blktrace: Fix uaf in blk_trace access after removing by sysfs

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

blktrace: Fix uaf in blk_trace access after removing by sysfs

There is an use-after-free problem triggered by following process:

P1(sda) P2(sdb)

echo 0 > /sys/block/sdb/trace/enable

blk_trace_remove_queue

synchronize_rcu

blk_trace_free

relay_close

rcu_read_lock

__blk_add_trace

trace_note_tsk

(Iterate running_trace_list)

relay_close_buf

relay_destroy_buf

kfree(buf)

trace_note(sdb's bt)

relay_reserve

buf->offset <- nullptr deference (use-after-free) !!!

rcu_read_unlock

[ 502.714379] BUG: kernel NULL pointer dereference, address:

0000000000000010

[ 502.715260] #PF: supervisor read access in kernel mode

[ 502.715903] #PF: error_code(0x0000) - not-present page

[ 502.716546] PGD 103984067 P4D 103984067 PUD 17592b067 PMD 0

[ 502.717252] Oops: 0000 [#1] SMP

[ 502.720308] RIP: 0010:trace_note.isra.0+0x86/0x360

[ 502.732872] Call Trace:

[ 502.733193] __blk_add_trace.cold+0x137/0x1a3

[ 502.733734] blk_add_trace_rq+0x7b/0xd0

[ 502.734207] blk_add_trace_rq_issue+0x54/0xa0

[ 502.734755] blk_mq_start_request+0xde/0x1b0

[ 502.735287] scsi_queue_rq+0x528/0x1140

...

[ 502.742704] sg_new_write.isra.0+0x16e/0x3e0

[ 502.747501] sg_ioctl+0x466/0x1100

Reproduce method:

ioctl(/dev/sda, BLKTRACESETUP, blk_user_trace_setup[buf_size=127])

ioctl(/dev/sda, BLKTRACESTART)

ioctl(/dev/sdb, BLKTRACESETUP, blk_user_trace_setup[buf_size=127])

ioctl(/dev/sdb, BLKTRACESTART)

echo 0 > /sys/block/sdb/trace/enable &

// Add delay(mdelay/msleep) before kernel enters blk_trace_free()

ioctl$SG_IO(/dev/sda, SG_IO, ...)

// Enters trace_note_tsk() after blk_trace_free() returned

// Use mdelay in rcu region rather than msleep(which may schedule out)

Remove blk_trace from running_list before calling blk_trace_free() by

sysfs if blk_trace is at Blktrace_running state.