Date:   Fri, 30 Sep 2022 14:15:37 -0400
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Kees Cook <keescook@...omium.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH v4 1/2] Avoid using EFI tables Xen may have clobbered

On Fri, Sep 30, 2022 at 06:25:53PM +0200, Ard Biesheuvel wrote:
> On Fri, 30 Sept 2022 at 01:02, Demi Marie Obenour
> <demi@...isiblethingslab.com> wrote:
> >
> > Memory of type EFI_CONVENTIONAL_MEMORY, EFI_LOADER_CODE, EFI_LOADER_DATA,
> > EFI_BOOT_SERVICES_CODE, and EFI_BOOT_SERVICES_DATA may be clobbered by
> > Xen before Linux gets to start using it.  Therefore, Linux under Xen
> > must not use EFI tables from such memory.  Most of the remaining EFI
> > memory types are not suitable for EFI tables, leaving only
> > EFI_ACPI_RECLAIM_MEMORY, EFI_RUNTIME_SERVICES_DATA, and
> > EFI_RUNTIME_SERVICES_CODE.  When running under Xen, Linux should only
> > use tables that are located in one of these types of memory.
> >
> > This patch ensures this, and also adds a function
> > (xen_config_table_memory_region_max()) that will be used later to
> > replace the usage of the EFI memory map in esrt.c when running under
> > Xen.  This function can also be used in mokvar-table.c and efi-bgrt.c,
> > but I have not implemented this.
> >
> > Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
> > ---
> >  drivers/firmware/efi/efi.c |  8 +++++---
> >  drivers/xen/efi.c          | 35 +++++++++++++++++++++++++++++++++++
> >  include/linux/efi.h        |  9 +++++++++
> >  3 files changed, 49 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> > index e4080ad96089abd7f84745dd8461c548bcbb7685..d344f3ff73d1c5ed0c67e3251a9502e66719741d 100644
> > --- a/drivers/firmware/efi/efi.c
> > +++ b/drivers/firmware/efi/efi.c
> > @@ -574,7 +574,6 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
> >         unsigned long table;
> >         int i;
> >
> > -       pr_info("");
> 
> Why are you removing these prints?

If I left them, I would need to include a pr_cont("\n") later.
Checkpatch recommends against that.  What is the purpose of this print?
I assumed that since it prints an empty string it is superfluous.

> >         for (i = 0; i < count; i++) {
> >                 if (!IS_ENABLED(CONFIG_X86)) {
> >                         guid = &config_tables[i].guid;
> > @@ -585,7 +584,6 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
> >
> >                         if (IS_ENABLED(CONFIG_X86_32) &&
> >                             tbl64[i].table > U32_MAX) {
> > -                               pr_cont("\n");
> >                                 pr_err("Table located above 4GB, disabling EFI.\n");
> >                                 return -EINVAL;
> >                         }
> > @@ -594,10 +592,14 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
> >                         table = tbl32[i].table;
> >                 }
> >
> > +#ifdef CONFIG_XEN_EFI
> 
> We tend to prefer IS_ENABLED() for cases such as this one. That way,
> the compiler always gets to see the code inside the conditional block,
> which gives better build test coverage (even if CONFIG_XEN_EFI is
> disabled).

Can I count on the compiler eliminating the code as unreachable?  With
CONFIG_XEN_EFI disabled xen_config_table_memory_region_max() would be an
undefined symbol.

> > +               if (efi_enabled(EFI_PARAVIRT) && !xen_config_table_memory_region_max(table))
> 
> So the question here is whether Xen thinks the table should be
> disregarded or not. So let's define a prototype that reflects that
> purpose, and let the implementation reason about how this should be
> achieved.

xen_config_table_memory_region_max() doesn’t just return whether the
table should be disregarded, but also (if the table should not be
ignored) the end of the memory region containing it.  I will make
xen_efi_config_table_valid() a wrapper around
xen_config_table_memory_region_max(), as the former also needs to print
a warning if the table is in an invalid location.

> So
> 
> if (IS_ENABLED(CONFIG_XEN_EFI) &&
>     efi_enabled(EFI_PARAVIRT) &&
>     xen_efi_config_table_valid(guid, table)
>         continue
> 
> I should note here, though, that EFI_PARAViRT is only set on x86 not
> on other architectures that enable CONFIG_XEN_EFI so this will not
> work anywhere else.

What should I use instead?

