bpf: Verify scalar ids mapping in regsafe() using check_ids()

Make sure that the following unsafe example is rejected by verifier:

1: r9 = ... some pointer with range X ...
2: r6 = ... unbound scalar ID=a ...
3: r7 = ... unbound scalar ID=b ...
4: if (r6 > r7) goto +1
5: r6 = r7
6: if (r6 > X) goto ...
--- checkpoint ---
7: r9 += r7
8: *(u64 *)r9 = Y

This example is unsafe because not all execution paths verify r7 range.
Because of the jump at (4) the verifier would arrive at (6) in two states:
I.  r6{.id=b}, r7{.id=b} via path 1-6;
II. r6{.id=a}, r7{.id=b} via path 1-4, 6.

Currently regsafe() does not call check_ids() for scalar registers,
thus from POV of regsafe() states (I) and (II) are identical. If the
path 1-6 is taken by verifier first, and checkpoint is created at (6)
the path [1-4, 6] would be considered safe.

Changes in this commit:
- check_ids() is modified to disallow mapping multiple old_id to the
  same cur_id.
- check_scalar_ids() is added, unlike check_ids() it treats ID zero as
  a unique scalar ID.
- check_scalar_ids() needs to generate temporary unique IDs, field
  'tmp_id_gen' is added to bpf_verifier_env::idmap_scratch to
  facilitate this.
- regsafe() is updated to:
  - use check_scalar_ids() for precise scalar registers.
  - compare scalar registers using memcmp only for explore_alu_limits
    branch. This simplifies control flow for scalar case, and has no
    measurable performance impact.
- check_alu_op() is updated to avoid generating bpf_reg_state::id for
  constant scalar values when processing BPF_MOV. ID is needed to
  propagate range information for identical values, but there is
  nothing to propagate for constants.

Fixes: 75748837b7e5 ("bpf: Propagate scalar ranges through register assignments.")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230613153824.3324830-4-eddyz87@gmail.com

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 22fb13c..f70f9ac 100644 (file)
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -313,11 +313,6 @@ struct bpf_idx_pair {
        u32 idx;
 };
 
-struct bpf_id_pair {
-       u32 old;
-       u32 cur;
-};
-
 #define MAX_CALL_FRAMES 8
 /* Maximum number of register states that can exist at once */
 #define BPF_ID_MAP_SIZE ((MAX_BPF_REG + MAX_BPF_STACK / BPF_REG_SIZE) * MAX_CALL_FRAMES)
@@ -557,6 +552,16 @@ struct backtrack_state {
        u64 stack_masks[MAX_CALL_FRAMES];
 };
 
+struct bpf_id_pair {
+       u32 old;
+       u32 cur;
+};
+
+struct bpf_idmap {
+       u32 tmp_id_gen;
+       struct bpf_id_pair map[BPF_ID_MAP_SIZE];
+};
+
 struct bpf_idset {
        u32 count;
        u32 ids[BPF_ID_MAP_SIZE];
@@ -594,7 +599,7 @@ struct bpf_verifier_env {
        struct bpf_verifier_log log;
        struct bpf_subprog_info subprog_info[BPF_MAX_SUBPROGS + 1];
        union {
-               struct bpf_id_pair idmap_scratch[BPF_ID_MAP_SIZE];
+               struct bpf_idmap idmap_scratch;
                struct bpf_idset idset_scratch;
        };
        struct {

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 064aef5..fa43dc8 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12934,12 +12934,14 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
                if (BPF_SRC(insn->code) == BPF_X) {
                        struct bpf_reg_state *src_reg = regs + insn->src_reg;
                        struct bpf_reg_state *dst_reg = regs + insn->dst_reg;
+                       bool need_id = src_reg->type == SCALAR_VALUE && !src_reg->id &&
+                                      !tnum_is_const(src_reg->var_off);
 
