Re: [PATCH v3 1/3] x86/ptrace: Stop setting TS_COMPAT in ptrace code

Andy Lutomirski
2016-07-24T18:48:20Z

On Mon, Jun 20, 2016 at 4:39 PM, Andy Lutomirski <luto@kernel.org> wrote:
> Setting TS_COMPAT in ptrace is wrong: if we happen to do it during
> syscall entry, then we'll confuse seccomp and audit. (The former
> isn't a security problem: seccomp is currently entirely insecure if a
> malicious ptracer is attached.) As a minimal fix, this patch adds a
> new flag TS_I386_REGS_POKED that handles the ptrace special case.

Hi Ingo-

Could you apply this one patch for 4.8? While I don't think it's a
significant security issue in 4.7 or earlier, leaving it unfixed in
4.8 will introduce a potentially unpleasant interaction with some
seccomp changes that are queued up in the
security tree for 4.8.

It will have a trivially-resolvable conflict with -mm.

The rest of the series this is in can wait.

--Andy

>
> Cc: Pedro Alves <palves@redhat.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> arch/x86/entry/common.c | 6 +++++-
> arch/x86/include/asm/syscall.h | 5 +----
> arch/x86/include/asm/thread_info.h | 3 +++
> arch/x86/kernel/ptrace.c | 15 +++++++++------
> arch/x86/kernel/signal.c | 26 ++++++++++++++++++++++++--
> 5 files changed, 42 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index ec138e538c44..0db497a8ff19 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -270,8 +270,12 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
> * handling, because syscall restart has a fixup for compat
> * syscalls. The fixup is exercised by the ptrace_syscall_32
> * selftest.
> + *
> + * We also need to clear TS_REGS_POKED_I386: the 32-bit tracer
> + * special case only applies after poking regs and before the
> + * very next return to user mode.
> */
> - ti->status &= ~TS_COMPAT;
> + ti->status &= ~(TS_COMPAT|TS_I386_REGS_POKED);
> #endif
>
> user_enter();
> diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
> index 999b7cd2e78c..4e23dd15c661 100644
> --- a/arch/x86/include/asm/syscall.h
> +++ b/arch/x86/include/asm/syscall.h
> @@ -60,7 +60,7 @@ static inline long syscall_get_error(struct task_struct *task,
> * TS_COMPAT is set for 32-bit syscall entries and then
> * remains set until we return to user mode.
> */
> - if (task_thread_info(task)->status & TS_COMPAT)
> + if (task_thread_info(task)->status & (TS_COMPAT|TS_I386_REGS_POKED))
> /*
> * Sign-extend the value so (int)-EFOO becomes (long)-EFOO
> * and will match correctly in comparisons.
> @@ -239,9 +239,6 @@ static inline int syscall_get_arch(void)
> * TS_COMPAT is set for 32-bit syscall entry and then
> * remains set until we return to user mode.
> *
> - * TIF_IA32 tasks should always have TS_COMPAT set at
> - * system call time.
> - *
> * x32 tasks should be considered AUDIT_ARCH_X86_64.
> */
> if (task_thread_info(current)->status & TS_COMPAT)
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index 30c133ac05cd..4bca518d11f4 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -228,6 +228,9 @@ static inline unsigned long current_stack_pointer(void)
> * have to worry about atomic accesses.
> */
> #define TS_COMPAT 0x0002 /* 32bit syscall active (64BIT)*/
> +#ifdef CONFIG_COMPAT
> +#define TS_I386_REGS_POKED 0x0004 /* regs poked by 32-bit ptracer */
> +#endif
> #define TS_RESTORE_SIGMASK 0x0008 /* restore signal mask in do_signal() */
>
> #ifndef __ASSEMBLY__
> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
> index 600edd225e81..f79576a541ff 100644
> --- a/arch/x86/kernel/ptrace.c
> +++ b/arch/x86/kernel/ptrace.c
> @@ -923,15 +923,18 @@ static int putreg32(struct task_struct *child, unsigned regno, u32 value)
>
> case offsetof(struct user32, regs.orig_eax):
> /*
> - * A 32-bit debugger setting orig_eax means to restore
> - * the state of the task restarting a 32-bit syscall.
> - * Make sure we interpret the -ERESTART* codes correctly
> - * in case the task is not actually still sitting at the
> - * exit from a 32-bit syscall with TS_COMPAT still set.
> + * Warning: bizarre corner case fixup here. A 32-bit
> + * debugger setting orig_eax to -1 wants to disable
> + * syscall restart. Make sure that the syscall
> + * restart code sign-extends orig_ax. Also make sure
> + * we interpret the -ERESTART* codes correctly if
> + * loaded into regs->ax in case the task is not
> + * actually still sitting at the exit from a 32-bit
> + * syscall with TS_COMPAT still set.
> */
> regs->orig_ax = value;
> if (syscall_get_nr(child, regs) >= 0)
> - task_thread_info(child)->status |= TS_COMPAT;
> + task_thread_info(child)->status |= TS_I386_REGS_POKED;
> break;
>
> case offsetof(struct user32, regs.eflags):
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index 22cc2f9f8aec..6b952e1d8db8 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -760,8 +760,30 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
>
> static inline unsigned long get_nr_restart_syscall(const struct pt_regs *regs)
> {
> -#ifdef CONFIG_X86_64
> - if (in_ia32_syscall())
> + /*
> + * 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.
> + */
> +#ifdef CONFIG_IA32_EMULATION
> + if (current_thread_info()->status & (TS_COMPAT|TS_I386_REGS_POKED))
> return __NR_ia32_restart_syscall;
> #endif
> #ifdef CONFIG_X86_X32_ABI
> --
> 2.5.5
>

--
Andy Lutomirski
AMA Capital Management, LLC