CVE-2021-46984

In the Linux kernel, the following vulnerability has been resolved: kyber:
fix out of bounds access when preempted __blk_mq_sched_bio_merge() gets the
ctx and hctx for the current CPU and passes the hctx to ->bio_merge().
kyber_bio_merge() then gets the ctx for the current CPU again and uses that
to get the corresponding Kyber context in the passed hctx. However, the
thread may be preempted between the two calls to blk_mq_get_ctx(), and the
ctx returned the second time may no longer correspond to the passed hctx.
This “works” accidentally most of the time, but it can cause us to read
garbage if the second ctx came from an hctx with more ctx’s than the first
one (i.e., if ctx->index_hw[hctx->type] > hctx->nr_ctx). This manifested as
this UBSAN array index out of bounds error reported by Jakub: UBSAN:
array-index-out-of-bounds in …/kernel/locking/qspinlock.c:130:9 index
13106 is out of range for type ‘long unsigned int [128]’ Call Trace:
dump_stack+0xa4/0xe5 ubsan_epilogue+0x5/0x40
__ubsan_handle_out_of_bounds.cold.13+0x2a/0x34
queued_spin_lock_slowpath+0x476/0x480 do_raw_spin_lock+0x1c2/0x1d0
kyber_bio_merge+0x112/0x180 blk_mq_submit_bio+0x1f5/0x1100
submit_bio_noacct+0x7b0/0x870 submit_bio+0xc2/0x3a0
btrfs_map_bio+0x4f0/0x9d0 btrfs_submit_data_bio+0x24e/0x310
submit_one_bio+0x7f/0xb0 submit_extent_page+0xc4/0x440
__extent_writepage_io+0x2b8/0x5e0 __extent_writepage+0x28d/0x6e0
extent_write_cache_pages+0x4d7/0x7a0 extent_writepages+0xa2/0x110
do_writepages+0x8f/0x180 __writeback_single_inode+0x99/0x7f0
writeback_sb_inodes+0x34e/0x790 __writeback_inodes_wb+0x9e/0x120
wb_writeback+0x4d2/0x660 wb_workfn+0x64d/0xa10 process_one_work+0x53a/0xa80
worker_thread+0x69/0x5b0 kthread+0x20b/0x240 ret_from_fork+0x1f/0x30 Only
Kyber uses the hctx, so fix it by passing the request_queue to
->bio_merge() instead. BFQ and mq-deadline just use that, and Kyber can map
the queues itself to avoid the mismatch.