90 lines
2.9 KiB
Diff
90 lines
2.9 KiB
Diff
From b7e2fb6a680447b7bb7eb18bb7570afa8d2b7f09 Mon Sep 17 00:00:00 2001
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From: Gary Lin <glin@suse.com>
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Date: Thu, 10 Aug 2023 10:19:29 +0800
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Subject: [PATCH 4/4] diskfilter: look up cryptodisk devices first
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When using disk auto-unlocking with TPM 2.0, the typical grub.cfg may
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look like this:
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tpm2_key_protector_init --tpm2key=(hd0,gpt1)/boot/grub2/sealed.tpm
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cryptomount -u <PART-UUID> -P tpm2
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search --fs-uuid --set=root <FS-UUID>
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Since the disk search order is based on the order of module loading, the
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attacker could insert a malicious disk with the same FS-UUID root to
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trick grub2 to boot into th malicious root and further dump memory to
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steal the unsealed key.
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To defend such attack, we can specify the hint provided by 'grub-probe'
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to search the encrypted partition first:
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search --fs-uuid --set=root --hint='cryptouuid/<PART-UUID>' <FS-UUID>
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However, for LVM on a encrypted partition, the search hint provided by
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'grub-probe' is:
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--hint='lvmid/<VG-UUID>/<LV-UUID>'
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It doesn't guarantee to look up the logical volume from the encrypted
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partition, so the attacker may have the chance to fool grub2 to boot
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into the malicious disk.
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To mininize the attack surface, this commit tweaks the disk device search
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in diskfilter to look up cryptodisk devices first and then others, so
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that the auto-unlocked disk will be found first, not the attacker's disk.
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Signed-off-by: Gary Lin <glin@suse.com>
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---
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grub-core/disk/diskfilter.c | 35 ++++++++++++++++++++++++++---------
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1 file changed, 26 insertions(+), 9 deletions(-)
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diff --git a/grub-core/disk/diskfilter.c b/grub-core/disk/diskfilter.c
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index 61a311efd..94832c8dd 100644
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--- a/grub-core/disk/diskfilter.c
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+++ b/grub-core/disk/diskfilter.c
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@@ -226,15 +226,32 @@ scan_devices (const char *arname)
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int need_rescan;
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for (pull = 0; pull < GRUB_DISK_PULL_MAX; pull++)
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- for (p = grub_disk_dev_list; p; p = p->next)
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- if (p->id != GRUB_DISK_DEVICE_DISKFILTER_ID
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- && p->disk_iterate)
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- {
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- if ((p->disk_iterate) (scan_disk_hook, NULL, pull))
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- return;
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- if (arname && is_lv_readable (find_lv (arname), 1))
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- return;
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- }
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+ {
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+ /* look up the crytodisk devices first */
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+ for (p = grub_disk_dev_list; p; p = p->next)
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+ if (p->id == GRUB_DISK_DEVICE_CRYPTODISK_ID
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+ && p->disk_iterate)
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+ {
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+ if ((p->disk_iterate) (scan_disk_hook, NULL, pull))
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+ return;
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+ if (arname && is_lv_readable (find_lv (arname), 1))
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+ return;
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+ break;
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+ }
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+
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+ /* check the devices other than crytodisk */
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+ for (p = grub_disk_dev_list; p; p = p->next)
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+ if (p->id == GRUB_DISK_DEVICE_CRYPTODISK_ID)
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+ continue;
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+ else if (p->id != GRUB_DISK_DEVICE_DISKFILTER_ID
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+ && p->disk_iterate)
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+ {
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+ if ((p->disk_iterate) (scan_disk_hook, NULL, pull))
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+ return;
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+ if (arname && is_lv_readable (find_lv (arname), 1))
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+ return;
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+ }
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+ }
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scan_depth = 0;
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need_rescan = 1;
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--
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2.35.3
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