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Message-ID: <34b2e80343549454d6c026237f248c86.squirrel@webmail.greenhost.nl>
Date: Wed, 14 Mar 2012 08:31:07 +0100
From: "Indan Zupancic" <indan@....nu>
To: "Will Drewry" <wad@...omium.org>
Cc: linux-kernel@...r.kernel.org,
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"Will Drewry" <wad@...omium.org>
Subject: Re: [PATCH v14 10/13] ptrace,seccomp: Add PTRACE_SECCOMP support
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.
>
> 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.)
>
> 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@...hat.com)
> v11: - invert the logic to just make it a PTRACE_SYSCALL accelerator
> (indan@....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@...omium.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.
>
> #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.)
>
> /* 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 */
>
> /* 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.
> + 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?
> + if (fatal_signal_pending(current))
> + break;
> + return 0;
> case SECCOMP_RET_ALLOW:
> return 0;
> case SECCOMP_RET_KILL:
Greetings,
Indan
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