[PATCH 1/3] efi/libstub: Don't replace an existing LINUX_EFI_MEMRESERVE table

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

The LINUX_EFI_MEMRESERVE table is the root of a linked list of memory
reservations that need to persist across kexec, such as the GICv3 LPI
pending and property tables, which cannot be moved once the
redistributors have been enabled. Each kernel adds its own reservations
to the list that it finds via the EFI configuration table, and honours
the ones made by its predecessors.

The EFI stub installs a new, empty table unconditionally, replacing any
existing one. This is fine on a cold boot, but not if the stub is being
executed by a loader that passes on the configuration tables of an
earlier kernel, e.g., a kexec loader that runs the incoming kernel's EFI
stub, as the reservations made by earlier kernels would be lost.

So leave an existing table in place, and only install a new one if none
exists yet.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/efi-stub.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/firmware/efi/libstub/efi-stub.c b/drivers/firmware/efi/libstub/efi-stub.c
index 975148f986bf..6db10590f89b 100644
--- a/drivers/firmware/efi/libstub/efi-stub.c
+++ b/drivers/firmware/efi/libstub/efi-stub.c
@@ -83,6 +83,14 @@ static void install_memreserve_table(void)
 	struct linux_efi_memreserve *rsv;
 	efi_status_t status;
 
+	/*
+	 * If an earlier boot stage installed a memreserve table already, e.g.,
+	 * a kexec loader passing on the persistent reservations made by the
+	 * previous kernel, use it as is: installing a new one would replace it.
+	 */
+	if (get_efi_config_table(memreserve_table_guid))
+		return;
+
 	status = efi_bs_call(allocate_pool, EFI_LOADER_DATA, sizeof(*rsv),
 			     (void **)&rsv);
 	if (status != EFI_SUCCESS) {
-- 
2.56.0.385.gd3acb90ef8-goog