[PATCH 6.4 162/800] bpf: Use scalar ids in mark_chain_precision()

Greg Kroah-Hartman
2023-07-16T20:00:22Z

From: Eduard Zingerman <eddyz87@gmail.com>

[ Upstream commit 904e6ddf4133c52fdb9654c2cd2ad90f320d48b9 ]

Change mark_chain_precision() to track precision in situations
like below:

r2 = unknown value
...
--- state #0 ---
...
r1 = r2 // r1 and r2 now share the same ID
...
--- state #1 {r1.id = A, r2.id = A} ---
...
if (r2 > 10) goto exit; // find_equal_scalars() assigns range to r1
...
--- state #2 {r1.id = A, r2.id = A} ---
r3 = r10
r3 += r1 // need to mark both r1 and r2

At the beginning of the processing of each state, ensure that if a
register with a scalar ID is marked as precise, all registers sharing
this ID are also marked as precise.

This property would be used by a follow-up change in regsafe().

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/20230613153824.3324830-2-eddyz87@gmail.com
Stable-dep-of: 1ffc85d9298e ("bpf: Verify scalar ids mapping in regsafe() using check_ids()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/bpf_verifier.h | 10 +-
kernel/bpf/verifier.c | 115 ++++++++++++++++++
.../testing/selftests/bpf/verifier/precise.c | 8 +-
3 files changed, 128 insertions(+), 5 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 5b11a3b0fec0b..22fb13c738a9a 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -557,6 +557,11 @@ struct backtrack_state {
u64 stack_masks[MAX_CALL_FRAMES];
};

+struct bpf_idset {
+ u32 count;
+ u32 ids[BPF_ID_MAP_SIZE];
+};
+
/* single container for all structs
* one verifier_env per bpf_check() call
*/
@@ -588,7 +593,10 @@ struct bpf_verifier_env {
const struct bpf_line_info *prev_linfo;
struct bpf_verifier_log log;
struct bpf_subprog_info subprog_info[BPF_MAX_SUBPROGS + 1];
- struct bpf_id_pair idmap_scratch[BPF_ID_MAP_SIZE];
+ union {
+ struct bpf_id_pair idmap_scratch[BPF_ID_MAP_SIZE];
+ struct bpf_idset idset_scratch;
+ };
struct {
int *insn_state;
int *insn_stack;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8e4ccbc0ae604..70056b7d5960c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3603,6 +3603,96 @@ static void mark_all_scalars_imprecise(struct bpf_verifier_env *env, struct bpf_
}
}

+static bool idset_contains(struct bpf_idset *s, u32 id)
+{
+ u32 i;
+
+ for (i = 0; i < s->count; ++i)
+ if (s->ids[i] == id)
+ return true;
+
+ return false;
+}
+
+static int idset_push(struct bpf_idset *s, u32 id)
+{
+ if (WARN_ON_ONCE(s->count >= ARRAY_SIZE(s->ids)))
+ return -EFAULT;
+ s->ids[s->count++] = id;
+ return 0;
+}
+
+static void idset_reset(struct bpf_idset *s)
+{
+ s->count = 0;
+}
+
+/* Collect a set of IDs for all registers currently marked as precise in env->bt.
+ * Mark all registers with these IDs as precise.
+ */
+static int mark_precise_scalar_ids(struct bpf_verifier_env *env, struct bpf_verifier_state *st)
+{
+ struct bpf_idset *precise_ids = &env->idset_scratch;
+ struct backtrack_state *bt = &env->bt;
+ struct bpf_func_state *func;
+ struct bpf_reg_state *reg;
+ DECLARE_BITMAP(mask, 64);
+ int i, fr;
+
+ idset_reset(precise_ids);
+
+ for (fr = bt->frame; fr >= 0; fr--) {
+ func = st->frame[fr];
+
+ bitmap_from_u64(mask, bt_frame_reg_mask(bt, fr));
+ for_each_set_bit(i, mask, 32) {
+ reg = &func->regs[i];
+ if (!reg->id || reg->type != SCALAR_VALUE)
+ continue;
+ if (idset_push(precise_ids, reg->id))
+ return -EFAULT;
+ }
+
+ bitmap_from_u64(mask, bt_frame_stack_mask(bt, fr));
+ for_each_set_bit(i, mask, 64) {
+ if (i >= func->allocated_stack / BPF_REG_SIZE)
+ break;
+ if (!is_spilled_scalar_reg(&func->stack[i]))
+ continue;
+ reg = &func->stack[i].spilled_ptr;
+ if (!reg->id)
+ continue;
+ if (idset_push(precise_ids, reg->id))
+ return -EFAULT;
+ }
+ }
+
+ for (fr = 0; fr <= st->curframe; ++fr) {
+ func = st->frame[fr];
+
+ for (i = BPF_REG_0; i < BPF_REG_10; ++i) {
+ reg = &func->regs[i];
+ if (!reg->id)
+ continue;
+ if (!idset_contains(precise_ids, reg->id))
+ continue;
+ bt_set_frame_reg(bt, fr, i);
+ }
+ for (i = 0; i < func->allocated_stack / BPF_REG_SIZE; ++i) {
+ if (!is_spilled_scalar_reg(&func->stack[i]))
+ continue;
+ reg = &func->stack[i].spilled_ptr;
+ if (!reg->id)
+ continue;
+ if (!idset_contains(precise_ids, reg->id))
+ continue;
+ bt_set_frame_slot(bt, fr, i);
+ }
+ }
+
+ return 0;
+}
+
/*
* __mark_chain_precision() backtracks BPF program instruction sequence and
* chain of verifier states making sure that register *regno* (if regno >= 0)
@@ -3733,6 +3823,31 @@ static int __mark_chain_precision(struct bpf_verifier_env *env, int regno)
bt->frame, last_idx, first_idx);
}

