Re: [Qemu-devel] [PATCH 3/5] seccomp: add elevateprivileges argument to command line

[Paolo Bonzini]

On 14/03/2017 12:32, Eduardo Otubo wrote:
> This patch introduces the new argument [,elevateprivileges=deny] to the
> `-sandbox on'. It avoids Qemu process to elevate its privileges by
> blacklisting all set*uid|gid system calls
> 
> Signed-off-by: Eduardo Otubo <address@hidden>
> ---
>  include/sysemu/seccomp.h |  1 +
>  qemu-options.hx          |  8 ++++++--
>  qemu-seccomp.c           | 28 ++++++++++++++++++++++++++++
>  vl.c                     | 11 +++++++++++
>  4 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/include/sysemu/seccomp.h b/include/sysemu/seccomp.h
> index 7a7bde246b..e6e78d85ce 100644
> --- a/include/sysemu/seccomp.h
> +++ b/include/sysemu/seccomp.h
> @@ -16,6 +16,7 @@
>  #define QEMU_SECCOMP_H
>  
>  #define OBSOLETE    0x0001
> +#define PRIVILEGED  0x0010
>  
>  #include <seccomp.h>
>  
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 1403d0c85f..47018db5aa 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -3732,8 +3732,10 @@ Old param mode (ARM only).
>  ETEXI
>  
>  DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \
> -    "-sandbox on[,obsolete=allow]  Enable seccomp mode 2 system call filter 
> (default 'off').\n" \
> -    "                              obsolete: Allow obsolete system calls",
> +    "-sandbox on[,obsolete=allow][,elevateprivileges=deny]\n" \
> +    "                               Enable seccomp mode 2 system call filter 
> (default 'off').\n" \
> +    "                               obsolete: Allow obsolete system calls\n" 
> \
> +    "                               elevateprivileges: avoids Qemu process 
> to elevate its privileges by blacklisting all set*uid|gid system calls",

Why allow these by default?

Paolo

>      QEMU_ARCH_ALL)
>  STEXI
>  @item -sandbox @var{arg}[,address@hidden
> @@ -3743,6 +3745,8 @@ disable it.  The default is 'off'.
>  @table @option
>  @item address@hidden
>  Enable Obsolete system calls
> address@hidden address@hidden
> +Disable set*uid|gid systema calls
>  @end table
>  ETEXI
>  
> diff --git a/qemu-seccomp.c b/qemu-seccomp.c
> index 5ef36890da..5aa6590386 100644
> --- a/qemu-seccomp.c
> +++ b/qemu-seccomp.c
> @@ -31,6 +31,19 @@ struct QemuSeccompSyscall {
>      uint8_t priority;
>  };
>  
> +static const struct QemuSeccompSyscall privileged_syscalls[] = {
> +    { SCMP_SYS(setuid), 255 },
> +    { SCMP_SYS(setgid), 255 },
> +    { SCMP_SYS(setpgid), 255 },
> +    { SCMP_SYS(setsid), 255 },
> +    { SCMP_SYS(setreuid), 255 },
> +    { SCMP_SYS(setregid), 255 },
> +    { SCMP_SYS(setresuid), 255 },
> +    { SCMP_SYS(setresgid), 255 },
> +    { SCMP_SYS(setfsuid), 255 },
> +    { SCMP_SYS(setfsgid), 255 },
> +};
> +
>  static const struct QemuSeccompSyscall obsolete[] = {
>      { SCMP_SYS(readdir), 255 },
>      { SCMP_SYS(_sysctl), 255 },
> @@ -125,6 +138,21 @@ int seccomp_start(uint8_t seccomp_opts)
>          }
>      }
>  
> +    if (seccomp_opts & PRIVILEGED) {
> +        for (i = 0; i < ARRAY_SIZE(privileged_syscalls); i++) {
> +            rc = seccomp_rule_add(ctx, SCMP_ACT_KILL, 
> privileged_syscalls[i].num, 0);
> +            if (rc < 0) {
> +                goto seccomp_return;
> +            }
> +            rc = seccomp_syscall_priority(ctx, privileged_syscalls[i].num,
> +                    privileged_syscalls[i].priority);
> +            if (rc < 0) {
> +                goto seccomp_return;
> +            }
> +        }
> +    }
> +
> +
>      rc = seccomp_load(ctx);
>  
>    seccomp_return:
> diff --git a/vl.c b/vl.c
> index 7b08b3383b..d071e240b0 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -273,6 +273,10 @@ static QemuOptsList qemu_sandbox_opts = {
>              .name = "obsolete",
>              .type = QEMU_OPT_STRING,
>          },
> +        {
> +            .name = "elevateprivileges",
> +            .type = QEMU_OPT_STRING,
> +        },
>          { /* end of list */ }
>      },
>  };
> @@ -1045,6 +1049,13 @@ static int parse_sandbox(void *opaque, QemuOpts *opts, 
> Error **errp)
>              }
>          }
>  
> +        value = qemu_opt_get(opts,"elevateprivileges");
> +        if (value) {
> +            if (strcmp(value, "deny") == 0) {
> +                seccomp_opts |= PRIVILEGED;
> +            }
> +        }
> +
>          if (seccomp_start(seccomp_opts) < 0) {
>              error_report("failed to install seccomp syscall filter "
>                           "in the kernel");
>