[tip: efi/urgent] efivar/ssdt: Don't iterate over EFI vars if no SSDT override was specified

[tip-bot2 for Ard Biesheuvel]

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

Commit-ID:     c05f8f92b701576b615f30aac31fabdc0648649b
Gitweb:        https://git.kernel.org/tip/c05f8f92b701576b615f30aac31fabdc0648649b
Author:        Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
AuthorDate:    Wed, 02 Oct 2019 18:58:59 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Mon, 07 Oct 2019 15:24:35 +02:00

efivar/ssdt: Don't iterate over EFI vars if no SSDT override was specified

The kernel command line option efivar_ssdt= allows the name to be
specified of an EFI variable containing an ACPI SSDT table that should
be loaded into memory by the OS, and treated as if it was provided by
the firmware.

Currently, that code will always iterate over the EFI variables and
compare each name with the provided name, even if the command line
option wasn't set to begin with.

So bail early when no variable name was provided. This works around a
boot regression on the 2012 Mac Pro, as reported by Scott.

Tested-by: Scott Talbert <swt@xxxxxxxxxx>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v4.9+
Cc: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx>
Cc: Dave Young <dyoung@xxxxxxxxxx>
Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
Cc: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Lukas Wunner <lukas@xxxxxxxxx>
Cc: Lyude Paul <lyude@xxxxxxxxxx>
Cc: Matthew Garrett <mjg59@xxxxxxxxxx>
Cc: Octavian Purdila <octavian.purdila@xxxxxxxxx>
Cc: Peter Jones <pjones@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-efi@xxxxxxxxxxxxxxx
Cc: linux-integrity@xxxxxxxxxxxxxxx
Fixes: 475fb4e8b2f4 ("efi / ACPI: load SSTDs from EFI variables")
Link: https://lkml.kernel.org/r/20191002165904.8819-3-ard.biesheuvel@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 drivers/firmware/efi/efi.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 8d3e778..69f00f7 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -267,6 +267,9 @@ static __init int efivar_ssdt_load(void)
 	void *data;
 	int ret;
 
+	if (!efivar_ssdt[0])
+		return 0;
+
 	ret = efivar_init(efivar_ssdt_iter, &entries, true, &entries);
 
 	list_for_each_entry_safe(entry, aux, &entries, list) {