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sandbox: allow restrictive nested seccomp filters without BPF JIT #43

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46 changes: 24 additions & 22 deletions src/action_runner.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -820,24 +820,20 @@ fn actionAuditArchitecture() u32 {
};
}

fn actionSeccompFilterInstructions() [23]SeccompFilterInstruction {
fn actionSeccompFilterInstructions() [19]SeccompFilterInstruction {
const linux = std.os.linux;
return .{
.{ .code = classic_bpf_load_absolute_word, .jump_true = 0, .jump_false = 0, .data = 4 },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 0, .jump_false = 19, .data = actionAuditArchitecture() },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 0, .jump_false = 15, .data = actionAuditArchitecture() },
.{ .code = classic_bpf_load_absolute_word, .jump_true = 0, .jump_false = 0, .data = 0 },
.{ .code = classic_bpf_bitwise_and_constant, .jump_true = 0, .jump_false = 0, .data = ~x86_x32_syscall_bit },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 6, .jump_false = 0, .data = @intFromEnum(linux.SYS.socket) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 14, .jump_false = 0, .data = @intFromEnum(linux.SYS.io_uring_setup) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 13, .jump_false = 0, .data = @intFromEnum(linux.SYS.seccomp) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 12, .jump_false = 0, .data = @intFromEnum(linux.SYS.bpf) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 4, .jump_false = 0, .data = @intFromEnum(linux.SYS.prctl) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 5, .jump_false = 0, .data = @intFromEnum(linux.SYS.setsockopt) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 4, .jump_false = 0, .data = @intFromEnum(linux.SYS.socket) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 10, .jump_false = 0, .data = @intFromEnum(linux.SYS.io_uring_setup) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 9, .jump_false = 0, .data = @intFromEnum(linux.SYS.bpf) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 3, .jump_false = 0, .data = @intFromEnum(linux.SYS.setsockopt) },
.{ .code = classic_bpf_return_constant, .jump_true = 0, .jump_false = 0, .data = linux.SECCOMP.RET.ALLOW },
.{ .code = classic_bpf_load_absolute_word, .jump_true = 0, .jump_false = 0, .data = 16 },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 7, .jump_false = 9, .data = linux.AF.VSOCK },
.{ .code = classic_bpf_load_absolute_word, .jump_true = 0, .jump_false = 0, .data = 16 },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 5, .jump_false = 7, .data = @intCast(@intFromEnum(linux.PR.SET_SECCOMP)) },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 5, .jump_false = 7, .data = linux.AF.VSOCK },
.{ .code = classic_bpf_load_absolute_word, .jump_true = 0, .jump_false = 0, .data = 24 },
.{ .code = classic_bpf_jump_equal_constant, .jump_true = 0, .jump_false = 5, .data = linux.SOL.SOCKET },
.{ .code = classic_bpf_load_absolute_word, .jump_true = 0, .jump_false = 0, .data = 32 },
Expand Down Expand Up @@ -1554,25 +1550,15 @@ test "sandbox seccomp rejects AF_VSOCK and io_uring_setup" {
));
}

test "sandbox seccomp rejects untrusted classic BPF JIT entry points" {
test "sandbox seccomp rejects untrusted BPF and socket filter entry points" {
const linux = std.os.linux;
const denied = @as(u32, linux.SECCOMP.RET.ERRNO) | @as(u32, @intFromEnum(std.posix.E.PERM));

try std.testing.expectEqual(denied, try evaluateActionSeccompFilterForTest(
actionAuditArchitecture(),
@intFromEnum(linux.SYS.seccomp),
.{ 0, 0, 0, 0, 0, 0 },
));
try std.testing.expectEqual(denied, try evaluateActionSeccompFilterForTest(
actionAuditArchitecture(),
@intFromEnum(linux.SYS.bpf),
.{ 0, 0, 0, 0, 0, 0 },
));
try std.testing.expectEqual(denied, try evaluateActionSeccompFilterForTest(
actionAuditArchitecture(),
@intFromEnum(linux.SYS.prctl),
.{ @intCast(@intFromEnum(linux.PR.SET_SECCOMP)), 0, 0, 0, 0, 0 },
));
for ([_]u32{ linux.SO.ATTACH_FILTER, linux.SO.ATTACH_REUSEPORT_CBPF }) |option| {
try std.testing.expectEqual(denied, try evaluateActionSeccompFilterForTest(
actionAuditArchitecture(),
Expand All @@ -1582,6 +1568,22 @@ test "sandbox seccomp rejects untrusted classic BPF JIT entry points" {
}
}

