Re: [edk2-devel] [RFC 1/2] efi/memattr: Use desc_size instead of total size to check for corruption

Re: [edk2-devel] [RFC 1/2] efi/memattr: Use desc_size instead of total size to check for corruption

[Ard Biesheuvel via groups.io]

On Wed, 8 Jan 2025 at 23:00, Usama Arif <usamaarif642@gmail.com> wrote:
>
> The commit in [1] introduced a check to see if EFI memory attributes
> table was corrupted. It assumed that efi.memmap.nr_map remains
> constant, but it changes during late boot.
> Hence, the check is valid during cold boot, but not in the subsequent
> kexec boot.
>
> This is best explained with an exampled. At cold boot, for a test
> machine:
> efi.memmap.nr_map=91,
> memory_attributes_table->num_entries=48,
> desc_size = 48
> Hence, the check introduced in [1] where 3x the size of the
> entire EFI memory map is a reasonable upper bound for the size of this
> table is valid.
>
> In late boot __efi_enter_virtual_mode calls 2 functions that updates
> efi.memmap.nr_map:
> - efi_map_regions which reduces the `count` of map entries
>   (for e.g. if should_map_region returns false) and which is reflected
>   in efi.memmap by __efi_memmap_init.
>   At this point efi.memmap.nr_map becomes 46 in the test machine.
> - efi_free_boot_services which also reduces the number of memory regions
>   available (for e.g. if md->type or md->attribute is not the right value).
>   At this point efi.memmap.nr_map becomes 9 in the test machine.
> Hence when you kexec into a new kernel and pass efi.memmap, the
> paramaters that are compared are:
> efi.memmap.nr_map=9,
> memory_attributes_table->num_entries=48,
> desc_size = 48
> where the check in [1] is no longer valid with such a low efi.memmap.nr_map
> as it was reduced due to efi_map_regions and efi_free_boot_services.
>
> A more appropriate check is to see if the description size reported by
> efi and memory attributes table is the same.
>
> [1] https://lore.kernel.org/all/20241031175822.2952471-2-ardb+git@google.com/
>
> Fixes: 8fbe4c49c0cc ("efi/memattr: Ignore table if the size is clearly bogus")
> Reported-by: Breno Leitao <leitao@debian.org>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> ---
>  drivers/firmware/efi/memattr.c | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
>

The more I think about this, the more I feel that kexec on x86 should
simply discard this table, and run with the firmware code RWX (which
is not the end of the world).

The main reason is that the EFI memory map and the EFI memory
attributes table are supposed to be a matched pair, where each RTcode
entry in the former is broken down into multiple code and data
segments in the latter. The amount of mangling that the x86 arch code
does of the EFI memory map makes it intractable to ensure that they
remain in sync, and so it is better not to bother.


> diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c
> index c38b1a335590..d3bc161361fb 100644
> --- a/drivers/firmware/efi/memattr.c
> +++ b/drivers/firmware/efi/memattr.c
> @@ -40,21 +40,17 @@ int __init efi_memattr_init(void)
>                 goto unmap;
>         }
>
> -
>         /*
> -        * Sanity check: the Memory Attributes Table contains up to 3 entries
> -        * for each entry of type EfiRuntimeServicesCode in the EFI memory map.
> -        * So if the size of the table exceeds 3x the size of the entire EFI
> -        * memory map, there is clearly something wrong, and the table should
> -        * just be ignored altogether.
> +        * Sanity check: the Memory Attributes Table desc_size and
> +        * efi.memmap.desc_size should match.
>          */
> -       size = tbl->num_entries * tbl->desc_size;
> -       if (size > 3 * efi.memmap.nr_map * efi.memmap.desc_size) {
> -               pr_warn(FW_BUG "Corrupted EFI Memory Attributes Table detected! (version == %u, desc_size == %u, num_entries == %u)\n",
> -                       tbl->version, tbl->desc_size, tbl->num_entries);
> +       if (efi.memmap.desc_size != tbl->desc_size) {
> +               pr_warn(FW_BUG "Corrupted EFI Memory Attributes Table detected! (version == %u, table desc_size == %u, efi.memmap.desc_size == %lu, table num_entries == %u)\n",
> +                       tbl->version, tbl->desc_size, efi.memmap.desc_size, tbl->num_entries);
>                 goto unmap;
>         }
>
> +       size = tbl->num_entries * tbl->desc_size;
>         tbl_size = sizeof(*tbl) + size;
>         memblock_reserve(efi_mem_attr_table, tbl_size);
>         set_bit(EFI_MEM_ATTR, &efi.flags);
> --
> 2.43.5
>


