CVE-2024-50254: bpf: Free dynamically allocated bits in bpf_iter_bits_destroy()

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

bpf: Free dynamically allocated bits in bpfiterbitsdestroy()

bpfiterbitsdestroy() uses "kit->nrbits <= 64" to check whether the bits are dynamically allocated. However, the check is incorrect and may cause a kmemleak as shown below:

unreferenced object 0xffff88812628c8c0 (size 32): comm "swapper/0", pid 1, jiffies 4294727320 hex dump (first 32 bytes): b0 c1 55 f5 81 88 ff ff f0 f0 f0 f0 f0 f0 f0 f0 ..U........... f0 f0 f0 f0 f0 f0 f0 f0 00 00 00 00 00 00 00 00 .............. backtrace (crc 781e32cc): [<00000000c452b4ab>] kmemleakalloc+0x4b/0x80 [<0000000004e09f80>] kmallocnodenoprof+0x480/0x5c0 [<00000000597124d6>] alloc.isra.0+0x89/0xb0 [<000000004ebfffcd>] allocbulk+0x2af/0x720 [<00000000d9c10145>] prefillmemcache+0x7f/0xb0 [<00000000ff9738ff>] bpfmemallocinit+0x3e2/0x610 [<000000008b616eac>] bpfglobalmainit+0x19/0x30 [<00000000fc473efc>] dooneinitcall+0xd3/0x3c0 [<00000000ec81498c>] kernelinitfreeable+0x66a/0x940 [<00000000b119f72f>] kernelinit+0x20/0x160 [<00000000f11ac9a7>] retfromfork+0x3c/0x70 [<0000000004671da4>] retfromforkasm+0x1a/0x30

That is because nrbits will be set as zero in bpfiterbitsnext() after all bits have been iterated.

Fix the issue by setting kit->bit to kit->nrbits instead of setting kit->nrbits to zero when the iteration completes in bpfiterbitsnext(). In addition, use "!nrbits || bits >= nrbits" to check whether the iteration is complete and still use "nrbits > 64" to indicate whether bits are dynamically allocated. The "!nrbits" check is necessary because bpfiterbitsnew() may fail before setting kit->nrbits, and this condition will stop the iteration early instead of accessing the zeroed or freed kit->bits.

Considering the initial value of kit->bits is -1 and the type of kit->nrbits is unsigned int, change the type of kit->nrbits to int. The potential overflow problem will be handled in the following patch.