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    SubjectRe: [PATCH v14 10/13] ptrace,seccomp: Add PTRACE_SECCOMP support
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    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@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.

    >
    > #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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