                        if (BPF_CLASS(insn->code) == BPF_ALU64) {
                                /* case: R1 = R2
                                 * copy register state to dest reg
                                 */
-                               if (src_reg->type == SCALAR_VALUE && !src_reg->id)
+                               if (need_id)
                                        /* Assign src and dst registers the same ID
                                         * that will be used by find_equal_scalars()
                                         * to propagate min/max range.
@@ -12958,7 +12960,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
                                } else if (src_reg->type == SCALAR_VALUE) {
                                        bool is_src_reg_u32 = src_reg->umax_value <= U32_MAX;
 
-                                       if (is_src_reg_u32 && !src_reg->id)
+                                       if (is_src_reg_u32 && need_id)
                                                src_reg->id = ++env->id_gen;
                                        copy_register_state(dst_reg, src_reg);
                                        /* Make sure ID is cleared if src_reg is not in u32 range otherwise
@@ -15114,8 +15116,9 @@ static bool range_within(struct bpf_reg_state *old,
  * So we look through our idmap to see if this old id has been seen before.  If
  * so, we require the new id to match; otherwise, we add the id pair to the map.
  */
-static bool check_ids(u32 old_id, u32 cur_id, struct bpf_id_pair *idmap)
+static bool check_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
 {
+       struct bpf_id_pair *map = idmap->map;
        unsigned int i;
 
        /* either both IDs should be set or both should be zero */
@@ -15126,20 +15129,34 @@ static bool check_ids(u32 old_id, u32 cur_id, struct bpf_id_pair *idmap)
                return true;
 
        for (i = 0; i < BPF_ID_MAP_SIZE; i++) {
-               if (!idmap[i].old) {
+               if (!map[i].old) {
                        /* Reached an empty slot; haven't seen this id before */
-                       idmap[i].old = old_id;
-                       idmap[i].cur = cur_id;
+                       map[i].old = old_id;
+                       map[i].cur = cur_id;
                        return true;
                }
-               if (idmap[i].old == old_id)
-                       return idmap[i].cur == cur_id;
+               if (map[i].old == old_id)
+                       return map[i].cur == cur_id;
+               if (map[i].cur == cur_id)
+                       return false;
        }
        /* We ran out of idmap slots, which should be impossible */
        WARN_ON_ONCE(1);
        return false;
 }
 
+/* Similar to check_ids(), but allocate a unique temporary ID
+ * for 'old_id' or 'cur_id' of zero.
+ * This makes pairs like '0 vs unique ID', 'unique ID vs 0' valid.
+ */
+static bool check_scalar_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
+{
+       old_id = old_id ? old_id : ++idmap->tmp_id_gen;
+       cur_id = cur_id ? cur_id : ++idmap->tmp_id_gen;
+
+       return check_ids(old_id, cur_id, idmap);
+}
+
 static void clean_func_state(struct bpf_verifier_env *env,
                             struct bpf_func_state *st)
 {
@@ -15238,7 +15255,7 @@ next:
 
 static bool regs_exact(const struct bpf_reg_state *rold,
                       const struct bpf_reg_state *rcur,
-                      struct bpf_id_pair *idmap)
+                      struct bpf_idmap *idmap)
 {
        return memcmp(rold, rcur, offsetof(struct bpf_reg_state, id)) == 0 &&
               check_ids(rold->id, rcur->id, idmap) &&
@@ -15247,7 +15264,7 @@ static bool regs_exact(const struct bpf_reg_state *rold,
 
 /* Returns true if (rold safe implies rcur safe) */
 static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
-                   struct bpf_reg_state *rcur, struct bpf_id_pair *idmap)
+                   struct bpf_reg_state *rcur, struct bpf_idmap *idmap)
 {
        if (!(rold->live & REG_LIVE_READ))
                /* explored state didn't use this */
@@ -15284,15 +15301,42 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
 
