FIx/workaround vchan "loopback" connection

[marmarek]

Reported by marmarek on 9 Feb 2015 21:15 UTC
Currently Xen implementation of vchan in R3 crashes when connection is made back to the source domain. This is apparently not supported by xen-gntalloc driver.

The exact message is:

[    9.937990] BUG: Bad page map in process qrexec-agent  pte:80000000f9d41167 pmd:131c3067
[    9.938010] page:ffffea00036a6638 count:1 mapcount:-1 mapping:          (null) index:0xffffffffffffffff
[    9.938018] page flags: 0x4000000000000c14(referenced|dirty|reserved|private)
[    9.938033] addr:00007fa856d47000 vm_flags:140400fb anon_vma:          (null) mapping:ffff880011efe940 index:11
[    9.938042] vma->vm_ops->fault:           (null)
[    9.938057] vma->vm_file->f_op->mmap: gntalloc_mmap+0x0/0x1c0 [   9.938066](xen_gntalloc]
[) CPU: 0 PID: 1108 Comm: qrexec-agent Tainted: G           O 3.12.23-1.pvops.qubes.x86_64 #1
[    9.938074]  ffff8800131f3818 ffff88001316fc78 ffffffff814db550 00007fa856d47000
[    9.938085]  ffff88001316fcb8 ffffffff81139413 ffff880011efe940 ffff8800131c3a38
[    9.938096]  ffffea00036a6638 00007fa856d47000 00007fa856d57000 ffff88001316fe18
[    9.938107] Call Trace:
[    9.938117]  [dump_stack+0x45/0x56
[    9.938126](<ffffffff814db550>])  [print_bad_pte+0x1a3/0x240
[    9.938133](<ffffffff81139413>])  [unmap_page_range+0x6ee/0x7d0
[    9.938142](<ffffffff8113ac9e>])  [unmap_single_vma+0x76/0xa0
[    9.938149](<ffffffff8113adf6>])  [unmap_vmas+0x49/0x90
[    9.938157](<ffffffff8113be09>])  [exit_mmap+0x9c/0x170
[    9.938166](<ffffffff8114443c>])  [mmput+0x5c/0x110
[    9.938175](<ffffffff8105950c>])  [do_exit+0x27c/0xa20
[    9.938184](<ffffffff8105d74c>])  [? vtime_account_user+0x4f/0x60
[    9.938194](<ffffffff810908ef>])  [? context_tracking_user_exit+0x52/0xc0
[    9.938203](<ffffffff81116502>])  [do_group_exit+0x3a/0xa0
[    9.938211](<ffffffff8105ed2a>])  [SyS_exit_group+0xf/0x10
[    9.938220](<ffffffff8105ed9f>])  [<ffffffff814ea907>] tracesys+0xdd/0xe2

Needs either fix in the kernel, or some special case in vchan-xen code (use simple shm instead of Xen shared memory?).

Migrated-From: https://wiki.qubes-os.org/ticket/951

[marmarek]

Currently vchan reject to open such connection, so at least there is no kernel crash. But still, it would be nice to fix this properly.

[marmarek]

Currently vchan reject to open such connection, so at least there is no kernel crash. But still, it would be nice to fix this properly.