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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     linux-efi@vger.kernel.org, linux-crypto@vger.kernel.org,
        patches@lists.linux.dev, linux-kernel@vger.kernel.org,
        ardb@kernel.org
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: [PATCH v3 1/5] efi: vars: prohibit reading random seed variables
Date:   Tue, 22 Nov 2022 03:04:00 +0100
Message-Id: <20221122020404.3476063-2-Jason@zx2c4.com>
In-Reply-To: <20221122020404.3476063-1-Jason@zx2c4.com>
References: <20221122020404.3476063-1-Jason@zx2c4.com>
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In anticipation of putting random seeds in EFI variables, it's important
that the random GUID namespace of variables remains hidden from
userspace. We accomplish this by not populating efivarfs with entries
from that GUID, as well as denying the creation of new ones in that
GUID.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 fs/efivarfs/inode.c | 4 ++++
 fs/efivarfs/super.c | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/fs/efivarfs/inode.c b/fs/efivarfs/inode.c
index 939e5e242b98..617f3ad2485e 100644
--- a/fs/efivarfs/inode.c
+++ b/fs/efivarfs/inode.c
@@ -91,6 +91,10 @@ static int efivarfs_create(struct user_namespace *mnt_userns, struct inode *dir,
 	err = guid_parse(dentry->d_name.name + namelen + 1, &var->var.VendorGuid);
 	if (err)
 		goto out;
+	if (guid_equal(&var->var.VendorGuid, &LINUX_EFI_RANDOM_SEED_TABLE_GUID)) {
+		err = -EPERM;
+		goto out;
+	}
 
 	if (efivar_variable_is_removable(var->var.VendorGuid,
 					 dentry->d_name.name, namelen))
diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
index 6780fc81cc11..07e82e246666 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -116,6 +116,9 @@ static int efivarfs_callback(efi_char16_t *name16, efi_guid_t vendor,
 	int err = -ENOMEM;
 	bool is_removable = false;
 
+	if (guid_equal(&vendor, &LINUX_EFI_RANDOM_SEED_TABLE_GUID))
+		return 0;
+
 	entry = kzalloc(sizeof(*entry), GFP_KERNEL);
 	if (!entry)
 		return err;
-- 
2.38.1