CVE-2021-47375

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(sdbs 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.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/3815fe7371d2411ce164281cef40d9fc7b323dee
• https://git.kernel.org/stable/c/488da313edf3abea7f7733efe011c96b23740ab5
• https://git.kernel.org/stable/c/5afedf670caf30a2b5a52da96eb7eac7dee6a9c9
• https://git.kernel.org/stable/c/677e362ba807f3aafe6f405c07e0b37244da5222
• https://git.kernel.org/stable/c/a5f8e86192612d0183047448d8bbe7918b3f1a26
• https://git.kernel.org/stable/c/d56171d9360c0170c5c5f8f7e2362a2e999eca40
• https://git.kernel.org/stable/c/dacfd5e4d1142bfb3809aab3634a375f6f373269
• https://git.kernel.org/stable/c/ebb8d26d93c3ec3c7576c52a8373a2309423c069
• https://git.kernel.org/stable/c/3815fe7371d2411ce164281cef40d9fc7b323dee
• https://git.kernel.org/stable/c/488da313edf3abea7f7733efe011c96b23740ab5
• https://git.kernel.org/stable/c/5afedf670caf30a2b5a52da96eb7eac7dee6a9c9
• https://git.kernel.org/stable/c/677e362ba807f3aafe6f405c07e0b37244da5222
• https://git.kernel.org/stable/c/a5f8e86192612d0183047448d8bbe7918b3f1a26
• https://git.kernel.org/stable/c/d56171d9360c0170c5c5f8f7e2362a2e999eca40
• https://git.kernel.org/stable/c/dacfd5e4d1142bfb3809aab3634a375f6f373269
• https://git.kernel.org/stable/c/ebb8d26d93c3ec3c7576c52a8373a2309423c069