Re: [kernel-hardening] Re: [PATCH v14 10/13] ptrace,seccomp: AddPTRACE_SECCOMP support

From: Will Drewry
Date: Wed Mar 14 2012 - 11:03:45 EST

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On Wed, Mar 14, 2012 at 2:31 AM, Indan Zupancic <indan@xxxxxx> 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@xxxxxxxxxx)
>> v11: - invert the logic to just make it a PTRACE_SYSCALL accelerator
>> (indan@xxxxxx)
>> 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@xxxxxxxxxxxx>
>> ---
>> 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.

>
>>
>> /* 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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Next message: Daniel Walker: "Re: [PATCH v3 2/2] DMAEngine: Add DMAEngine driver based on old MSMDMA APIs"
Previous message: Peter Zijlstra: "Re: [RFC PATCH 14/14] sched: implement usage tracking"
In reply to: Indan Zupancic: "Re: [PATCH v14 10/13] ptrace,seccomp: Add PTRACE_SECCOMP support"
Next in thread: Will Drewry: "Re: [kernel-hardening] Re: [PATCH v14 10/13] ptrace,seccomp: AddPTRACE_SECCOMP support"
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