+ /* If some register with scalar ID is marked as precise,
+ * make sure that all registers sharing this ID are also precise.
+ * This is needed to estimate effect of find_equal_scalars().
+ * Do this at the last instruction of each state,
+ * bpf_reg_state::id fields are valid for these instructions.
+ *
+ * Allows to track precision in situation like below:
+ *
+ * r2 = unknown value
+ * ...
+ * --- state #0 ---
+ * ...
+ * r1 = r2 // r1 and r2 now share the same ID
+ * ...
+ * --- state #1 {r1.id = A, r2.id = A} ---
+ * ...
+ * if (r2 > 10) goto exit; // find_equal_scalars() assigns range to r1
+ * ...
+ * --- state #2 {r1.id = A, r2.id = A} ---
+ * r3 = r10
+ * r3 += r1 // need to mark both r1 and r2
+ */
+ if (mark_precise_scalar_ids(env, st))
+ return -EFAULT;
+
if (last_idx < 0) {
/* we are at the entry into subprog, which
* is expected for global funcs, but only if
diff --git a/tools/testing/selftests/bpf/verifier/precise.c b/tools/testing/selftests/bpf/verifier/precise.c
index a928c53432999..ac5eeea094683 100644
--- a/tools/testing/selftests/bpf/verifier/precise.c
+++ b/tools/testing/selftests/bpf/verifier/precise.c
@@ -46,7 +46,7 @@
mark_precise: frame0: regs=r2 stack= before 20\
mark_precise: frame0: parent state regs=r2 stack=:\
mark_precise: frame0: last_idx 19 first_idx 10\
- mark_precise: frame0: regs=r2 stack= before 19\
+ mark_precise: frame0: regs=r2,r9 stack= before 19\
mark_precise: frame0: regs=r9 stack= before 18\
mark_precise: frame0: regs=r8,r9 stack= before 17\
mark_precise: frame0: regs=r0,r9 stack= before 15\
@@ -106,10 +106,10 @@
mark_precise: frame0: regs=r2 stack= before 22\
mark_precise: frame0: parent state regs=r2 stack=:\
mark_precise: frame0: last_idx 20 first_idx 20\
- mark_precise: frame0: regs=r2 stack= before 20\
- mark_precise: frame0: parent state regs=r2 stack=:\
+ mark_precise: frame0: regs=r2,r9 stack= before 20\
+ mark_precise: frame0: parent state regs=r2,r9 stack=:\
mark_precise: frame0: last_idx 19 first_idx 17\
- mark_precise: frame0: regs=r2 stack= before 19\
+ mark_precise: frame0: regs=r2,r9 stack= before 19\
mark_precise: frame0: regs=r9 stack= before 18\
mark_precise: frame0: regs=r8,r9 stack= before 17\
mark_precise: frame0: parent state regs= stack=:",
--
2.39.2