bpf: prevent out-of-bounds speculation

Under speculation, CPUs may mis-predict branches in bounds checks. Thus,
memory accesses under a bounds check may be speculated even if the
bounds check fails, providing a primitive for building a side channel.

To avoid leaking kernel data round up array-based maps and mask the index
after bounds check, so speculated load with out of bounds index will load
either valid value from the array or zero from the padded area.

Unconditionally mask index for all array types even when max_entries
are not rounded to power of 2 for root user.
When map is created by unpriv user generate a sequence of bpf insns
that includes AND operation to make sure that JITed code includes
the same 'index & index_mask' operation.

If prog_array map is created by unpriv user replace
  bpf_tail_call(ctx, map, index);
with
  if (index >= max_entries) {
    index &= map->index_mask;
    bpf_tail_call(ctx, map, index);
  }
(along with roundup to power 2) to prevent out-of-bounds speculation.
There is secondary redundant 'if (index >= max_entries)' in the interpreter
and in all JITs, but they can be optimized later if necessary.

Other array-like maps (cpumap, devmap, sockmap, perf_event_array, cgroup_array)
cannot be used by unpriv, so no changes there.

That fixes bpf side of "Variant 1: bounds check bypass (CVE-2017-5753)" on
all architectures with and without JIT.

v2->v3:
Daniel noticed that attack potentially can be crafted via syscall commands
without loading the program, so add masking to those paths as well.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 36 insertions, 11 deletions

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 7c25426d3cf5..aaa319848e7d 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -53,9 +53,10 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
 {
         bool percpu = attr->map_type == BPF_MAP_TYPE_PERCPU_ARRAY;
         int numa_node = bpf_map_attr_numa_node(attr);
+        u32 elem_size, index_mask, max_entries;
+        bool unpriv = !capable(CAP_SYS_ADMIN);
         struct bpf_array *array;
         u64 array_size;
-        u32 elem_size;
 
         /* check sanity of attributes */
         if (attr->max_entries == 0 || attr->key_size != 4 ||
@@ -72,11 +73,20 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
 
         elem_size = round_up(attr->value_size, 8);
 
+        max_entries = attr->max_entries;
+        index_mask = roundup_pow_of_two(max_entries) - 1;
+
+        if (unpriv)
+                /* round up array size to nearest power of 2,
+                 * since cpu will speculate within index_mask limits
+                 */
+                max_entries = index_mask + 1;
+
         array_size = sizeof(*array);
         if (percpu)
-                array_size += (u64) attr->max_entries * sizeof(void *);
+                array_size += (u64) max_entries * sizeof(void *);
         else
-                array_size += (u64) attr->max_entries * elem_size;
+                array_size += (u64) max_entries * elem_size;
 
         /* make sure there is no u32 overflow later in round_up() */
         if (array_size >= U32_MAX - PAGE_SIZE)
@@ -86,6 +96,8 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
         array = bpf_map_area_alloc(array_size, numa_node);
         if (!array)
                 return ERR_PTR(-ENOMEM);
+        array->index_mask = index_mask;
+        array->map.unpriv_array = unpriv;
 
         /* copy mandatory map attributes */
         array->map.map_type = attr->map_type;
@@ -121,12 +133,13 @@ static void *array_map_lookup_elem(struct bpf_map *map, void *key)
         if (unlikely(index >= array->map.max_entries))
                 return NULL;
 
-        return array->value + array->elem_size * index;
+        return array->value + array->elem_size * (index & array->index_mask);
 }
 
 /* emit BPF instructions equivalent to C code of array_map_lookup_elem() */
 static u32 array_map_gen_lookup(struct bpf_map *map, struct bpf_insn *insn_buf)
 {
+        struct bpf_array *array = container_of(map, struct bpf_array, map);
         struct bpf_insn *insn = insn_buf;
         u32 elem_size = round_up(map->value_size, 8);
         const int ret = BPF_REG_0;
@@ -135,7 +148,12 @@ static u32 array_map_gen_lookup(struct bpf_map *map, struct bpf_insn *insn_buf)
 
