Re: [PATCH v2] xen: fix XEN_DOMCTL_gdbsx_guestmemio crash

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 19/04/2022 11:18, Juergen Gross wrote:
> A hypervisor built without CONFIG_GDBSX will crash in case the
> XEN_DOMCTL_gdbsx_guestmemio domctl is being called, as the call will
> end up in iommu_do_domctl() with d == NULL:
>
> (XEN) CPU:    6
> (XEN) RIP:    e008:[<ffff82d040269984>] iommu_do_domctl+0x4/0x30
> (XEN) RFLAGS: 0000000000010202   CONTEXT: hypervisor (d0v0)
> (XEN) rax: 00000000000003e8   rbx: ffff830856277ef8   rcx: ffff830856277fff
> ...
> (XEN) Xen call trace:
> (XEN)    [<ffff82d040269984>] R iommu_do_domctl+0x4/0x30
> (XEN)    [<ffff82d04035cd5f>] S arch_do_domctl+0x7f/0x2330
> (XEN)    [<ffff82d040239e46>] S do_domctl+0xe56/0x1930
> (XEN)    [<ffff82d040238ff0>] S do_domctl+0/0x1930
> (XEN)    [<ffff82d0402f8c59>] S pv_hypercall+0x99/0x110
> (XEN)    [<ffff82d0402f5161>] S 
> arch/x86/pv/domain.c#_toggle_guest_pt+0x11/0x90
> (XEN)    [<ffff82d040366288>] S lstar_enter+0x128/0x130
> (XEN)
> (XEN) Pagetable walk from 0000000000000144:
> (XEN)  L4[0x000] = 0000000000000000 ffffffffffffffff
> (XEN)
> (XEN) ****************************************
> (XEN) Panic on CPU 6:
> (XEN) FATAL PAGE FAULT
> (XEN) [error_code=0000]
> (XEN) Faulting linear address: 0000000000000144
>
> Fix this issue by modifying the interface of gdbsx_guest_mem_io() to
> take the already known domain pointer instead of the domid.

There is some explanation missing here.  The adjustments you make are
within CONFIG_GDBSX, with the exception of the final hunk.


The actual bug is that non-IOMMU subops end up in iommu_do_domctl(), so
while this is good cleanup to gdbsx_guest_mem_io() it, along with Jan's
adjustment to iommu_do_domctl(), are not suitable fixes to the crash as
reported.

So someone's going to have to write a 3rd patch which actually fixes the
root of the crash, and this will become cleanup.

~Andrew

> diff --git a/xen/arch/x86/debug.c b/xen/arch/x86/debug.c
> index d90dc93056..c0dd6eaf15 100644
> --- a/xen/arch/x86/debug.c
> +++ b/xen/arch/x86/debug.c
> @@ -159,17 +159,11 @@ static unsigned int dbg_rw_guest_mem(struct domain *dp, 
> unsigned long addr,
>   * Returns: number of bytes remaining to be copied.
>   */
>  unsigned int dbg_rw_mem(unsigned long gva, XEN_GUEST_HANDLE_PARAM(void) buf,
> -                        unsigned int len, domid_t domid, bool toaddr,
> +                        unsigned int len, struct domain *d, bool toaddr,
>                          uint64_t pgd3)
>  {
> -    struct domain *d = rcu_lock_domain_by_id(domid);
> -
> -    if ( d )
> -    {
> -        if ( !d->is_dying )
> +    if ( d && !d->is_dying )
>              len = dbg_rw_guest_mem(d, gva, buf, len, toaddr, pgd3);

This needs re-indenting.