-debugger
Unexpected: retval 1, errno 22
erestartsys-trap-debugger: ptrace-tests/tests/erestartsys-trap-debugger.c:421

Fixes: 609c19a385c8 ("x86/ptrace: Stop setting TS_COMPAT in ptrace code")
Reported-by: Jan Kratochvil <jan.kratochvil@xxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20210201174709.GA17895@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
arch/x86/include/asm/thread_info.h | 14 +++++++++++++-
arch/x86/kernel/signal.c | 24 +-----------------------
2 files changed, 14 insertions(+), 24 deletions(-)

--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -236,10 +236,22 @@ static inline int arch_within_stack_fram
#define TS_COMPAT 0x0002 /* 32bit syscall active (64BIT)*/

+#ifndef __ASSEMBLY__
#define TS_I386_REGS_POKED 0x0004 /* regs poked by 32-bit ptracer */
+#define TS_COMPAT_RESTART 0x0008
+#define arch_set_restart_data arch_set_restart_data
+static inline void arch_set_restart_data(struct restart_block *restart)
+ struct thread_info *ti = current_thread_info();
+ if (ti->status & TS_COMPAT)
+ ti->status |= TS_COMPAT_RESTART;
+ else
+ ti->status &= ~TS_COMPAT_RESTART;
-#ifndef __ASSEMBLY__

#ifdef CONFIG_X86_32
#define in_ia32_syscall() true
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -776,30 +776,8 @@ handle_signal(struct ksignal *ksig, stru

static inline unsigned long get_nr_restart_syscall(const struct pt_regs *regs)
- /*
- * This function is fundamentally broken as currently
- * implemented.
- *
- * The idea is that we want to trigger a call to the
- * restart_block() syscall and that we want in_ia32_syscall(),
- * in_x32_syscall(), etc. to match whatever they were in the
- * syscall being restarted. We assume that the syscall
- * instruction at (regs->ip - 2) matches whatever syscall
- * instruction we used to enter in the first place.
- *
- * The problem is that we can get here when ptrace pokes
- * syscall-like values into regs even if we're not in a syscall
- * at all.
- *
- * For now, we maintain historical behavior and guess based on
- * stored state. We could do better by saving the actual
- * syscall arch in restart_block or (with caveats on x32) by
- * checking if regs->ip points to 'int $0x80'. The current
- * behavior is incorrect if a tracer has a different bitness
- * than the tracee.
- */
- if (current_thread_info()->status & (TS_COMPAT|TS_I386_REGS_POKED))
+ if (current_thread_info()->status & TS_COMPAT_RESTART)
return __NR_ia32_restart_syscall;
#ifdef CONFIG_X86_X32_ABI