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Re: [edk2-devel] [RFC 1/2] efi/memattr: Use desc_size instead of total size to check for corruption

[Ard Biesheuvel via groups.io]

On Thu, 9 Jan 2025 at 17:36, Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 09/01/2025 15:45, Ard Biesheuvel wrote:
> > On Wed, 8 Jan 2025 at 23:00, Usama Arif <usamaarif642@gmail.com> wrote:
> >>
> >> The commit in [1] introduced a check to see if EFI memory attributes
> >> table was corrupted. It assumed that efi.memmap.nr_map remains
> >> constant, but it changes during late boot.
> >> Hence, the check is valid during cold boot, but not in the subsequent
> >> kexec boot.
> >>
> >> This is best explained with an exampled. At cold boot, for a test
> >> machine:
> >> efi.memmap.nr_map=91,
> >> memory_attributes_table->num_entries=48,
> >> desc_size = 48
> >> Hence, the check introduced in [1] where 3x the size of the
> >> entire EFI memory map is a reasonable upper bound for the size of this
> >> table is valid.
> >>
> >> In late boot __efi_enter_virtual_mode calls 2 functions that updates
> >> efi.memmap.nr_map:
> >> - efi_map_regions which reduces the `count` of map entries
> >>   (for e.g. if should_map_region returns false) and which is reflected
> >>   in efi.memmap by __efi_memmap_init.
> >>   At this point efi.memmap.nr_map becomes 46 in the test machine.
> >> - efi_free_boot_services which also reduces the number of memory regions
> >>   available (for e.g. if md->type or md->attribute is not the right value).
> >>   At this point efi.memmap.nr_map becomes 9 in the test machine.
> >> Hence when you kexec into a new kernel and pass efi.memmap, the
> >> paramaters that are compared are:
> >> efi.memmap.nr_map=9,
> >> memory_attributes_table->num_entries=48,
> >> desc_size = 48
> >> where the check in [1] is no longer valid with such a low efi.memmap.nr_map
> >> as it was reduced due to efi_map_regions and efi_free_boot_services.
> >>
> >> A more appropriate check is to see if the description size reported by
> >> efi and memory attributes table is the same.
> >>
> >> [1] https://lore.kernel.org/all/20241031175822.2952471-2-ardb+git@google.com/
> >>
> >> Fixes: 8fbe4c49c0cc ("efi/memattr: Ignore table if the size is clearly bogus")
> >> Reported-by: Breno Leitao <leitao@debian.org>
> >> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> >> ---
> >>  drivers/firmware/efi/memattr.c | 16 ++++++----------
> >>  1 file changed, 6 insertions(+), 10 deletions(-)
> >>
> >
> > The more I think about this, the more I feel that kexec on x86 should
> > simply discard this table, and run with the firmware code RWX (which
> > is not the end of the world).
>
>
> By discard this table, do you mean kexec not use e820_table_firmware?

No, I mean kexec ignores the memory attributes table.

> Also a very basic question, what do you mean by run with the firmware RWX?
>

The memory attributes table is an overlay for the EFI memory map that
describes which runtime code regions may be mapped with restricted
permissions. Without this table, everything will be mapped writable as
well as executable, but only in the EFI page tables, which are only
active when an EFI call is in progress.

