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.