CVE-2024-53125: bpf: sync_linked_regs() must preserve subreg_def

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

bpf: synclinkedregs() must preserve subregdef

Range propagation must not affect subregdef marks, otherwise the following example is rewritten by verifier incorrectly when BPFFTESTRNDHI32 flag is set:

0: call bpfktimegetns call bpfktimegetns 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 subregdef is overridden at (3) by copyregisterstate(); - w1 is read at (5) but markinsnzext() does not mark (2) for zero extension, because w1 subregdef is not set; - because of BPFFTESTRNDHI32 flag verifier inserts random value for hi32 bits of (2) (marked (r)); - this random value is read at (5).