Re: [Qemu-devel] [PATCH 1.1] scsi: Add assertion for use-after-free errors

[Stefan Weil <sw@weilnetz.de>]

Am 03.05.2012 19:36, schrieb Stefan Weil:
> The QEMU emulation which is currently used with Raspberry PI images
> (qemu-system-arm -M versatilepb ...) accesses memory which was freed.
>
> Valgrind output (extract):
>
> ==17857== Invalid write of size 4
> ==17857== at 0x24EB06: scsi_req_unref (scsi-bus.c:1273)
> ==17857== by 0x24FFAE: scsi_read_complete (scsi-disk.c:277)
> ==17857== by 0x152ACC: bdrv_co_em_bh (block.c:3363)
> ==17857== by 0x13D49C: qemu_bh_poll (async.c:71)
> ==17857== by 0x211A8C: main_loop_wait (main-loop.c:503)
> ==17857== by 0x207954: main_loop (vl.c:1555)
> ==17857== by 0x20E9C9: main (vl.c:3653)
> ==17857== Address 0x1c54383c is 12 bytes inside a block of size 260 free'd
> ==17857== at 0x4824B3A: free (vg_replace_malloc.c:366)
> ==17857== by 0x20ADFA: free_and_trace (vl.c:2250)
> ==17857== by 0x4899FC5: g_free (in /lib/libglib-2.0.so.0.2400.1)
> ==17857== by 0x24EB3B: scsi_req_unref (scsi-bus.c:1277)
> ==17857== by 0x24F003: scsi_req_complete (scsi-bus.c:1383)
> ==17857== by 0x25022A: scsi_read_data (scsi-disk.c:334)
> ==17857== by 0x24EB9F: scsi_req_continue (scsi-bus.c:1289)
> ==17857== by 0x1C7787: lsi_do_dma (lsi53c895a.c:575)
> ==17857== by 0x1C8CDA: lsi_execute_script (lsi53c895a.c:1147)
> ==17857== by 0x1C74EA: lsi_resume_script (lsi53c895a.c:510)
> ==17857== by 0x1C7ECD: lsi_transfer_data (lsi53c895a.c:746)
> ==17857== by 0x24EC90: scsi_req_data (scsi-bus.c:1307)


Hi Paolo,

this is the result of a bisect to narrow the source of problem:

ac6684264642f1aea7cba5c0c3907409b1f7f904 is the first bad commit
commit ac6684264642f1aea7cba5c0c3907409b1f7f904
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Thu Apr 19 11:55:28 2012 +0200

     scsi: support FUA on reads

     To force unit access on reads, flush the cache *before* doing the read.

     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Regards,

Stefan


>
> (There are some more similar messages.)
>
> This patch adds an assertion which also detects those errors:
>
> Calling scsi_req_unref is not allowed when the previous call
> of that function has decremented refcount to 0, because in this
> case req was freed.
>
> Signed-off-by: Stefan Weil <sw@weilnetz.de>
> ---
>
> There are chances that this patch breaks some test scenarios,
> but that is intentional: we should not pretend that there are
> no errors when there are some.
>
> The Raspberry PI emulation with QEMU is currently used by
> a lot of people.
>
> Please apply this patch for the tests of QEMU 1.1.
>
> Of course we should also fix the problem which triggers the
> assertion. I still don't know whether it is caused by
> lsi53c895a.c or by the scsi code.

It is the scsi code, see git bisect result.