The efivarfs pseudo-filesystems exposes all EFI variables as world-readable, and carries some logic to prevent accidental deletion from bricking a system, by setting the immutable flag on all variables whose purpose is unknown. When the RandomSeed support was added, we decided not to expose this variable via efivarfs at all, given that the kernel itself was intended to both produce and consume it directly, without involvement from user space. This removed the need to deal with the world-readable default permissions (which would be undesirable for the random seed that will be used on the next boot), as this would require special handling of the RandomSeed variable, given that we cannot restrict those permissions for all EFI variables without running the risk of breaking user space. Now that the producer side of this mechanism has been reverted [0], it is no longer possible to set the RandomSeed variable at all. Whether and how we will replace the in-kernel producer with something more robust is still under discussion, but in the mean time, let's relax the efivarfs restriction on any direct access of the variable, and instead, ensure that it is created as user read-write only, both when the EFI varstore is enumerated (at mount time) and when the file is created explicitly by user space. Also ensure that the file is not created with the immutable flag set so that user space can manipulate the file as usual. [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=69cbeb61ff9093 Cc: Jason A. Donenfeld <Jason@xxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> --- fs/efivarfs/inode.c | 4 ++-- fs/efivarfs/super.c | 9 ++++++--- 2 files changed, 8 insertions(+), 5 deletions(-) diff --git a/fs/efivarfs/inode.c b/fs/efivarfs/inode.c index b973a2c03dde825e..00cf368fb0ea7315 100644 --- a/fs/efivarfs/inode.c +++ b/fs/efivarfs/inode.c @@ -92,8 +92,8 @@ static int efivarfs_create(struct mnt_idmap *idmap, struct inode *dir, if (err) goto out; if (guid_equal(&var->var.VendorGuid, &LINUX_EFI_RANDOM_SEED_TABLE_GUID)) { - err = -EPERM; - goto out; + mode &= S_IFMT | S_IRUSR | S_IWUSR; + is_removable = true; } if (efivar_variable_is_removable(var->var.VendorGuid, diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c index 482d612b716bb1f0..f98597ec2105ffbb 100644 --- a/fs/efivarfs/super.c +++ b/fs/efivarfs/super.c @@ -115,9 +115,12 @@ static int efivarfs_callback(efi_char16_t *name16, efi_guid_t vendor, int len; int err = -ENOMEM; bool is_removable = false; + umode_t mode = S_IRUGO | S_IWUSR; - if (guid_equal(&vendor, &LINUX_EFI_RANDOM_SEED_TABLE_GUID)) - return 0; + if (guid_equal(&vendor, &LINUX_EFI_RANDOM_SEED_TABLE_GUID)) { + mode = S_IRUSR | S_IWUSR; + is_removable = true; + } entry = kzalloc(sizeof(*entry), GFP_KERNEL); if (!entry) @@ -147,7 +150,7 @@ static int efivarfs_callback(efi_char16_t *name16, efi_guid_t vendor, /* replace invalid slashes like kobject_set_name_vargs does for /sys/firmware/efi/vars. */ strreplace(name, '/', '!'); - inode = efivarfs_get_inode(sb, d_inode(root), S_IFREG | 0644, 0, + inode = efivarfs_get_inode(sb, d_inode(root), S_IFREG | mode, 0, is_removable); if (!inode) goto fail_name; -- 2.39.2
