blk-throttle: fix access race during throttle policy activation

Summary

In the Linux kernel, the following vulnerability has been resolved: blk-throttle: fix access race during throttle policy activation On repeated cold boots we occasionally hit a NULL pointer crash in blk_should_throtl() when throttling is consulted before the throttle policy is fully enabled for the queue. Checking only q->td != NULL is insufficient during early initialization, so blkg_to_pd() for the throttle policy can still return NULL and blkg_to_tg() becomes NULL, which later gets dereferenced. Unable to handle kernel NULL pointer dereference at virtual address 0000000000000156 ... pc : submit_bio_noacct+0x14c/0x4c8 lr : submit_bio_noacct+0x48/0x4c8 sp : ffff800087f0b690 x29: ffff800087f0b690 x28: 0000000000005f90 x27: ffff00068af393c0 x26: 0000000000080000 x25: 000000000002fbc0 x24: ffff000684ddcc70 x23: 0000000000000000 x22: 0000000000000000 x21: 0000000000000000 x20: 0000000000080000 x19: ffff000684ddcd08 x18: ffffffffffffffff x17: 0000000000000000 x16: ffff80008132a550 x15: 0000ffff98020fff x14: 0000000000000000 x13: 1fffe000d11d7021 x12: ffff000688eb810c x11: ffff00077ec4bb80 x10: ffff000688dcb720 x9 : ffff80008068ef60 x8 : 00000a6fb8a86e85 x7 : 000000000000111e x6 : 0000000000000002 x5 : 0000000000000246 x4 : 0000000000015cff x3 : 0000000000394500 x2 : ffff000682e35e40 x1 : 0000000000364940 x0 : 000000000000001a Call trace: submit_bio_noacct+0x14c/0x4c8 verity_map+0x178/0x2c8 __map_bio+0x228/0x250 dm_submit_bio+0x1c4/0x678 __submit_bio+0x170/0x230 submit_bio_noacct_nocheck+0x16c/0x388 submit_bio_noacct+0x16c/0x4c8 submit_bio+0xb4/0x210 f2fs_submit_read_bio+0x4c/0xf0 f2fs_mpage_readpages+0x3b0/0x5f0 f2fs_readahead+0x90/0xe8 Tighten blk_throtl_activated() to also require that the throttle policy bit is set on the queue: return q->td != NULL && test_bit(blkcg_policy_throtl.plid, q->blkcg_pols); This prevents blk_should_throtl() from accessing throttle group state until policy data has been attached to blkgs.