bpf: sync_linked_regs() must preserve subreg_def

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: 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).

References

• https://git.kernel.org/stable/c/dadf82c1b2608727bcc306843b540cd7414055a7
• https://git.kernel.org/stable/c/b57ac2d92c1f565743f6890a5b9cf317ed856b09
• https://git.kernel.org/stable/c/60fd3538d2a8fd44c41d25088c0ece3e1fd30659
• https://git.kernel.org/stable/c/bfe9446ea1d95f6cb7848da19dfd58d2eec6fd84
• https://git.kernel.org/stable/c/e2ef0f317a52e678fe8fa84b94d6a15b466d6ff0
• https://git.kernel.org/stable/c/e9bd9c498cb0f5843996dbe5cbce7a1836a83c70