> Sorry for the very basic questions above!
>

No worries.


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Re: [edk2-devel] [RFC 1/2] efi/memattr: Use desc_size instead of total size to check for corruption

[Ard Biesheuvel via groups.io]

On Fri, 10 Jan 2025 at 11:53, Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 10/01/2025 07:21, Ard Biesheuvel wrote:
> > On Thu, 9 Jan 2025 at 17:36, Usama Arif <usamaarif642@gmail.com> wrote:
> >>
> >>
> >>
> >> On 09/01/2025 15:45, Ard Biesheuvel wrote:
> >>> On Wed, 8 Jan 2025 at 23:00, Usama Arif <usamaarif642@gmail.com> wrote:
> >>>>
> >>>> The commit in [1] introduced a check to see if EFI memory attributes
> >>>> table was corrupted. It assumed that efi.memmap.nr_map remains
> >>>> constant, but it changes during late boot.
> >>>> Hence, the check is valid during cold boot, but not in the subsequent
> >>>> kexec boot.
> >>>>
> >>>> This is best explained with an exampled. At cold boot, for a test
> >>>> machine:
> >>>> efi.memmap.nr_map=91,
> >>>> memory_attributes_table->num_entries=48,
> >>>> desc_size = 48
> >>>> Hence, the check introduced in [1] where 3x the size of the
> >>>> entire EFI memory map is a reasonable upper bound for the size of this
> >>>> table is valid.
> >>>>
> >>>> In late boot __efi_enter_virtual_mode calls 2 functions that updates
> >>>> efi.memmap.nr_map:
> >>>> - efi_map_regions which reduces the `count` of map entries
> >>>>   (for e.g. if should_map_region returns false) and which is reflected
> >>>>   in efi.memmap by __efi_memmap_init.
> >>>>   At this point efi.memmap.nr_map becomes 46 in the test machine.
> >>>> - efi_free_boot_services which also reduces the number of memory regions
> >>>>   available (for e.g. if md->type or md->attribute is not the right value).
> >>>>   At this point efi.memmap.nr_map becomes 9 in the test machine.
> >>>> Hence when you kexec into a new kernel and pass efi.memmap, the
> >>>> paramaters that are compared are:
> >>>> efi.memmap.nr_map=9,
> >>>> memory_attributes_table->num_entries=48,
> >>>> desc_size = 48
> >>>> where the check in [1] is no longer valid with such a low efi.memmap.nr_map
> >>>> as it was reduced due to efi_map_regions and efi_free_boot_services.
> >>>>
> >>>> A more appropriate check is to see if the description size reported by
> >>>> efi and memory attributes table is the same.
> >>>>
> >>>> [1] https://lore.kernel.org/all/20241031175822.2952471-2-ardb+git@google.com/
> >>>>
> >>>> Fixes: 8fbe4c49c0cc ("efi/memattr: Ignore table if the size is clearly bogus")
> >>>> Reported-by: Breno Leitao <leitao@debian.org>
> >>>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> >>>> ---
> >>>>  drivers/firmware/efi/memattr.c | 16 ++++++----------
> >>>>  1 file changed, 6 insertions(+), 10 deletions(-)
> >>>>
> >>>
> >>> The more I think about this, the more I feel that kexec on x86 should
> >>> simply discard this table, and run with the firmware code RWX (which
> >>> is not the end of the world).
> >>
> >>
> >> By discard this table, do you mean kexec not use e820_table_firmware?
> >
> > No, I mean kexec ignores the memory attributes table.
> >
> >> Also a very basic question, what do you mean by run with the firmware RWX?
> >>
> >
> > The memory attributes table is an overlay for the EFI memory map that
> > describes which runtime code regions may be mapped with restricted
> > permissions. Without this table, everything will be mapped writable as
> > well as executable, but only in the EFI page tables, which are only
> > active when an EFI call is in progress.
> >
>
> Thanks for explaining!
>
> So basically get rid of memattr.c :)
>
> Do you mean get rid of it only for kexec, or not do it for any
> boot (including cold boot)?

