Register range information is copied in several places. The intent is
to transfer range/id information from one register/stack spill to
another. Currently this is done using direct register assignment, e.g.:

static void find_equal_scalars(..., struct bpf_reg_state *known_reg)
{
	...
	struct bpf_reg_state *reg;
	...
			*reg = *known_reg;
	...
}

However, such assignments also copy the following bpf_reg_state fields:

struct bpf_reg_state {
	...
	struct bpf_reg_state *parent;
	...
	enum bpf_reg_liveness live;
	...
};

Copying of these fields is accidental and incorrect, as could be
demonstrated by the following example:

     0: call ktime_get_ns()
     1: r6 = r0
     2: call ktime_get_ns()
     3: r7 = r0
     4: if r0 > r6 goto +1             ; r0 & r6 are unbound thus generated
                                       ; branch states are identical
     5: *(u64 *)(r10 - 8) = 0xdeadbeef ; 64-bit write to fp[-8]
    --- checkpoint ---
     6: r1 = 42                        ; r1 marked as written
     7: *(u8 *)(r10 - 8) = r1          ; 8-bit write, fp[-8] parent & live
                                       ; overwritten
     8: r2 = *(u64 *)(r10 - 8)
     9: r0 = 0
    10: exit

This example is unsafe because 64-bit write to fp[-8] at (5) is
conditional, thus not all bytes of fp[-8] are guaranteed to be set
when it is read at (8). However, currently the example passes
verification.

First, the execution path 1-10 is examined by verifier.
Suppose that a new checkpoint is created by is_state_visited() at (6).
After checkpoint creation:
- r1.parent points to checkpoint.r1,
- fp[-8].parent points to checkpoint.fp[-8].
At (6) the r1.live is set to REG_LIVE_WRITTEN.
At (7) the fp[-8].parent is set to r1.parent and fp[-8].live is set to
REG_LIVE_WRITTEN, because of the following code called in
check_stack_write_fixed_off():

static void save_register_state(struct bpf_func_state *state,
				int spi, struct bpf_reg_state *reg,
				int size)
{
	...
	state->stack[spi].spilled_ptr = *reg;  // <--- parent & live copied
	if (size == BPF_REG_SIZE)
		state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN;
	...
}

Note the intent to mark stack spill as written only if 8 bytes are
spilled to a slot, however this intent is spoiled by a 'live' field copy.
At (8) the checkpoint.fp[-8] should be marked as REG_LIVE_READ but
this does not happen:
- fp[-8] in a current state is already marked as REG_LIVE_WRITTEN;
- fp[-8].parent points to checkpoint.r1, parentage chain is used by
  mark_reg_read() to mark checkpoint states.
At (10) the verification is finished for path 1-10 and jump 4-6 is
examined. The checkpoint.fp[-8] never gets REG_LIVE_READ mark and this
spill is pruned from the cached states by clean_live_states(). Hence
verifier state obtained via path 1-4,6 is deemed identical to one
obtained via path 1-6 and program marked as safe.

Note: the example should be executed with BPF_F_TEST_STATE_FREQ flag
set to force creation of intermediate verifier states.

This commit revisits the locations where bpf_reg_state instances are
copied and replaces the direct copies with a call to a function
copy_register_state(dst, src) that preserves 'parent' and 'live'
fields of the 'dst'.

Fixes: 679c782d ("bpf/verifier: per-register parent pointers")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230106142214.1040390-2-eddyz87@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>