On 08.09.2017 13:15, Eduardo Otubo wrote: > On Fri, Sep 08, 2017 at 11:50:12AM +0200, Thomas Huth wrote: >> On 08.09.2017 11:10, Eduardo Otubo wrote: >>> This patch adds [,spawn=deny] argument to `-sandbox on' option. It >>> blacklists fork and execve system calls, avoiding Qemu to spawn new >>> threads or processes. >>> >>> Signed-off-by: Eduardo Otubo <[email protected]> >>> --- >>> include/sysemu/seccomp.h | 1 + >>> qemu-options.hx | 9 +++++++-- >>> qemu-seccomp.c | 12 ++++++++++++ >>> vl.c | 16 ++++++++++++++++ >>> 4 files changed, 36 insertions(+), 2 deletions(-) >>> >>> diff --git a/include/sysemu/seccomp.h b/include/sysemu/seccomp.h >>> index 4a9e63c7cd..3ab5fc4f61 100644 >>> --- a/include/sysemu/seccomp.h >>> +++ b/include/sysemu/seccomp.h >>> @@ -18,6 +18,7 @@ >>> #define QEMU_SECCOMP_SET_DEFAULT (1 << 0) >>> #define QEMU_SECCOMP_SET_OBSOLETE (1 << 1) >>> #define QEMU_SECCOMP_SET_PRIVILEGED (1 << 2) >>> +#define QEMU_SECCOMP_SET_SPAWN (1 << 3) >>> >>> #include <seccomp.h> >>> >>> diff --git a/qemu-options.hx b/qemu-options.hx >>> index 5c1b163fb5..2b04b9f170 100644 >>> --- a/qemu-options.hx >>> +++ b/qemu-options.hx >>> @@ -4018,6 +4018,7 @@ ETEXI >>> >>> DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \ >>> "-sandbox >>> on[,obsolete=allow|deny][,elevateprivileges=allow|deny|children]\n" \ >>> + " [,spawn=allow|deny]\n" \ >>> " Enable seccomp mode 2 system call filter (default >>> 'off').\n" \ >>> " use 'obsolete' to allow obsolete system calls that >>> are provided\n" \ >>> " by the kernel, but typically no longer used by >>> modern\n" \ >>> @@ -4025,10 +4026,12 @@ DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \ >>> " use 'elevateprivileges' to allow or deny QEMU process >>> to elevate\n" \ >>> " its privileges by blacklisting all set*uid|gid >>> system calls.\n" \ >>> " The value 'children' will deny set*uid|gid system >>> calls for\n" \ >>> - " main QEMU process but will allow forks and >>> execves to run unprivileged\n", >>> + " main QEMU process but will allow forks and >>> execves to run unprivileged\n" \ >>> + " use 'spawn' to avoid QEMU to spawn new threads or >>> processes by\n" \ >>> + " blacklisting *fork and execve\n", >>> QEMU_ARCH_ALL) >>> STEXI >>> -@item -sandbox >>> @var{arg}[,obsolete=@var{string}][,elevateprivileges=@var{string}] >>> +@item -sandbox >>> @var{arg}[,obsolete=@var{string}][,elevateprivileges=@var{string}][,spawn=@var{string}] >>> @findex -sandbox >>> Enable Seccomp mode 2 system call filter. 'on' will enable syscall >>> filtering and 'off' will >>> disable it. The default is 'off'. >>> @@ -4037,6 +4040,8 @@ disable it. The default is 'off'. >>> Enable Obsolete system calls >>> @item elevateprivileges=@var{string} >>> Disable set*uid|gid system calls >>> +@item spawn=@var{string} >>> +Disable *fork and execve >>> @end table >>> ETEXI >>> >>> diff --git a/qemu-seccomp.c b/qemu-seccomp.c >>> index 2bad16cafb..4c169febf8 100644 >>> --- a/qemu-seccomp.c >>> +++ b/qemu-seccomp.c >>> @@ -79,6 +79,10 @@ static const struct QemuSeccompSyscall blacklist[] = { >>> { SCMP_SYS(setresgid), 4, QEMU_SECCOMP_SET_PRIVILEGED }, >>> { SCMP_SYS(setfsuid), 4, QEMU_SECCOMP_SET_PRIVILEGED }, >>> { SCMP_SYS(setfsgid), 4, QEMU_SECCOMP_SET_PRIVILEGED }, >>> + /* spawn */ >>> + { SCMP_SYS(fork), 8, QEMU_SECCOMP_SET_SPAWN }, >>> + { SCMP_SYS(vfork), 8, QEMU_SECCOMP_SET_SPAWN }, >>> + { SCMP_SYS(execve), 8, QEMU_SECCOMP_SET_SPAWN }, >>> }; >>> >>> >>> @@ -109,6 +113,14 @@ int seccomp_start(uint32_t seccomp_opts) >>> } >>> >>> break; >>> + case QEMU_SECCOMP_SET_SPAWN: >>> + if (seccomp_opts & QEMU_SECCOMP_SET_SPAWN) { >>> + break; >>> + } else { >>> + continue; >>> + } >>> + >> >> Remove the above empty line? >> >> Anyway, it's somewhat ugly that you need a switch-case statement here at >> all. Couldn't you simply check it like this: >> >> if (!(seccomp_opts & blacklist[i].set)) { >> continue; >> } >> ? >> >> You then just have to invert the meaning of the >> QEMU_SECCOMP_SET_OBSOLETE bit in the second patch, so that this bit is >> treated in the same way as the others (i.e. use >> uint32_t seccomp_opts = QEMU_SECCOMP_SET_OBSOLETE; >> instead of >> uint32_t seccomp_opts = 0x00000; >> in vl.c in the second patch). > > That's indeed much better, but perhaps: > uint32_t seccomp_opts = QEMU_SECCOMP_SET_DEFAULT | QEMU_SECCOMP_SET_OBSOLETE;
Right, the default set should be excluded by default of course, too! :-) Thomas