Only for kexec, and only on x86

> I do like this idea! I couldn't find this in the git history,
> but do you know if this was added in the linux kernel just
> because EFI spec added support for it, or if there was a
> specific security problem?
>

Mapping memory RWX is generally a bad idea, so we should avoid it if
possible. But EFI runtime memory regions are only mapped during a EFI
runtime call, and these don't happen often at all, so the benefit is
only marginal. (In the early days of EFI, it was more common for the
OS to map these regions permanently, but we stopped doing that a long
time ago)


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Re: [edk2-devel] [RFC 2/2] efi/memattr: add efi_mem_attr_table as a reserved region in 820_table_firmware

[Ard Biesheuvel via groups.io]

On Wed, 8 Jan 2025 at 23:00, Usama Arif <usamaarif642@gmail.com> wrote:
>
> When this area is not reserved, it comes up as usable in
> /sys/firmware/memmap. This means that kexec, which uses that memmap
> to find usable memory regions, can select the region where
> efi_mem_attr_table is and overwrite it and relocate_kernel.
>
> Since the patch in [1] was merged, all boots after kexec
> are producing the warning that it introduced.
>
> Having a fix in firmware can be difficult to get through.

I don't follow. I don't think there is anything wrong with the
firmware here. Could you elaborate?


> The next ideal place would be in libstub. However, it looks like
> InstallMemoryAttributesTable [2] is not available as a boot service
> call option [3], [4], and install_configuration_table does not
> seem to work as a valid substitute.
>

To do what, exactly?

