Re: [Qemu-devel] [PATCH v3 2/6] seccomp: add obsolete argument to command line

[Daniel P. Berrange]

On Fri, Jul 28, 2017 at 02:10:36PM +0200, Eduardo Otubo wrote:
> This patch introduces the argument [,obsolete=allow] to the `-sandbox on'
> option. It allows Qemu to run safely on old system that still relies on
> old system calls.
> 
> Signed-off-by: Eduardo Otubo <address@hidden>
> ---
>  include/sysemu/seccomp.h |  4 +++-
>  qemu-options.hx          |  9 +++++++--
>  qemu-seccomp.c           | 32 +++++++++++++++++++++++++++++++-
>  vl.c                     | 16 +++++++++++++++-
>  4 files changed, 56 insertions(+), 5 deletions(-)
> 
> diff --git a/include/sysemu/seccomp.h b/include/sysemu/seccomp.h
> index cfc06008cb..7a7bde246b 100644
> --- a/include/sysemu/seccomp.h
> +++ b/include/sysemu/seccomp.h
> @@ -15,7 +15,9 @@
>  #ifndef QEMU_SECCOMP_H
>  #define QEMU_SECCOMP_H
>  
> +#define OBSOLETE    0x0001

Please namespace this - its far too generic a term to expose to other
source files. I'd suggest 

  QEMU_SECCOMP_SET_OBSOLETE

> -int seccomp_start(void);
> +int seccomp_start(uint8_t seccomp_opts);

This only allows for 8 sets. Perhaps its enough, but I'd suggest
just using a uint32_t straight away.

> diff --git a/qemu-options.hx b/qemu-options.hx
> index 746b5fa75d..54e492f36a 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4004,13 +4004,18 @@ Old param mode (ARM only).
>  ETEXI
>  
>  DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \
> -    "-sandbox <arg>  Enable seccomp mode 2 system call filter (default 
> 'off').\n",
> +    "-sandbox on[,obsolete=allow]  Enable seccomp mode 2 system call filter 
> (default 'off').\n" \
> +    "                obsolete: Allow obsolete system calls\n",
>      QEMU_ARCH_ALL)
>  STEXI
> address@hidden -sandbox @var{arg}
> address@hidden -sandbox @var{arg}[,address@hidden
>  @findex -sandbox
>  Enable Seccomp mode 2 system call filter. 'on' will enable syscall filtering 
> and 'off' will
>  disable it.  The default is 'off'.
> address@hidden @option
> address@hidden address@hidden
> +Enable Obsolete system calls

Lets explain this a bit more.

E obsolete system calls that are provided by the kernel, but typically no
longer used by modern C library implementations. 

> address@hidden table
>  ETEXI
>  
>  DEF("readconfig", HAS_ARG, QEMU_OPTION_readconfig,
> diff --git a/qemu-seccomp.c b/qemu-seccomp.c
> index f8877b07b5..c6a8b28260 100644
> --- a/qemu-seccomp.c
> +++ b/qemu-seccomp.c
> @@ -31,6 +31,20 @@ struct QemuSeccompSyscall {
>      uint8_t priority;
>  };
>  
> +static const struct QemuSeccompSyscall obsolete[] = {
> +    { SCMP_SYS(readdir), 255 },
> +    { SCMP_SYS(_sysctl), 255 },
> +    { SCMP_SYS(bdflush), 255 },
> +    { SCMP_SYS(create_module), 255 },
> +    { SCMP_SYS(get_kernel_syms), 255 },
> +    { SCMP_SYS(query_module), 255 },
> +    { SCMP_SYS(sgetmask), 255 },
> +    { SCMP_SYS(ssetmask), 255 },
> +    { SCMP_SYS(sysfs), 255 },
> +    { SCMP_SYS(uselib), 255 },
> +    { SCMP_SYS(ustat), 255 },
> +};
> +
>  static const struct QemuSeccompSyscall blacklist[] = {
>      { SCMP_SYS(reboot), 255 },
>      { SCMP_SYS(swapon), 255 },
> @@ -56,7 +70,20 @@ static const struct QemuSeccompSyscall blacklist[] = {
>      { SCMP_SYS(vserver), 255 },
>  };
>  
> -int seccomp_start(void)
> +static int is_obsolete(int syscall)
> +{
> +    unsigned int i = 0;
> +
> +    for (i = 0; i < ARRAY_SIZE(obsolete); i++) {
> +        if (syscall == obsolete[i].num) {
> +            return 1;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
> +int seccomp_start(uint8_t seccomp_opts)
>  {
>      int rc = 0;
>      unsigned int i = 0;
> @@ -69,6 +96,9 @@ int seccomp_start(void)
>      }
>  
>      for (i = 0; i < ARRAY_SIZE(blacklist); i++) {
> +        if ((seccomp_opts & OBSOLETE) && is_obsolete(blacklist[i].num)) {
> +            continue;
> +        }

IMHO this is leading to a rather inefficient approach. Why not extend
QemuSeccompSyscall struct so that it has another field to list which
set it belongs to. Then you can do


  static const struct QemuSeccompSyscall blacklist[] = {
    { SCMP_SYS(reboot), 255, QEMU_SECCOMP_SET_DEFAULT },
    { SCMP_SYS(swapon), 255, QEMU_SECCOMP_SET_DEFAULT },
     ....
    { SCMP_SYS(readdir), 255, QEMU_SECCOMP_SET_OBSOLETE },
    { SCMP_SYS(_sysctl), 255, QEMU_SECCOMP_SET_OBSOLETE },
    ...

And then to process this you can do

      for (i = 0; i < ARRAY_SIZE(blacklist); i++) {
          if (blacklist[i].set != QEMU_SECCOMP_SET_OBSOLETE &&
              blacklist[i].set & seccomp_opts) {
              continue;
          }


>          rc = seccomp_rule_add(ctx, SCMP_ACT_KILL, blacklist[i].num, 0);
>          if (rc < 0) {
>              goto seccomp_return;
> diff --git a/vl.c b/vl.c
> index 15b98800e9..cbe09c94af 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -271,6 +271,10 @@ static QemuOptsList qemu_sandbox_opts = {
>              .name = "enable",
>              .type = QEMU_OPT_BOOL,
>          },
> +        {
> +            .name = "obsolete",
> +            .type = QEMU_OPT_STRING,
> +        },
>          { /* end of list */ }
>      },
>  };
> @@ -1032,7 +1036,17 @@ static int parse_sandbox(void *opaque, QemuOpts *opts, 
> Error **errp)
>  {
>      if (qemu_opt_get_bool(opts, "enable", false)) {
>  #ifdef CONFIG_SECCOMP
> -        if (seccomp_start() < 0) {
> +        uint8_t seccomp_opts = 0x0000;
> +        const char *value = NULL;
> +
> +        value = qemu_opt_get(opts, "obsolete");
> +        if (value) {
> +            if (strcmp(value, "allow") == 0) {
> +                seccomp_opts |= OBSOLETE;
> +            }
> +        }

IIUC, the values will all be booleans, so we should just use

   if (qemu_opt_get_bool(opts, "obsolete", false))
       seccomp_opts |= OBSOLETE;

> +
> +        if (seccomp_start(seccomp_opts) < 0) {
>              error_report("failed to install seccomp syscall filter "
>                           "in the kernel");
>              return -1;

Regards,
Daniel
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