[PATCH] fs/efivarfs: Permit read-only access on platforms that lack SetVariable

[Ard Biesheuvel <ardb@kernel.org>]

Commit

  bf67fad19e493b ("efi: Use more granular check for availability for variable services")

introduced a check into the efivarfs init code that aborts loading of the
module if not all three variable runtime services (GetVariable, SetVariable
and GetNextVariable) are supported. However, this results in efivarfs being
unavailable entirely if only SetVariable support is missing, which is only
needed if you want to make any modifications.

So let's relax this restriction, and only require the GetVariable services,
but force efivarfs to be mounted read-only if SetVariable is unsupported.

Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Fixes: bf67fad19e493b ("efi: Use more granular check for availability for variable services")
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 fs/efivarfs/super.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
index 12c66f5d92dd..c12608f0dcd9 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -201,6 +201,9 @@ static int efivarfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_d_op		= &efivarfs_d_ops;
 	sb->s_time_gran         = 1;
 
+	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_SET_VARIABLE))
+		sb->s_flags |= SB_RDONLY;
+
 	inode = efivarfs_get_inode(sb, NULL, S_IFDIR | 0755, 0, true);
 	if (!inode)
 		return -ENOMEM;
@@ -252,7 +255,8 @@ static struct file_system_type efivarfs_type = {
 
 static __init int efivarfs_init(void)
 {
-	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_VARIABLE_SERVICES))
+	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE |
+				       EFI_RT_SUPPORTED_GET_NEXT_VARIABLE_NAME))
 		return -ENODEV;
 
 	if (!efivars_kobject())
-- 
2.17.1