        switch (base_type(rold->type)) {
        case SCALAR_VALUE:
-               if (regs_exact(rold, rcur, idmap))
-                       return true;
-               if (env->explore_alu_limits)
-                       return false;
+               if (env->explore_alu_limits) {
+                       /* explore_alu_limits disables tnum_in() and range_within()
+                        * logic and requires everything to be strict
+                        */
+                       return memcmp(rold, rcur, offsetof(struct bpf_reg_state, id)) == 0 &&
+                              check_scalar_ids(rold->id, rcur->id, idmap);
+               }
                if (!rold->precise)
                        return true;
-               /* new val must satisfy old val knowledge */
+               /* Why check_ids() for scalar registers?
+                *
+                * Consider the following BPF code:
+                *   1: r6 = ... unbound scalar, ID=a ...
+                *   2: r7 = ... unbound scalar, ID=b ...
+                *   3: if (r6 > r7) goto +1
+                *   4: r6 = r7
+                *   5: if (r6 > X) goto ...
+                *   6: ... memory operation using r7 ...
+                *
+                * First verification path is [1-6]:
+                * - at (4) same bpf_reg_state::id (b) would be assigned to r6 and r7;
+                * - at (5) r6 would be marked <= X, find_equal_scalars() would also mark
+                *   r7 <= X, because r6 and r7 share same id.
+                * Next verification path is [1-4, 6].
+                *
+                * Instruction (6) would be reached in two states:
+                *   I.  r6{.id=b}, r7{.id=b} via path 1-6;
+                *   II. r6{.id=a}, r7{.id=b} via path 1-4, 6.
+                *
+                * Use check_ids() to distinguish these states.
+                * ---
+                * Also verify that new value satisfies old value range knowledge.
+                */
                return range_within(rold, rcur) &&
-                      tnum_in(rold->var_off, rcur->var_off);
+                      tnum_in(rold->var_off, rcur->var_off) &&
+                      check_scalar_ids(rold->id, rcur->id, idmap);
        case PTR_TO_MAP_KEY:
        case PTR_TO_MAP_VALUE:
        case PTR_TO_MEM:
@@ -15338,7 +15382,7 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
 }
 
 static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
-                     struct bpf_func_state *cur, struct bpf_id_pair *idmap)
+                     struct bpf_func_state *cur, struct bpf_idmap *idmap)
 {
        int i, spi;
 
@@ -15441,7 +15485,7 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 }
 
 static bool refsafe(struct bpf_func_state *old, struct bpf_func_state *cur,
-                   struct bpf_id_pair *idmap)
+                   struct bpf_idmap *idmap)
 {
        int i;
 
@@ -15489,13 +15533,13 @@ static bool func_states_equal(struct bpf_verifier_env *env, struct bpf_func_stat
 
        for (i = 0; i < MAX_BPF_REG; i++)
                if (!regsafe(env, &old->regs[i], &cur->regs[i],
-                            env->idmap_scratch))
+                            &env->idmap_scratch))
                        return false;
 
-       if (!stacksafe(env, old, cur, env->idmap_scratch))
+       if (!stacksafe(env, old, cur, &env->idmap_scratch))
                return false;
 
-       if (!refsafe(old, cur, env->idmap_scratch))
+       if (!refsafe(old, cur, &env->idmap_scratch))
                return false;
 
        return true;
@@ -15510,7 +15554,8 @@ static bool states_equal(struct bpf_verifier_env *env,
        if (old->curframe != cur->curframe)
                return false;
 
-       memset(env->idmap_scratch, 0, sizeof(env->idmap_scratch));
+       env->idmap_scratch.tmp_id_gen = env->id_gen;
+       memset(&env->idmap_scratch.map, 0, sizeof(env->idmap_scratch.map));
 
        /* Verification state from speculative execution simulation
         * must never prune a non-speculative execution one.
@@ -15528,7 +15573,7 @@ static bool states_equal(struct bpf_verifier_env *env,
                return false;
 
        if (old->active_lock.id &&
-           !check_ids(old->active_lock.id, cur->active_lock.id, env->idmap_scratch))
+           !check_ids(old->active_lock.id, cur->active_lock.id, &env->idmap_scratch))
                return false;
 
        if (old->active_rcu_lock != cur->active_rcu_lock)