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SubjectRe: [kernel-hardening] Re: [PATCH v14 10/13] ptrace,seccomp: Add PTRACE_SECCOMP support
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On Wed, Mar 14, 2012 at 10:03 AM, Will Drewry <wad@chromium.org> wrote:
> On Wed, Mar 14, 2012 at 2:31 AM, Indan Zupancic <indan@nul.nu> wrote:
>> Hello,
>>
>> On Mon, March 12, 2012 22:28, Will Drewry wrote:
>>> This change adds support for a new ptrace option, PTRACE_O_TRACESECCOMP,
>>> and a new return value for seccomp BPF programs, SECCOMP_RET_TRACE.
>>>
>>> When a tracer specifies the PTRACE_O_TRACESECCOMP ptrace option, the
>>> tracer will be notified for any syscall that results in a BPF program
>>> returning SECCOMP_RET_TRACE.  The 16-bit SECCOMP_RET_DATA mask of the
>>> BPF program return value will be passed as the ptrace_message and may be
>>> retrieved using PTRACE_GETEVENTMSG.
>>
>> Maybe good to tell it gets notified with PTRACE_EVENT_SECCOMP.
>
> Good call - I'll update it.
>
>>>
>>> If the subordinate process is not using seccomp filter, then no
>>> system call notifications will occur even if the option is specified.
>>>
>>> If there is no attached tracer when SECCOMP_RET_TRACE is returned,
>>> the system call will not be executed and an -ENOSYS errno will be
>>> returned to userspace.
>>
>> When no tracer with PTRACE_O_TRACESECCOMP set is attached?
>> (Because that's what the code is doing.)
>
> That too :)
>
>>>
>>> This change adds a dependency on the system call slow path.  Any future
>>> efforts to use the system call fast path for seccomp filter will need to
>>> address this restriction.
>>>
>>> v14: - rebase/nochanges
>>> v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
>>>       (Brings back a change to ptrace.c and the masks.)
>>> v12: - rebase to linux-next
>>>     - use ptrace_event and update arch/Kconfig to mention slow-path dependency
>>>     - drop all tracehook changes and inclusion (oleg@redhat.com)
>>> v11: - invert the logic to just make it a PTRACE_SYSCALL accelerator
>>>       (indan@nul.nu)
>>> v10: - moved to PTRACE_O_SECCOMP / PT_TRACE_SECCOMP
>>> v9:  - n/a
>>> v8:  - guarded PTRACE_SECCOMP use with an ifdef
>>> v7:  - introduced
>>>
>>> Signed-off-by: Will Drewry <wad@chromium.org>
>>> ---
>>> arch/Kconfig            |   11 ++++++-----
>>> include/linux/ptrace.h  |    7 +++++--
>>> include/linux/seccomp.h |    1 +
>>> kernel/ptrace.c         |    3 +++
>>> kernel/seccomp.c        |   20 +++++++++++++++-----
>>> 5 files changed, 30 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/arch/Kconfig b/arch/Kconfig
>>> index d92a78e..3f8132c 100644
>>> --- a/arch/Kconfig
>>> +++ b/arch/Kconfig
>>> @@ -202,15 +202,16 @@ config HAVE_CMPXCHG_DOUBLE
>>> config HAVE_ARCH_SECCOMP_FILTER
>>>       bool
>>>       help
>>> -       This symbol should be selected by an architecure if it provides:
>>> -       asm/syscall.h:
>>> +       An arch should select this symbol if it provides all of these things:
>>>         - syscall_get_arch()
>>>         - syscall_get_arguments()
>>>         - syscall_rollback()
>>>         - syscall_set_return_value()
>>> -       SIGSYS siginfo_t support must be implemented.
>>> -       __secure_computing_int()/secure_computing()'s return value must be
>>> -       checked, with -1 resulting in the syscall being skipped.
>>> +       - SIGSYS siginfo_t support
>>> +       - uses __secure_computing_int() or secure_computing()
>>> +       - secure_computing is called from a ptrace_event()-safe context
>>> +       - secure_computing return value is checked and a return value of -1
>>> +         results in the system call being skipped immediately.