         *insn++ = BPF_ALU64_IMM(BPF_ADD, map_ptr, offsetof(struct bpf_array, value));
         *insn++ = BPF_LDX_MEM(BPF_W, ret, index, 0);
-        *insn++ = BPF_JMP_IMM(BPF_JGE, ret, map->max_entries, 3);
+        if (map->unpriv_array) {
+                *insn++ = BPF_JMP_IMM(BPF_JGE, ret, map->max_entries, 4);
+                *insn++ = BPF_ALU32_IMM(BPF_AND, ret, array->index_mask);
+        } else {
+                *insn++ = BPF_JMP_IMM(BPF_JGE, ret, map->max_entries, 3);
+        }
 
         if (is_power_of_2(elem_size)) {
                 *insn++ = BPF_ALU64_IMM(BPF_LSH, ret, ilog2(elem_size));
@@ -157,7 +175,7 @@ static void *percpu_array_map_lookup_elem(struct bpf_map *map, void *key)
         if (unlikely(index >= array->map.max_entries))
                 return NULL;
 
-        return this_cpu_ptr(array->pptrs[index]);
+        return this_cpu_ptr(array->pptrs[index & array->index_mask]);
 }
 
 int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value)
@@ -177,7 +195,7 @@ int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value)
          */
         size = round_up(map->value_size, 8);
         rcu_read_lock();
-        pptr = array->pptrs[index];
+        pptr = array->pptrs[index & array->index_mask];
         for_each_possible_cpu(cpu) {
                 bpf_long_memcpy(value + off, per_cpu_ptr(pptr, cpu), size);
                 off += size;
@@ -225,10 +243,11 @@ static int array_map_update_elem(struct bpf_map *map, void *key, void *value,
                 return -EEXIST;
 
         if (array->map.map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
-                memcpy(this_cpu_ptr(array->pptrs[index]),
+                memcpy(this_cpu_ptr(array->pptrs[index & array->index_mask]),
                        value, map->value_size);
         else
-                memcpy(array->value + array->elem_size * index,
+                memcpy(array->value +
+                       array->elem_size * (index & array->index_mask),
                        value, map->value_size);
         return 0;
 }
@@ -262,7 +281,7 @@ int bpf_percpu_array_update(struct bpf_map *map, void *key, void *value,
          */
         size = round_up(map->value_size, 8);
         rcu_read_lock();
-        pptr = array->pptrs[index];
+        pptr = array->pptrs[index & array->index_mask];
         for_each_possible_cpu(cpu) {
                 bpf_long_memcpy(per_cpu_ptr(pptr, cpu), value + off, size);
                 off += size;
@@ -613,6 +632,7 @@ static void *array_of_map_lookup_elem(struct bpf_map *map, void *key)
 static u32 array_of_map_gen_lookup(struct bpf_map *map,
                                    struct bpf_insn *insn_buf)
 {
+        struct bpf_array *array = container_of(map, struct bpf_array, map);
         u32 elem_size = round_up(map->value_size, 8);
         struct bpf_insn *insn = insn_buf;
         const int ret = BPF_REG_0;
@@ -621,7 +641,12 @@ static u32 array_of_map_gen_lookup(struct bpf_map *map,
 
         *insn++ = BPF_ALU64_IMM(BPF_ADD, map_ptr, offsetof(struct bpf_array, value));
         *insn++ = BPF_LDX_MEM(BPF_W, ret, index, 0);
-        *insn++ = BPF_JMP_IMM(BPF_JGE, ret, map->max_entries, 5);
+        if (map->unpriv_array) {
+                *insn++ = BPF_JMP_IMM(BPF_JGE, ret, map->max_entries, 6);
+                *insn++ = BPF_ALU32_IMM(BPF_AND, ret, array->index_mask);
+        } else {
+                *insn++ = BPF_JMP_IMM(BPF_JGE, ret, map->max_entries, 5);
+        }
         if (is_power_of_2(elem_size))
                 *insn++ = BPF_ALU64_IMM(BPF_LSH, ret, ilog2(elem_size));
         else