bpf: sync_linked_regs() must preserve subreg_def

Range propagation must not affect subreg_def marks, otherwise the
following example is rewritten by verifier incorrectly when
BPF_F_TEST_RND_HI32 flag is set:

  0: call bpf_ktime_get_ns                   call bpf_ktime_get_ns
  1: r0 &= 0x7fffffff       after verifier   r0 &= 0x7fffffff
  2: w1 = w0                rewrites         w1 = w0
  3: if w0 < 10 goto +0     -------------->  r11 = 0x2f5674a6     (r)
  4: r1 >>= 32                               r11 <<= 32           (r)
  5: r0 = r1                                 r1 |= r11            (r)
  6: exit;                                   if w0 < 0xa goto pc+0
                                             r1 >>= 32
                                             r0 = r1
                                             exit

(or zero extension of w1 at (2) is missing for architectures that
 require zero extension for upper register half).

The following happens w/o this patch:
- r0 is marked as not a subreg at (0);
- w1 is marked as subreg at (2);
- w1 subreg_def is overridden at (3) by copy_register_state();
- w1 is read at (5) but mark_insn_zext() does not mark (2)
  for zero extension, because w1 subreg_def is not set;
- because of BPF_F_TEST_RND_HI32 flag verifier inserts random
  value for hi32 bits of (2) (marked (r));
- this random value is read at (5).

Fixes: 75748837 ("bpf: Propagate scalar ranges through register assignments.")
Reported-by: Lonial Con <kongln9170@gmail.com>
Signed-off-by: Lonial Con <kongln9170@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Closes: https://lore.kernel.org/bpf/7e2aa30a62d740db182c170fdd8f81c596df280d.camel@gmail.com
Link: https://lore.kernel.org/bpf/20240924210844.1758441-1-eddyz87@gmail.com

kernel/bpf/verifier.c

@@ -15326,8 +15326,12 @@ static void sync_linked_regs(struct bpf_verifier_state *vstate, struct bpf_reg_s
 			continue;
 		if ((!(reg->id & BPF_ADD_CONST) && !(known_reg->id & BPF_ADD_CONST)) ||
 		    reg->off == known_reg->off) {
+			s32 saved_subreg_def = reg->subreg_def;
+
 			copy_register_state(reg, known_reg);
+			reg->subreg_def = saved_subreg_def;
 		} else {
+			s32 saved_subreg_def = reg->subreg_def;
 			s32 saved_off = reg->off;
 
 			fake_reg.type = SCALAR_VALUE;
@@ -15340,6 +15344,7 @@ static void sync_linked_regs(struct bpf_verifier_state *vstate, struct bpf_reg_s
 			 * otherwise another sync_linked_regs() will be incorrect.
 			 */
 			reg->off = saved_off;
+			reg->subreg_def = saved_subreg_def;
 
 			scalar32_min_max_add(reg, &fake_reg);
 			scalar_min_max_add(reg, &fake_reg);