>>>
>>> config SECCOMP_FILTER
>>>       def_bool y
>>> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
>>> index c2f1f6a..84b3418 100644
>>> --- a/include/linux/ptrace.h
>>> +++ b/include/linux/ptrace.h
>>> @@ -62,8 +62,9 @@
>>> #define PTRACE_O_TRACEEXEC    0x00000010
>>> #define PTRACE_O_TRACEVFORKDONE       0x00000020
>>> #define PTRACE_O_TRACEEXIT    0x00000040
>>> +#define PTRACE_O_TRACESECCOMP        0x00000080
>>>
>>> -#define PTRACE_O_MASK                0x0000007f
>>> +#define PTRACE_O_MASK                0x000000ff
>>>
>>> /* Wait extended result codes for the above trace options.  */
>>> #define PTRACE_EVENT_FORK     1
>>> @@ -73,6 +74,7 @@
>>> #define PTRACE_EVENT_VFORK_DONE       5
>>> #define PTRACE_EVENT_EXIT     6
>>> #define PTRACE_EVENT_STOP     7
>>> +#define PTRACE_EVENT_SECCOMP 8
>>
>> I think PTRACE_EVENT_STOP is supposed to be hidden, it's never directly
>> seen by user space. Instead of doing the obvious thing, they went the
>> crazy PTRACE_INTERRUPT + PTRACE_LISTEN way.
>>
>> So it's better to add PTRACE_EVENT_SECCOMP as 8 and bump STOP one up.
>> But if PTRACE_EVENT_STOP is really hidden then it shouldn't show up in
>> the user space header at all, it should be after the ifdef KERNEL.
>
> yeah that's in the -next branch so it will be fixed on merge resolve.
>
>>>
>>> #include <asm/ptrace.h>
>>>
>>> @@ -101,8 +103,9 @@
>>> #define PT_TRACE_EXEC         PT_EVENT_FLAG(PTRACE_EVENT_EXEC)
>>> #define PT_TRACE_VFORK_DONE   PT_EVENT_FLAG(PTRACE_EVENT_VFORK_DONE)
>>> #define PT_TRACE_EXIT         PT_EVENT_FLAG(PTRACE_EVENT_EXIT)
>>> +#define PT_TRACE_SECCOMP     PT_EVENT_FLAG(PTRACE_EVENT_SECCOMP)
>>>
>>> -#define PT_TRACE_MASK        0x000003f4
>>> +#define PT_TRACE_MASK        0x00000ff4
>>
>> This is wrong. Shouldn't it be 0xbf4? (0x7f4 if you bump STOP up.)
>
> If I bump stop up, but in the resolved version the masks simplify.

Oops. To answer the question about 0xbf4 versus 0xff4. 0xbf4 would
make sense for filtering out STOP, but it seemed like overkill because
stop has already been moved out of the way in -next:
http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git;a=blob;f=include/linux/ptrace.h;h=5c719627c2aa7bc58431ba5aa66195f9f89113ee;hb=c3381bf07c37799a7893c39469ad892322f9ded2#l62

Merge should resolve this issue entirely.

>
>>
>>>
>>> /* single stepping state bits (used on ARM and PA-RISC) */
>>> #define PT_SINGLESTEP_BIT     31
>>> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
>>> index e6d4b56..f4c1774 100644
>>> --- a/include/linux/seccomp.h
>>> +++ b/include/linux/seccomp.h
>>> @@ -21,6 +21,7 @@
>>> #define SECCOMP_RET_KILL      0x00000000U /* kill the task immediately */
>>> #define SECCOMP_RET_TRAP      0x00020000U /* disallow and force a SIGSYS */
>>> #define SECCOMP_RET_ERRNO     0x00030000U /* returns an errno */
>>> +#define SECCOMP_RET_TRACE    0x7ffe0000U /* pass to a tracer or disallow */
>>> #define SECCOMP_RET_ALLOW     0x7fff0000U /* allow */
>>
>> Maybe a good idea to leave more gaps between all the return codes, in
>> case new return codes are added in the future that fall between existing
>> ones? E.g:
>>
>> #define SECCOMP_RET_KILL        0x00000000U /* kill the task immediately */
>> #define SECCOMP_RET_TRAP        0x00100000U /* disallow and force a SIGSYS */
>> #define SECCOMP_RET_ERRNO       0x00200000U /* returns an errno */
>> #define SECCOMP_RET_TRACE       0x00300000U /* pass to a tracer or disallow */
>> #define SECCOMP_RET_ALLOW       0x00400000U /* allow */
>
> The gaps are intentional.  Priority is from least permissive (0) to
> most permissive (0x7fff).  So the gaps are
> there for things between errno and trace. I could add more gaps on
> those sides, but I don't think there is much need to given the scope
> of what is possible in the middle.  I certainly wouldn't push ALLOW
> down to the bottom.
