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    Subject[PATCH v10 08/11] seccomp: Add SECCOMP_RET_TRAP
    Date
    Adds a new return value to seccomp filters that triggers a SIGSYS to be
    delivered with the new SYS_SECCOMP si_code.

    This allows in-process system call emulation, including just specifying
    an errno or cleanly dumping core, rather than just dying.

    v10: - use SIGSYS, syscall_get_arch, updates arch/Kconfig
    note suggested-by (though original suggestion had other behaviors)
    v9: - changes to SIGILL
    v8: - clean up based on changes to dependent patches
    v7: - introduction

    Suggested-by: Markus Gutschke <markus@chromium.org>
    Suggested-by: Julien Tinnes <jln@chromium.org>
    Signed-off-by: Will Drewry <wad@chromium.org>
    ---
    arch/Kconfig | 14 +++++++++-----
    include/linux/seccomp.h | 1 +
    kernel/seccomp.c | 28 ++++++++++++++++++++++++++++
    3 files changed, 38 insertions(+), 5 deletions(-)

    diff --git a/arch/Kconfig b/arch/Kconfig
    index aa00571..6d6d9dc 100644
    --- a/arch/Kconfig
    +++ b/arch/Kconfig
    @@ -202,11 +202,15 @@ 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, specifically syscall_get_arguments(),
    - syscall_get_arch(), and syscall_set_return_value(). Additionally,
    - its system call entry path must respect a return value of -1 from
    - __secure_computing_int() and/or secure_computing().
    + This symbol should be selected by an architecure if it provides:
    + asm/syscall.h:
    + - 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.

    config SECCOMP_FILTER
    def_bool y
    diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
    index 54ecb61..d039b7b 100644
    --- a/include/linux/seccomp.h
    +++ b/include/linux/seccomp.h
    @@ -19,6 +19,7 @@
    * selects the least permissive choice.
    */
    #define SECCOMP_RET_KILL 0x00000000U /* kill the task immediately */
    +#define SECCOMP_RET_TRAP 0x00020000U /* disallow and send sigtrap */
    #define SECCOMP_RET_ERRNO 0x00030000U /* returns an errno */
    #define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */

    diff --git a/kernel/seccomp.c b/kernel/seccomp.c
    index 23f1844..fc25d3a 100644
    --- a/kernel/seccomp.c
    +++ b/kernel/seccomp.c
    @@ -279,6 +279,26 @@ void copy_seccomp(struct seccomp *child,
    child->mode = prev->mode;
    child->filter = get_seccomp_filter(prev->filter);
    }
    +
    +/**
    + * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
    + * @syscall: syscall number to send to userland
    + * @reason: filter-supplied reason code to send to userland (via si_errno)
    + *
    + * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
    + */
    +static void seccomp_send_sigsys(int syscall, int reason)
    +{
    + struct siginfo info;
    + memset(&info, 0, sizeof(info));
    + info.si_signo = SIGSYS;
    + info.si_code = SYS_SECCOMP;
    + info.si_call_addr = (void __user *)KSTK_EIP(current);
    + info.si_errno = reason;
    + info.si_arch = syscall_get_arch(current, task_pt_regs(current));
    + info.si_syscall = syscall;
    + force_sig_info(SIGSYS, &info, current);
    +}
    #endif /* CONFIG_SECCOMP_FILTER */

    /*
    @@ -332,6 +352,14 @@ 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;
    + /* 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);
    + return -1;
    + }
    case SECCOMP_RET_ALLOW:
    return 0;
    case SECCOMP_RET_KILL:
    --
    1.7.5.4


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