[tip: efi/urgent] efi: efibc: check for efivars write capability

[tip-bot2 for Ard Biesheuvel]

The following commit has been merged into the efi/urgent branch of tip:

Commit-ID:     46908326c6b801201f1e46f5ed0db6e85bef74ae
Gitweb:        https://git.kernel.org/tip/46908326c6b801201f1e46f5ed0db6e85bef74ae
Author:        Ard Biesheuvel <ardb@xxxxxxxxxx>
AuthorDate:    Tue, 15 Sep 2020 18:12:09 +03:00
Committer:     Ard Biesheuvel <ardb@xxxxxxxxxx>
CommitterDate: Tue, 15 Sep 2020 18:22:47 +03:00

efi: efibc: check for efivars write capability

Branden reports that commit

  f88814cc2578c1 ("efi/efivars: Expose RT service availability via efivars abstraction")

regresses UEFI platforms that implement GetVariable but not SetVariable
when booting kernels that have EFIBC (bootloader control) enabled.

The reason is that EFIBC is a user of the efivars abstraction, which was
updated to permit users that rely only on the read capability, but not on
the write capability. EFIBC is in the latter category, so it has to check
explicitly whether efivars supports writes.

Fixes: f88814cc2578c1 ("efi/efivars: Expose RT service availability via efivars abstraction")
Tested-by: Branden Sherrell <sherrellbc@xxxxxxxxx>
Link: https://lore.kernel.org/linux-efi/AE217103-C96F-4AFC-8417-83EC11962004@xxxxxxxxx/
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 drivers/firmware/efi/efibc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/efibc.c b/drivers/firmware/efi/efibc.c
index 35dccc8..15a4753 100644
--- a/drivers/firmware/efi/efibc.c
+++ b/drivers/firmware/efi/efibc.c
@@ -84,7 +84,7 @@ static int __init efibc_init(void)
 {
 	int ret;
 
-	if (!efi_enabled(EFI_RUNTIME_SERVICES))
+	if (!efivars_kobject() || !efivar_supports_writes())
 		return -ENODEV;
 
 	ret = register_reboot_notifier(&efibc_reboot_notifier);