test "sandbox seccomp allows additional restrictive filters" {
const linux = std.os.linux;
const allowed: u32 = linux.SECCOMP.RET.ALLOW;

try std.testing.expectEqual(allowed, try evaluateActionSeccompFilterForTest(
actionAuditArchitecture(),
@intFromEnum(linux.SYS.seccomp),
.{ linux.SECCOMP.SET_MODE_FILTER, 0, 0, 0, 0, 0 },
));
try std.testing.expectEqual(allowed, try evaluateActionSeccompFilterForTest(
actionAuditArchitecture(),
@intFromEnum(linux.SYS.prctl),
.{ @intCast(@intFromEnum(linux.PR.SET_SECCOMP)), linux.SECCOMP.MODE.FILTER, 0, 0, 0, 0 },
));
}

test "sandbox seccomp allows unrelated socket options and prctl operations" {
const linux = std.os.linux;
const allowed: u32 = linux.SECCOMP.RET.ALLOW;
Expand Down
56 changes: 26 additions & 30 deletions tools/e2e_action_tool.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -259,37 +259,12 @@ fn expectBpfProgramsBlocked() !void {
return error.SeccompCheckFailed;
}

const blocked_calls = [_]struct { name: []const u8, result: usize }{
.{
.name = "seccomp",
.result = linux.seccomp(linux.SECCOMP.SET_MODE_FILTER, 0, null),
},
.{
.name = "prctl(PR_SET_SECCOMP)",
.result = linux.prctl(
@intFromEnum(linux.PR.SET_SECCOMP),
linux.SECCOMP.MODE.FILTER,
0,
0,
0,
),
},
.{
.name = "bpf",
.result = linux.syscall3(.bpf, 0, 0, 0),
switch (linux.errno(linux.syscall3(.bpf, 0, 0, 0))) {
.PERM, .ACCES => {},
else => |err| {
std.debug.print("sandboxed action unexpectedly reached bpf: {s}\n", .{@tagName(err)});
return error.SeccompCheckFailed;
},
};
for (blocked_calls) |blocked_call| {
switch (linux.errno(blocked_call.result)) {
.PERM, .ACCES => {},
else => |err| {
std.debug.print(
"sandboxed action unexpectedly reached {s}: {s}\n",
.{ blocked_call.name, @tagName(err) },
);
return error.SeccompCheckFailed;
},
}
}

const socket_rc = linux.socket(linux.AF.INET, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, 0);
Expand Down Expand Up @@ -333,6 +308,27 @@ fn expectBpfProgramsBlocked() !void {
std.debug.print("sandbox seccomp filter rejected SO_REUSEADDR\n", .{});
return error.SeccompCheckFailed;
}

const Instruction = extern struct { code: u16, jump_true: u8, jump_false: u8, data: u32 };
const Program = extern struct { length: u16, instructions: [*]const Instruction };
const instructions = [_]Instruction{.{
.code = 0x06,
.jump_true = 0,
.jump_false = 0,
.data = linux.SECCOMP.RET.ALLOW,
}};
const program = Program{ .length = instructions.len, .instructions = &instructions };
const nested = linux.prctl(
@intFromEnum(linux.PR.SET_SECCOMP),
linux.SECCOMP.MODE.FILTER,
@intFromPtr(&program),
0,
0,
);
if (linux.errno(nested) != .SUCCESS) {
std.debug.print("sandbox rejected an additional seccomp filter: {s}\n", .{@tagName(linux.errno(nested))});
return error.SeccompCheckFailed;
}
}

fn expectCgroupLimits(io: std.Io) !void {
Expand Down
2 changes: 1 addition & 1 deletion vm/linux.config
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ CONFIG_NR_CPUS=64
CONFIG_BINFMT_ELF=y
CONFIG_BINFMT_SCRIPT=y
CONFIG_BPF=y
CONFIG_BPF_JIT=y
# CONFIG_BPF_JIT is not set
CONFIG_BLOCK=y
CONFIG_BLK_DEV=y
CONFIG_BLK_DEV_SD=y
Expand Down