[libvirt] [PATCH 03/19] vircgroup: introduce virCgroupV2DeviceLoadProg

[Pavel Hrdina]

There are two possible ways how to create BPF program:

    - One way is to write simple C-like code which can by compiled into
      BPF object file which can be loaded into kernel using elfutils.

    - The second way is to define macros which looks like assembler
      instructions and can be used directly to create BPF program that
      can be directly loaded into kernel.

Since the program is not too complex we can use the second option.

If there is no program, all devices are allowed, if there is some
program it is executed and based on the exit status the access is
denied for 0 and allowed for 1.

Our program will follow these rules:

    - first it will try to look for the specific key using major and
      minor to see if there is any rule for that specific device

    - if there is no specific rule it will try to look for any rule that
      matches only major of the device

    - if there is no match with major it will try the same but with
      minor of the device

    - as the last attempt it will try to look for rule for all devices
      and if there is no match it will return 0 to deny that access

Signed-off-by: Pavel Hrdina <phrdina at redhat.com>
---
 src/util/vircgroupv2.c | 76 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)

diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c
index 0adc0d4d23..63e3123cd9 100644
--- a/src/util/vircgroupv2.c
+++ b/src/util/vircgroupv2.c
@@ -1595,6 +1595,82 @@ virCgroupV2GetCpuacctStat(virCgroupPtr group,
 }
 
 
+static int
+virCgroupV2DeviceLoadProg(int mapfd)
+{
+# define VIR_CGROUP_BPF_LOOKUP \
+    /* prepare key param on stack */ \
+    VIR_BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_2, -8), \
+    VIR_BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), \
+    VIR_BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), \
+    /* lookup key (major << 32) | minor in map */ \
+    VIR_BPF_LD_MAP_FD(BPF_REG_1, mapfd), \
+    VIR_BPF_CALL_INSN(BPF_FUNC_map_lookup_elem)
+
+# define VIR_CGROUP_BPF_CHECK_PERM \
+    /* if no key skip perm check */ \
+    VIR_BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 6), \
+    /* get perms from map */ \
+    VIR_BPF_LDX_MEM(BPF_W, BPF_REG_1, BPF_REG_0, 0), \
+    /* get perms from ctx */ \
+    VIR_BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_6, 0), \
+    /* (map perms) & (ctx perms) */ \
+    VIR_BPF_ALU64_REG(BPF_AND, BPF_REG_1, BPF_REG_2), \
+    /* if (map perms) & (ctx perms) == (ctx perms) exit, otherwise continue */ \
+    VIR_BPF_JMP_REG(BPF_JNE, BPF_REG_1, BPF_REG_2, 2), \
+    /* set ret 1 and exit */ \
+    VIR_BPF_MOV64_IMM(BPF_REG_0, 1), \
+    VIR_BPF_EXIT_INSN()
+
+    struct bpf_insn prog[] = {
+        /* save ctx, argument passed to BPF program */
+        VIR_BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),
+
+        /* get major from ctx and shift << 32 */
+        VIR_BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_6, 4),
+        VIR_BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 32),
+        /* get minor from ctx and | to shifted major */
+        VIR_BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_6, 8),
+        VIR_BPF_ALU64_REG(BPF_OR, BPF_REG_2, BPF_REG_3),
+        /* lookup ((major << 32) | minor) in map and check perms */
+        VIR_CGROUP_BPF_LOOKUP,
+        VIR_CGROUP_BPF_CHECK_PERM,
+
+        /* get major from ctx and shift << 32 */
+        VIR_BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_6, 4),
+        VIR_BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 32),
+        /* use -1 as minor and | to shifted major */
+        VIR_BPF_MOV32_IMM(BPF_REG_3, -1),
+        VIR_BPF_ALU64_REG(BPF_OR, BPF_REG_2, BPF_REG_3),
+        /* lookup ((major << 32) | -1) in map and check perms */
+        VIR_CGROUP_BPF_LOOKUP,
+        VIR_CGROUP_BPF_CHECK_PERM,
+
+        /* use -1 as major and shift << 32 */
+        VIR_BPF_MOV32_IMM(BPF_REG_2, -1),
+        VIR_BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 32),
+        /* get minor from ctx and | to shifted major */
+        VIR_BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_6, 8),
+        VIR_BPF_ALU64_REG(BPF_OR, BPF_REG_2, BPF_REG_3),
+        /* lookup ((-1 << 32) | minor) in map and check perms */
+        VIR_CGROUP_BPF_LOOKUP,
+        VIR_CGROUP_BPF_CHECK_PERM,
+
+        /* use -1 as key which means major = -1 and minor = -1 */
+        VIR_BPF_MOV64_IMM(BPF_REG_2, -1),
+        /* lookup -1 in map and check perms*/
+        VIR_CGROUP_BPF_LOOKUP,
+        VIR_CGROUP_BPF_CHECK_PERM,
+
+        /* no key was found, exit with 0 */
+        VIR_BPF_MOV64_IMM(BPF_REG_0, 0),
+        VIR_BPF_EXIT_INSN(),
+    };
+
+    return virBPFLoadProg(prog, BPF_PROG_TYPE_CGROUP_DEVICE, ARRAY_CARDINALITY(prog));
+}
+
+
 virCgroupBackend virCgroupV2Backend = {
     .type = VIR_CGROUP_BACKEND_TYPE_V2,
 
-- 
2.20.1