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([2620:10d:c092:500::6:97ef]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-ab2c90d52edsm490919366b.42.2025.01.13.04.00.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 13 Jan 2025 04:00:12 -0800 (PST) Message-ID: <138f28ec-341e-4c48-a14b-4371a8198de8@gmail.com> Date: Mon, 13 Jan 2025 12:00:12 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [edk2-devel] [RFC 1/2] efi/memattr: Use desc_size instead of total size to check for corruption From: "Usama Arif via groups.io" To: Dave Young , Ard Biesheuvel Cc: linux-efi@vger.kernel.org, devel@edk2.groups.io, kexec@lists.infradead.org, hannes@cmpxchg.org, x86@kernel.org, linux-kernel@vger.kernel.org, leitao@debian.org, gourry@gourry.net, kernel-team@meta.com References: <20250108215957.3437660-1-usamaarif642@gmail.com> <20250108215957.3437660-2-usamaarif642@gmail.com> <8613563a-ee7c-4271-b1f0-4d1ac365ad3a@gmail.com> In-Reply-To: Precedence: Bulk List-Subscribe: List-Help: Sender: devel@edk2.groups.io List-Id: Mailing-List: list devel@edk2.groups.io; contact devel+owner@edk2.groups.io Resent-Date: Wed, 15 Jan 2025 10:53:11 -0800 Resent-From: usamaarif642@gmail.com Reply-To: devel@edk2.groups.io,usamaarif642@gmail.com List-Unsubscribe-Post: List-Unsubscribe=One-Click List-Unsubscribe: Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-GND-Status: LEGIT Authentication-Results: spool.mail.gandi.net; dkim=pass header.d=groups.io header.s=20240830 header.b=AohLdwR4; dmarc=pass (policy=none) header.from=groups.io; spf=pass (spool.mail.gandi.net: domain of bounce@groups.io designates 45.79.224.7 as permitted sender) smtp.mailfrom=bounce@groups.io On 13/01/2025 11:27, Usama Arif wrote: >=20 >=20 > On 13/01/2025 02:33, Dave Young wrote: >> On Fri, 10 Jan 2025 at 18:54, Usama Arif wrote: >>> >>> >>> >>> On 10/01/2025 07:21, Ard Biesheuvel wrote: >>>> On Thu, 9 Jan 2025 at 17:36, Usama Arif wrote= : >>>>> >>>>> >>>>> >>>>> On 09/01/2025 15:45, Ard Biesheuvel wrote: >>>>>> On Wed, 8 Jan 2025 at 23:00, Usama Arif wro= te: >>>>>>> >>>>>>> The commit in [1] introduced a check to see if EFI memory attribute= s >>>>>>> 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 subseque= nt >>>>>>> kexec boot. >>>>>>> >>>>>>> This is best explained with an exampled. At cold boot, for a test >>>>>>> machine: >>>>>>> efi.memmap.nr_map=3D91, >>>>>>> memory_attributes_table->num_entries=3D48, >>>>>>> desc_size =3D 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 t= his >>>>>>> table is valid. >>>>>>> >>>>>>> In late boot __efi_enter_virtual_mode calls 2 functions that update= s >>>>>>> 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 reflec= ted >>>>>>> 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 re= gions >>>>>>> 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=3D9, >>>>>>> memory_attributes_table->num_entries=3D48, >>>>>>> desc_size =3D 48 >>>>>>> where the check in [1] is no longer valid with such a low efi.memma= p.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@g= oogle.com/ >>>>>>> >>>>>>> Fixes: 8fbe4c49c0cc ("efi/memattr: Ignore table if the size is clea= rly bogus") >>>>>>> Reported-by: Breno Leitao >>>>>>> Signed-off-by: Usama Arif >>>>>>> --- >>>>>>> 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 shoul= d >>>>>> 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)? >>> 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? >>> >> >> Usama, can you try the patch below? >> [ format is wrong due to webmail corruption. But if it works I can >> send a formal patch later ] >> >> $ git diff arch/x86 >> diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quir= ks.c >> index 846bf49f2508..58dc77c5210e 100644 >> --- a/arch/x86/platform/efi/quirks.c >> +++ b/arch/x86/platform/efi/quirks.c >> @@ -561,6 +561,11 @@ int __init efi_reuse_config(u64 tables, int nr_tabl= es) >> >> if (!efi_guidcmp(guid, SMBIOS_TABLE_GUID)) >> ((efi_config_table_64_t *)p)->table =3D data->sm= bios; >> + >> + /* Not bother to play with mem attr table across kexec *= / >> + if (!efi_guidcmp(guid, EFI_MEMORY_ATTRIBUTES_TABLE_GUID)= ) >> + ((efi_config_table_64_t *)p)->table =3D >> EFI_INVALID_TABLE_ADDR; >> + >> p +=3D sz; >> } >> >=20 > This would work, I am guessing it will have a similar effect to what I se= nt > last week in=20 > https://lore.kernel.org/all/fd63613c-fd26-42de-b5ed-cc734f72eb36@gmail.co= m/ >=20 > I think it needs to be wrapped in ifdef CONFIG_X86_64. >=20 IMO we should consider the 2 patches in this series first before disabling = it for kexec. These patches actually fix the issue. -=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#121018): https://edk2.groups.io/g/devel/message/121018 Mute This Topic: https://groups.io/mt/110517813/7686176 Group Owner: devel+owner@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [rebecca@openfw.io] -=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-=3D-