CVE-2025-38063

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

dm: fix unconditional IO throttle caused by REQ_PREFLUSH

When a bio with REQPREFLUSH is submitted to dm, _sendemptyflush() generates a flushbio with REQOPWRITE | REQPREFLUSH | REQSYNC, which causes the flushbio to be throttled by wbt_wait().

An example from v5.4, similar problem also exists in upstream:

crash> bt 2091206
PID: 2091206  TASK: ffff2050df92a300  CPU: 109  COMMAND: "kworker/u260:0"
 #0 [ffff800084a2f7f0] __switch_to at ffff80004008aeb8
 #1 [ffff800084a2f820] __schedule at ffff800040bfa0c4
 #2 [ffff800084a2f880] schedule at ffff800040bfa4b4
 #3 [ffff800084a2f8a0] io_schedule at ffff800040bfa9c4
 #4 [ffff800084a2f8c0] rq_qos_wait at ffff8000405925bc
 #5 [ffff800084a2f940] wbt_wait at ffff8000405bb3a0
 #6 [ffff800084a2f9a0] __rq_qos_throttle at ffff800040592254
 #7 [ffff800084a2f9c0] blk_mq_make_request at ffff80004057cf38
 #8 [ffff800084a2fa60] generic_make_request at ffff800040570138
 #9 [ffff800084a2fae0] submit_bio at ffff8000405703b4
#10 [ffff800084a2fb50] xlog_write_iclog at ffff800001280834 [xfs]
#11 [ffff800084a2fbb0] xlog_sync at ffff800001280c3c [xfs]
#12 [ffff800084a2fbf0] xlog_state_release_iclog at ffff800001280df4 [xfs]
#13 [ffff800084a2fc10] xlog_write at ffff80000128203c [xfs]
#14 [ffff800084a2fcd0] xlog_cil_push at ffff8000012846dc [xfs]
#15 [ffff800084a2fda0] xlog_cil_push_work at ffff800001284a2c [xfs]
#16 [ffff800084a2fdb0] process_one_work at ffff800040111d08
#17 [ffff800084a2fe00] worker_thread at ffff8000401121cc
#18 [ffff800084a2fe70] kthread at ffff800040118de4

After commit 2def2845cc33 ("xfs: don't allow log IO to be throttled"), the metadata submitted by xlogwriteiclog() should not be throttled. But due to the existence of the dm layer, throttling flush_bio indirectly causes the metadata bio to be throttled.

Fix this by conditionally adding REQIDLE to flushbio.biopf, which makes wbtshouldthrottle() return false to avoid wbtwait().