> As a last option for a fix, this patch marks that region as reserved in
> e820_table_firmware if it is currently E820_TYPE_RAM so that kexec doesn't
> use it for kernel segments.
>
> [1] https://lore.kernel.org/all/20241031175822.2952471-2-ardb+git@google.com/
> [2] https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c#L100
> [3] https://github.com/tianocore/edk2/blob/42a141800c0c26a09d2344e84a89ce4097a263ae/MdeModulePkg/Core/Dxe/DxeMain/DxeMain.c#L41
> [4] https://elixir.bootlin.com/linux/v6.12.6/source/drivers/firmware/efi/libstub/efistub.h#L327
>
> Reported-by: Breno Leitao <leitao@debian.org>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> ---
>  arch/x86/include/asm/e820/api.h | 2 ++
>  arch/x86/kernel/e820.c          | 6 ++++++
>  arch/x86/platform/efi/efi.c     | 9 +++++++++
>  drivers/firmware/efi/memattr.c  | 1 +
>  include/linux/efi.h             | 7 +++++++
>  5 files changed, 25 insertions(+)
>
> diff --git a/arch/x86/include/asm/e820/api.h b/arch/x86/include/asm/e820/api.h
> index 2e74a7f0e935..4e9aa24f03bd 100644
> --- a/arch/x86/include/asm/e820/api.h
> +++ b/arch/x86/include/asm/e820/api.h
> @@ -16,6 +16,8 @@ extern bool e820__mapped_all(u64 start, u64 end, enum e820_type type);
>
>  extern void e820__range_add   (u64 start, u64 size, enum e820_type type);
>  extern u64  e820__range_update(u64 start, u64 size, enum e820_type old_type, enum e820_type new_type);
> +extern u64  e820__range_update_firmware(u64 start, u64 size, enum e820_type old_type,
> +                                       enum e820_type new_type);
>  extern u64  e820__range_remove(u64 start, u64 size, enum e820_type old_type, bool check_type);
>  extern u64  e820__range_update_table(struct e820_table *t, u64 start, u64 size, enum e820_type old_type, enum e820_type new_type);
>
> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> index 82b96ed9890a..01d7d3c0d299 100644
> --- a/arch/x86/kernel/e820.c
> +++ b/arch/x86/kernel/e820.c
> @@ -538,6 +538,12 @@ u64 __init e820__range_update_table(struct e820_table *t, u64 start, u64 size,
>         return __e820__range_update(t, start, size, old_type, new_type);
>  }
>
> +u64 __init e820__range_update_firmware(u64 start, u64 size, enum e820_type old_type,
> +                                      enum e820_type new_type)
> +{
> +       return __e820__range_update(e820_table_firmware, start, size, old_type, new_type);
> +}
> +
>  /* Remove a range of memory from the E820 table: */
>  u64 __init e820__range_remove(u64 start, u64 size, enum e820_type old_type, bool check_type)
>  {
> diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
> index a7ff189421c3..13684c5d7c05 100644
> --- a/arch/x86/platform/efi/efi.c
> +++ b/arch/x86/platform/efi/efi.c
> @@ -168,6 +168,15 @@ static void __init do_add_efi_memmap(void)
>         e820__update_table(e820_table);
>  }
>
> +/* Reserve firmware area if it was marked as RAM */
> +void arch_update_firmware_area(u64 addr, u64 size)
> +{
> +       if (e820__get_entry_type(addr, addr + size) == E820_TYPE_RAM) {
> +               e820__range_update_firmware(addr, size, E820_TYPE_RAM, E820_TYPE_RESERVED);
> +               e820__update_table(e820_table_firmware);
> +       }
> +}
> +
>  /*
>   * Given add_efi_memmap defaults to 0 and there is no alternative
>   * e820 mechanism for soft-reserved memory, import the full EFI memory
> diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c
> index d3bc161361fb..d131781e2d7b 100644
> --- a/drivers/firmware/efi/memattr.c
> +++ b/drivers/firmware/efi/memattr.c
> @@ -53,6 +53,7 @@ int __init efi_memattr_init(void)
>         size = tbl->num_entries * tbl->desc_size;
>         tbl_size = sizeof(*tbl) + size;
>         memblock_reserve(efi_mem_attr_table, tbl_size);
> +       arch_update_firmware_area(efi_mem_attr_table, tbl_size);
>         set_bit(EFI_MEM_ATTR, &efi.flags);
>
>  unmap:
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index e5815867aba9..8eb9698bd6a4 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -1358,4 +1358,11 @@ extern struct blocking_notifier_head efivar_ops_nh;
>  void efivars_generic_ops_register(void);
>  void efivars_generic_ops_unregister(void);
>
> +#ifdef CONFIG_X86_64
> +void __init arch_update_firmware_area(u64 addr, u64 size);
> +#else
> +static inline void __init arch_update_firmware_area(u64 addr, u64 size)
> +{
> +}
> +#endif
>  #endif /* _LINUX_EFI_H */
> --
> 2.43.5
>


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Re: [edk2-devel] [RFC 2/2] efi/memattr: add efi_mem_attr_table as a reserved region in 820_table_firmware

[Ard Biesheuvel via groups.io]

On Thu, 9 Jan 2025 at 17:32, Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 09/01/2025 16:15, Ard Biesheuvel wrote:
> > On Wed, 8 Jan 2025 at 23:00, Usama Arif <usamaarif642@gmail.com> wrote:
> >>
> >> When this area is not reserved, it comes up as usable in
> >> /sys/firmware/memmap. This means that kexec, which uses that memmap
> >> to find usable memory regions, can select the region where
> >> efi_mem_attr_table is and overwrite it and relocate_kernel.
> >>
> >> Since the patch in [1] was merged, all boots after kexec
> >> are producing the warning that it introduced.
> >>
> >> Having a fix in firmware can be difficult to get through.
> >
> > I don't follow. I don't think there is anything wrong with the
> > firmware here. Could you elaborate?
> >
>
> So the problem is, kexec sees this memory as System RAM, and decides
> it can be used to place an image here.
>
> I guess the question is (and I actually don't know the answer here),
> whose responsibility is it to mark this region as reserved so that
> its not touched by anyone else. I would have thought it should be
> firmware?
>

No, it is the OS. The firmware only reserves regions that are needed
for its own correct operation at runtime. For informational tables
such as this one, it is up to the OS whether it wants to reserve it
and keep it in place, consume it and release the memory, or ignore it
altogether.

