Re: [PATCH v2 5/6] efi: xen: Implement memory descriptor lookup based on hypercall

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Mon, Oct 03, 2022 at 05:59:52PM +0200, Ard Biesheuvel wrote:
> On Mon, 3 Oct 2022 at 17:29, Demi Marie Obenour
> <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Oct 03, 2022 at 01:26:24PM +0200, Ard Biesheuvel wrote:
> > > Xen on x86 boots dom0 in EFI mode but without providing a memory map.
> > > This means that some sanity checks we would like to perform on
> > > configuration tables or other data structures in memory are not
> > > currently possible. Xen does, however, expose EFI memory descriptor info
> > > via a Xen hypercall, so let's wire that up instead.
> > >
> > > Co-developed-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > ---
> > >  drivers/firmware/efi/efi.c |  5 ++-
> > >  drivers/xen/efi.c          | 34 ++++++++++++++++++++
> > >  include/linux/efi.h        |  1 +
> > >  3 files changed, 39 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> > > index 55bd3f4aab28..2c12b1a06481 100644
> > > --- a/drivers/firmware/efi/efi.c
> > > +++ b/drivers/firmware/efi/efi.c
> > > @@ -456,7 +456,7 @@ void __init efi_find_mirror(void)
> > >   * and if so, populate the supplied memory descriptor with the 
> > > appropriate
> > >   * data.
> > >   */
> > > -int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
> > > +int __efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
> > >  {
> > >       efi_memory_desc_t *md;
> > >
> > > @@ -485,6 +485,9 @@ int efi_mem_desc_lookup(u64 phys_addr, 
> > > efi_memory_desc_t *out_md)
> > >       return -ENOENT;
> > >  }
> > >
> > > +extern int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
> > > +      __weak __alias(__efi_mem_desc_lookup);
> > > +
> > >  /*
> > >   * Calculate the highest address of an efi memory descriptor.
> > >   */
> > > diff --git a/drivers/xen/efi.c b/drivers/xen/efi.c
> > > index d1ff2186ebb4..74f3f6d8cdc8 100644
> > > --- a/drivers/xen/efi.c
> > > +++ b/drivers/xen/efi.c
> > > @@ -26,6 +26,7 @@
> > >
> > >  #include <xen/interface/xen.h>
> > >  #include <xen/interface/platform.h>
> > > +#include <xen/page.h>
> > >  #include <xen/xen.h>
> > >  #include <xen/xen-ops.h>
> > >
> > > @@ -292,3 +293,36 @@ void __init xen_efi_runtime_setup(void)
> > >       efi.get_next_high_mono_count    = xen_efi_get_next_high_mono_count;
> > >       efi.reset_system                = xen_efi_reset_system;
> > >  }
> > > +
> > > +int efi_mem_desc_lookup(u64 phys_addr, efi_memory_desc_t *out_md)
> > > +{
> > > +     static_assert(XEN_PAGE_SHIFT == EFI_PAGE_SHIFT,
> > > +                   "Mismatch between EFI_PAGE_SHIFT and XEN_PAGE_SHIFT");
> > > +     struct xen_platform_op op = {
> > > +             .cmd = XENPF_firmware_info,
> > > +             .u.firmware_info = {
> > > +                     .type = XEN_FW_EFI_INFO,
> > > +                     .index = XEN_FW_EFI_MEM_INFO,
> > > +                     .u.efi_info.mem.addr = phys_addr,
> > > +                     .u.efi_info.mem.size = U64_MAX - phys_addr,
> > > +             }
> > > +     };
> > > +     union xenpf_efi_info *info = &op.u.firmware_info.u.efi_info;
> > > +     int rc;
> > > +
> > > +     if (!efi_enabled(EFI_PARAVIRT) || efi_enabled(EFI_MEMMAP))
> > > +             return __efi_mem_desc_lookup(phys_addr, out_md);
> > > +
> > > +     rc = HYPERVISOR_platform_op(&op);
> > > +     if (rc) {
> > > +             pr_warn("Failed to lookup header 0x%llx in Xen memory map: 
> > > error %d\n",
> > > +                     phys_addr, rc);
> > > +     }
> > > +
> > > +     out_md->phys_addr       = info->mem.addr;
> >
> > This will be equal to phys_addr, not the actual start of the memory
> > region.
> >
> > > +     out_md->num_pages       = info->mem.size >> EFI_PAGE_SHIFT;
> >
> > Similarly, this will be the number of bytes in the memory region
> > after phys_addr, not the total number of bytes in the region.  These two
> > differences mean that this function is not strictly equivalent to the
> > original efi_mem_desc_lookup().
> >
> > I am not sure if this matters in practice, but I thought you would want
> > to be aware of it.
> 
> This is a bit disappointing. Is there no way to obtain this
> information via a Xen hypercall?

It is possible, but doing so is very complex (it essentially requires a
binary search).  This really should be fixed on the Xen side.

> In any case, it means we'll need to round down phys_addr to page size
> at the very least.

That makes sense.  Are there any callers that will be broken even with
this rounding?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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