Re: [PATCH] x86: make PTRACE_GETREGSET return 32-bit regs if 64-bit process entered kernel with int 80

Oleg Nesterov
2013-02-14T15:02:04Z

On 02/14, Denys Vlasenko wrote:
>
> Determining personality of a ptraced process is a murky area.
> On x86, for years strace was looking at segment selectors,
> which is conceptually wrong: see, for example,
> https://lkml.org/lkml/2012/1/18/320
>
> strace recently changed detection method and current git code
> (not released yet) does the following: it reads registers
> with PTRACE_GETREGSET, and looks at returned regset size.
> It is different for 64-bit and 32-bit processes,
> and appears to be a reliable way to determine personality:
> No need to check segment selectors for magic values.
>
> This works for well-behaving processes.
>
> But the hole described in the aforementioned lkml thread
> still remains: 64-bit processes can perform 32-bit syscalls
> using "int 80" entry method, and in this case, kernel returns
> 64-bit regset. For example, this:
>
> asm("int $0x80": :"a" (29)); /* 32-bit sys_pause */
>
> will be decoded by strace as a (64-bit) shmget syscall.
>
> This patch makes it so that in syscall-entry-stop caused by
> "int 80" instruction, PTRACE_GETREGSET returns 32-bit regset.

Not sure...

First of all, this is incompatible change. And to me, it doesn't look
correct anyway. Say, why the debugger can't modify r15 if a 64bit tracee
does int80 ? Or think about PTRACE_EVENT_FORK which can be reported with
TS_COMPAT set.

Probably is_ia32_task() should be reported "explicitely" as we already
discussed, and afaik you have other ideas.

Oleg.

> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
>
> --- linux-3.7.7/arch/x86/include/asm/uaccess.h
> +++ linux-3.7.7_regset/arch/x86/include/asm/uaccess.h
> @@ -594,5 +594,7 @@
> # include <asm/uaccess_64.h>
> #endif
>
> +#define ARCH_HAS_SYSCALL_USER_REGSET_VIEW 1
> +
> #endif /* _ASM_X86_UACCESS_H */
>
> --- linux-3.7.7/arch/x86/kernel/ptrace.c
> +++ linux-3.7.7_regset/arch/x86/kernel/ptrace.c
> @@ -1443,6 +1443,22 @@
> #endif
> }
>
> +const struct user_regset_view *syscall_user_regset_view(struct task_struct *task)
> +{
> +#ifdef CONFIG_IA32_EMULATION
> + /* Did task make 32-bit syscall just now?
> + * Task can still be 64-bit: think "int 0x80 on x86_64".
> + */
> + if (task_thread_info(task)->status & TS_COMPAT)
> +#endif
> +#if defined CONFIG_X86_32 || defined CONFIG_IA32_EMULATION
> + return &user_x86_32_view;
> +#endif
> +#ifdef CONFIG_X86_64
> + return &user_x86_64_view;
> +#endif
> +}
> +
> static void fill_sigtrap_info(struct task_struct *tsk,
> struct pt_regs *regs,
> int error_code, int si_code,
> --- linux-3.7.7/include/linux/regset.h
> +++ linux-3.7.7_regset/include/linux/regset.h
> @@ -204,6 +204,12 @@
> */
> const struct user_regset_view *task_user_regset_view(struct task_struct *tsk);
>
> +#ifdef ARCH_HAS_SYSCALL_USER_REGSET_VIEW
> +const struct user_regset_view *syscall_user_regset_view(struct task_struct *tsk);
> +#else
> +# define syscall_user_regset_view task_user_regset_view
> +#endif
> +
>
> /*
> * These are helpers for writing regset get/set functions in arch code.
> --- linux-3.7.7/kernel/ptrace.c
> +++ linux-3.7.7_regset/kernel/ptrace.c
> @@ -684,7 +684,7 @@
> static int ptrace_regset(struct task_struct *task, int req, unsigned int type,
> struct iovec *kiov)
> {
> - const struct user_regset_view *view = task_user_regset_view(task);
> + const struct user_regset_view *view = syscall_user_regset_view(task);
> const struct user_regset *regset = find_regset(view, type);
> int regset_no;
>