> > +                       continue;
> > +#endif
> > +
> >                 if (!match_config_table(guid, table, common_tables) && arch_tables)
> >                         match_config_table(guid, table, arch_tables);
> >         }
> > -       pr_cont("\n");
> >         set_bit(EFI_CONFIG_TABLES, &efi.flags);
> >
> >         if (efi_rng_seed != EFI_INVALID_TABLE_ADDR) {
> > diff --git a/drivers/xen/efi.c b/drivers/xen/efi.c
> > index d1ff2186ebb48a7c0981ecb6d4afcbbb25ffcea0..c2274ddfcc63304008ef0fd78fd9fa416f75d073 100644
> > --- a/drivers/xen/efi.c
> > +++ b/drivers/xen/efi.c
> > @@ -28,6 +28,7 @@
> >  #include <xen/interface/platform.h>
> >  #include <xen/xen.h>
> >  #include <xen/xen-ops.h>
> > +#include <xen/page.h>
> >
> >  #include <asm/page.h>
> >
> > @@ -271,6 +272,40 @@ static void xen_efi_reset_system(int reset_type, efi_status_t status,
> >         }
> >  }
> >
> > +__init u64 xen_config_table_memory_region_max(u64 addr)
> 
> It is more idiomatic for Linux to put __init after the return type.
> And if we adopt my suggestion above, this becomes
> 
> bool __init xen_efi_config_table_valid(const efi_guid_t *guid, u64 table)
> 
> Alternatively, you could pass the string identifier of the table
> instead of the guid (or both) to print in the diagnostic message.

Will fix in v5.

> > +{
> > +       static_assert(XEN_PAGE_SHIFT == EFI_PAGE_SHIFT,
> > +                     "Mismatch between EFI_PAGE_SHIFT and XEN_PAGE_SHIFT");
> 
> Is this the only place where this matters? And this never happens on x86, right?

My understanding is that it should never happen on any architecture.
That’s why I static_assert() it.  I have no idea if this is the only
place it matters, though.

> > +       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 = addr,
> > +                       .u.efi_info.mem.size = U64_MAX - addr,
> > +               }
> > +       };
> > +       union xenpf_efi_info *info = &op.u.firmware_info.u.efi_info;
> > +       int rc = HYPERVISOR_platform_op(&op);
> > +
> > +       if (rc) {
> > +               pr_warn("Failed to lookup header %llu in Xen memory map: error %d\n",
> > +                       (unsigned long long)addr, rc);
> > +               return 0;
> > +       }
> > +
> > +       switch (info->mem.type) {
> > +       case EFI_RUNTIME_SERVICES_CODE:
> > +       case EFI_RUNTIME_SERVICES_DATA:
> > +       case EFI_ACPI_RECLAIM_MEMORY:
> 
> If we are listing all memory types that Xen preserves, you might add
> EFI_RESERVED_MEMORY here. Otherwise, please only list the ones that
> you need to permit explicitly.

My understanding was that EFI_RESERVED_MEMORY should never be touched by
the OS, so I left it out.  Which types would you permit?

> > +               return info->mem.addr + info->mem.size;
> > +       default:
> > +               pr_warn("Table %llu is in memory of type %d, ignoring it\n",
> > +                       (unsigned long long)addr, info->mem.type);
> > +               return 0;
> > +       }
> > +}
> > +
> >  /*
> >   * Set XEN EFI runtime services function pointers. Other fields of struct efi,
> >   * e.g. efi.systab, will be set like normal EFI.
> > diff --git a/include/linux/efi.h b/include/linux/efi.h
> > index d2b84c2fec39f0268324d1a38a73ed67786973c9..fc81e4b984398cdb399e7886b2cae7f33bf91613 100644
> > --- a/include/linux/efi.h
> > +++ b/include/linux/efi.h
> > @@ -1324,4 +1324,13 @@ struct linux_efi_coco_secret_area {
> >  /* Header of a populated EFI secret area */
> >  #define EFI_SECRET_TABLE_HEADER_GUID   EFI_GUID(0x1e74f542, 0x71dd, 0x4d66,  0x96, 0x3e, 0xef, 0x42, 0x87, 0xff, 0x17, 0x3b)
> >
> > +#ifdef CONFIG_XEN_EFI
> 
> Please drop this #ifdef

Will fix in v5.

> > +/*
> > + * Returns the end of the memory region containing the given config table,
> > + * or 0 if the given address does not reside in memory that can validly
> > + * contain EFI configuration tables.
> > + */
> > +__init u64 xen_config_table_memory_region_max(u64 addr);
> 
> You can drop the __init here

Will fix in v5.

> > +#endif
> > +
> >  #endif /* _LINUX_EFI_H */
> > --
> > Sincerely,
> > Demi Marie Obenour (she/her/hers)
> > Invisible Things Lab
> >

-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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