bpf: sync_linked_regs() must preserve subreg_def

commit e9bd9c498cb0f5843996dbe5cbce7a1836a83c70 upstream.

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: 75748837b7e5 ("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
[ shung-hsi.yu: sync_linked_regs() was called find_equal_scalars() before commit
  4bf79f9be434 ("bpf: Track equal scalars history on per-instruction level"), and
  modification is done because there is only a single call to
  copy_register_state() before commit 98d7ca374ba4 ("bpf: Track delta between
  "linked" registers."). ]
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b68572c41e96409359f9b174ef691dfa97f80c06..bdd5105337dc1a5a9db9ee5f69953c0d61e39cd1 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10350,8 +10350,11 @@ static void find_equal_scalars(struct bpf_verifier_state *vstate,
        struct bpf_reg_state *reg;
 
        bpf_for_each_reg_in_vstate(vstate, state, reg, ({
-               if (reg->type == SCALAR_VALUE && reg->id == known_reg->id)
+               if (reg->type == SCALAR_VALUE && reg->id == known_reg->id) {
+                       s32 saved_subreg_def = reg->subreg_def;
                        copy_register_state(reg, known_reg);
+                       reg->subreg_def = saved_subreg_def;
+               }
        }));
 }