efi: Add efi= parameter parsing to the EFI boot stub

We need a way to customize the behaviour of the EFI boot stub, in
particular, we need a way to disable the "chunking" workaround, used
when reading files from the EFI System Partition.

One of my machines doesn't cope well when reading files in 1MB chunks to
a buffer above the 4GB mark - it appears that the "chunking" bug
workaround triggers another firmware bug. This was only discovered with
commit 4bf7111f5016 ("x86/efi: Support initrd loaded above 4G"), and
that commit is perfectly valid. The symptom I observed was a corrupt
initrd rather than any kind of crash.

efi= is now used to specify EFI parameters in two very different
execution environments, the EFI boot stub and during kernel boot.

There is also a slight performance optimization by enabling efi=nochunk,
but that's offset by the fact that you're more likely to run into
firmware issues, at least on x86. This is the rationale behind leaving
the workaround enabled by default.

Also provide some documentation for EFI_READ_CHUNK_SIZE and why we're
using the current value of 1MB.

Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Roy Franz <roy.franz@linaro.org>
Cc: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Borislav Petkov <bp@suse.de>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 60 insertions, 2 deletions

diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
index 32d5cca30f49..a920fec8fe88 100644
--- a/drivers/firmware/efi/libstub/efi-stub-helper.c
+++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
@@ -15,8 +15,23 @@
 
 #include "efistub.h"
 
+/*
+ * Some firmware implementations have problems reading files in one go.
+ * A read chunk size of 1MB seems to work for most platforms.
+ *
+ * Unfortunately, reading files in chunks triggers *other* bugs on some
+ * platforms, so we provide a way to disable this workaround, which can
+ * be done by passing "efi=nochunk" on the EFI boot stub command line.
+ *
+ * If you experience issues with initrd images being corrupt it's worth
+ * trying efi=nochunk, but chunking is enabled by default because there
+ * are far more machines that require the workaround than those that
+ * break with it enabled.
+ */
 #define EFI_READ_CHUNK_SIZE	(1024 * 1024)
 
+static unsigned long __chunk_size = EFI_READ_CHUNK_SIZE;
+
 struct file_info {
 	efi_file_handle_t *handle;
 	u64 size;
@@ -281,6 +296,49 @@ void efi_free(efi_system_table_t *sys_table_arg, unsigned long size,
 	efi_call_early(free_pages, addr, nr_pages);
 }
 
+/*
+ * Parse the ASCII string 'cmdline' for EFI options, denoted by the efi=
+ * option, e.g. efi=nochunk.
+ *
+ * It should be noted that efi= is parsed in two very different
+ * environments, first in the early boot environment of the EFI boot
+ * stub, and subsequently during the kernel boot.
+ */
+efi_status_t efi_parse_options(char *cmdline)
+{
+	char *str;
+
+	/*
+	 * If no EFI parameters were specified on the cmdline we've got
+	 * nothing to do.
+	 */
+	str = strstr(cmdline, "efi=");
+	if (!str)
+		return EFI_SUCCESS;
+
+	/* Skip ahead to first argument */
+	str += strlen("efi=");
+
+	/*
+	 * Remember, because efi= is also used by the kernel we need to
+	 * skip over arguments we don't understand.
+	 */
+	while (*str) {
+		if (!strncmp(str, "nochunk", 7)) {
+			str += strlen("nochunk");
+			__chunk_size = -1UL;
+		}
+
+		/* Group words together, delimited by "," */
+		while (*str && *str != ',')
+			str++;
+
+		if (*str == ',')
+			str++;
+	}
+
+	return EFI_SUCCESS;
+}
 
 /*
  * Check the cmdline for a LILO-style file= arguments.
@@ -423,8 +481,8 @@ efi_status_t handle_cmdline_files(efi_system_table_t *sys_table_arg,
 			size = files[j].size;
 			while (size) {
 				unsigned long chunksize;
-				if (size > EFI_READ_CHUNK_SIZE)
-					chunksize = EFI_READ_CHUNK_SIZE;
+				if (size > __chunk_size)
+					chunksize = __chunk_size;
 				else
 					chunksize = size;