>
>>>
>>> /* Masks for the return value sections. */
>>> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
>>> index 00ab2ca..8cf6da1 100644
>>> --- a/kernel/ptrace.c
>>> +++ b/kernel/ptrace.c
>>> @@ -551,6 +551,9 @@ static int ptrace_setoptions(struct task_struct *child, unsigned
>> long data)
>>>       if (data & PTRACE_O_TRACEEXIT)
>>>               child->ptrace |= PT_TRACE_EXIT;
>>>
>>> +     if (data & PTRACE_O_TRACESECCOMP)
>>> +             child->ptrace |= PT_TRACE_SECCOMP;
>>> +
>>>       return (data & ~PTRACE_O_MASK) ? -EINVAL : 0;
>>> }
>>>
>>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>>> index 140490a..ddacc68 100644
>>> --- a/kernel/seccomp.c
>>> +++ b/kernel/seccomp.c
>>> @@ -17,13 +17,13 @@
>>> #include <linux/audit.h>
>>> #include <linux/compat.h>
>>> #include <linux/filter.h>
>>> +#include <linux/ptrace.h>
>>> #include <linux/sched.h>
>>> #include <linux/seccomp.h>
>>> #include <linux/security.h>
>>> #include <linux/slab.h>
>>> #include <linux/uaccess.h>
>>>
>>> -#include <linux/tracehook.h>
>>> #include <asm/syscall.h>
>>>
>>> /* #define SECCOMP_DEBUG 1 */
>>> @@ -389,14 +389,24 @@ int __secure_computing_int(int this_syscall)
>>>                                                -(action & SECCOMP_RET_DATA),
>>>                                                0);
>>>                       return -1;
>>> -             case SECCOMP_RET_TRAP: {
>>> -                     int reason_code = action & SECCOMP_RET_DATA;
>>> +             case SECCOMP_RET_TRAP:
>>>                       /* Show the handler the original registers. */
>>>                       syscall_rollback(current, task_pt_regs(current));
>>>                       /* Let the filter pass back 16 bits of data. */
>>> -                     seccomp_send_sigsys(this_syscall, reason_code);
>>> +                     seccomp_send_sigsys(this_syscall,
>>> +                                         action & SECCOMP_RET_DATA);
>>>                       return -1;
>>> -             }
>>
>> These are unrelated changes and probably shouldn't be here. It just makes
>> it harder to review the code if you change it in a later patch for no
>> apparent reason.
>>
>>> +             case SECCOMP_RET_TRACE:
>>> +                     /* Skip these calls if there is no tracer. */
>>> +                     if (!ptrace_event_enabled(current,
>>> +                                               PTRACE_EVENT_SECCOMP))
>>
>> One line please, it's 81 chars.
>
> Ok :)
>
>>> +                             return -1;
>>> +                     /* Allow the BPF to provide the event message */
>>> +                     ptrace_event(PTRACE_EVENT_SECCOMP,
>>> +                                  action & SECCOMP_RET_DATA);
>>
>> Why not move "int reason_code = action & SECCOMP_RET_DATA;" to the start
>> of the function out of the if checks, instead of duplicating the code?
>
> Yeah I am cleaning this all up in the next rev. This was just ugly.
>
>>> +                     if (fatal_signal_pending(current))
>>> +                             break;
>>> +                     return 0;
>>>               case SECCOMP_RET_ALLOW:
>>>                       return 0;
>>>               case SECCOMP_RET_KILL:
>>
>
> Thanks!
> will
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