> Maybe its not the firmwares' job to mark it as reserved, but just pass
> it to kernel and the kernel is supposed to make sure it gets reserved
> in a proper way, even across kexecs.
>

Indeed.

> I think in the end whoevers' responsibility it is, the easiest path forward
> seems to be in kernel? (and not firmware or libstub)
>

Agreed. But as I pointed out in the other thread, the memory
attributes table only augments the memory map with permission
information, and can be disregarded, and given how badly we mangle the
memory map on x86, maybe this is the right choice here.

> >
> >> The next ideal place would be in libstub. However, it looks like
> >> InstallMemoryAttributesTable [2] is not available as a boot service
> >> call option [3], [4], and install_configuration_table does not
> >> seem to work as a valid substitute.
> >>
> >
> > To do what, exactly?
> >
>
> To change the memory type from System RAM to either reserved or
> something more appropriate, i.e. any type that is not touched by
> kexec or any other userspace.
>
> Basically the example code I attached at the end of the cover letter in
> https://lore.kernel.org/all/20250108215957.3437660-1-usamaarif642@gmail.com/
> It could be EFI_ACPI_RECLAIM_MEMORY or EFI_RESERVED_TYPE, both of which aren't
> touched by kexec.
>

This is a kexec problem (on x86 only) so let's fix it there.


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Re: [edk2-devel] [RFC 2/2] efi/memattr: add efi_mem_attr_table as a reserved region in 820_table_firmware

[Ard Biesheuvel via groups.io]

On Fri, 10 Jan 2025 at 12:36, Breno Leitao <leitao@debian.org> wrote:
>
> Hello Ard,
>
> On Fri, Jan 10, 2025 at 08:32:08AM +0100, Ard Biesheuvel wrote:
> > On Thu, 9 Jan 2025 at 17:32, Usama Arif <usamaarif642@gmail.com> wrote:
>
> > > I think in the end whoevers' responsibility it is, the easiest path forward
> > > seems to be in kernel? (and not firmware or libstub)
> > >
> >
> > Agreed. But as I pointed out in the other thread, the memory
> > attributes table only augments the memory map with permission
> > information, and can be disregarded, and given how badly we mangle the
> > memory map on x86, maybe this is the right choice here.
>
> If this augmented memory is not preserved accross kexec, then the next
> kexec'ed kernel will be able to find the original table?
>
> I understand that the memattr region(s) need to be always (in each kexec
> instances) `memblocked_reserved` to protect it from being used as a
> System RAM, right?
>
> Thus, if it is not passed throught e820, kexec'ed kernel needs to fetch
> it again from original EFI table at kexec/boot time.
>

Not sure what 'fetching' means here.

> This brings me another question.
>
> If the kexec'ed kernel sees the original memory, why can't it
> augment/update the RX permissions *again*, instead of passing the
> previous augmented version from previous kernel in this crazy dance.
>

I don't understand what original memory means. I think we're talking
past each other tbh.

> > This is a kexec problem (on x86 only) so let's fix it there.
>
> Would you mind explaining what kexec needs to be done differently?
> Should it preserve the augmented memattr table independently if it is
> mapped in e820?
>

I don't know what 'mapped in e820' means.

Let's forget about what the memory attributes table actually contains,
and just assume we can live without it, ok?

So when booting x86 via kexec (which is already detected in
arch/x86/platform/efi/efi.c), the kernel should pretend that the table
does not exist.


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