0010-shim-another-attempt-to-fix-load-options-handling.patch

From 4d64389c6c941d21548b06423b8131c872e3c3c7 Mon Sep 17 00:00:00 2001
From: Chris Coulson <chris.coulson@canonical.com>
Date: Mon, 7 Jun 2021 16:34:18 +0100
Subject: [PATCH 10/26] shim: another attempt to fix load options handling

The load options handling is quite complicated and tries to accomodate
several scenarios, but there are currently multiple issues:

- If the supplied LoadOptions is an EFI_LOAD_OPTION structure,
second_stage gets initialized to the entire contents of the OptionalData
field and load_options is initialized to NULL, which means it isn't
possible to pass additional options to the second stage loader (and it
looks like the intention is for this to be supported).

- If the supplied LoadOptions contains 2 or more strings, the code seems
to assume that shim was executed from the UEFI shell and that the first
argument is the path of the shim executable, so it's ignored. But this
breaks the ability to pass additional options to the second stage loader
from BDS on firmware implementations that initialize LoadOptions to just
the OptionalData field of the EFI_LOAD_OPTION, which is what EDK2 seems
to do.

This is moot anyway because this case (strings == 2) doesn't actually seem
to work, as nothing sets loader_len and therefore second_stage is not set
to the custom loader path.

- If the supplied LoadOptions contains a single string that isn't shim's
path, nothing sets loader_len and therefore second_stage isn't set at the
end of set_second_stage.

- set_second_stage replaces L' ' characters with L'\0' - whilst this is
useful to NULL terminate the path for the second stage, it doesn't seem
quite right to do this for the remaining LoadOptions data. Grub's
chainloader command supplies additional arguments as a NULL-terminated
space-delimited string via LoadOptions. Making it NULL-delimited seems to
be incompatible with the kernel's commandline handling, which wouldn't
work for scenarios where you might want to direct-boot a kernel image
(wrapped in systemd's EFI stub) from shim.

- handle_image passes the original LoadOptions to the second stage if
load_options is NULL, which means that the second stage currently always
gets shim's load options.

I've made an attempt to try to fix things. After the initial
checks in set_second_stage, it now does this:

- Tries to parse LoadOptions as an EFI_LOAD_OPTION in order to extract
the OptionalData if it is.
- If it's not an EFI_LOAD_OPTION, check if the first string is the
current shim path and ignore it if it is (the UEFI shell case).
- Split LoadOptions in to a single NULL terminated string (used to
initialize second_stage) and the unmodified remaining data (used to
initialize load_options and load_options_size).

I've also modified handle_image to always set LoadOptions and
LoadOptionsSize. If shim is executed with no options, or is only
executed with a single option to override the second stage loader
path, the second stage is executed with LoadOptions = NULL and
LoadOptionsSize = 0 now.

I've tested this on EDK2 and I can load a custom loader with extra
options from both BDS and the UEFI shell:

FS0:\> shimx64.efi test.efi
LoadOptionsSize: 0
LoadOptions: (null)
FS0:\> shimx64.efi       test.efi
LoadOptionsSize: 0
LoadOptions: (null)
FS0:\> shimx64.efi test.efi foo bar
LoadOptionsSize: 16
LoadOptions: foo bar
---
 include/ucs2.h |  27 -------
 pe.c           |   6 +-
 shim.c         | 200 ++++++++++++++++++++++---------------------------
 3 files changed, 92 insertions(+), 141 deletions(-)

diff --git a/include/ucs2.h b/include/ucs2.h
index e43c341..ee038ce 100644
--- a/include/ucs2.h
+++ b/include/ucs2.h
@@ -81,31 +81,4 @@ is_all_nuls(UINT8 *data, UINTN data_size)
 	return true;
 }
 
-static inline UINTN
-__attribute__((__unused__))
-count_ucs2_strings(UINT8 *data, UINTN data_size)
-{
-	UINTN pos = 0;
-	UINTN last_nul_pos = 0;
-	UINTN num_nuls = 0;
-	UINTN i;
-
-	if (data_size % 2 != 0)
-		return 0;
-
-	for (i = pos; i < data_size; i++) {
-		if (i % 2 != 0) {
-			if (data[i] != 0)
-				return 0;
-		} else if (data[i] == 0) {
-			last_nul_pos = i;
-			num_nuls++;
-		}
-		pos = i;
-	}
-	if (num_nuls > 0 && last_nul_pos != pos - 1)
-		return 0;
-	return num_nuls;
-}
-
 #endif /* SHIM_UCS2_H */
diff --git a/pe.c b/pe.c
index 365e32a..13bc397 100644
--- a/pe.c
+++ b/pe.c
@@ -1144,10 +1144,8 @@ handle_image (void *data, unsigned int datasize,
 	li->ImageSize = context.ImageSize;
 
 	/* Pass the load options to the second stage loader */
-	if ( load_options ) {
-		li->LoadOptions = load_options;
-		li->LoadOptionsSize = load_options_size;
-	}
+	li->LoadOptions = load_options;
+	li->LoadOptionsSize = load_options_size;
 
 	if (!found_entry_point) {
 		perror(L"Entry point is not within sections\n");
diff --git a/shim.c b/shim.c
index 40e4894..ecf6ee5 100644
--- a/shim.c
+++ b/shim.c
@@ -1241,9 +1241,13 @@ EFI_STATUS init_grub(EFI_HANDLE image_handle)
 	return efi_status;
 }
 
+/*
+ * Extract the OptionalData and OptionalData fields from an
+ * EFI_LOAD_OPTION.
+ */
 static inline EFI_STATUS
-get_load_option_optional_data(UINT8 *data, UINTN data_size,
-			      UINT8 **od, UINTN *ods)
+get_load_option_optional_data(VOID *data, UINT32 data_size,
+			      VOID **od, UINT32 *ods)
 {
 	/*
 	 * If it's not at least Attributes + FilePathListLength +
@@ -1253,7 +1257,8 @@ get_load_option_optional_data(UINT8 *data, UINTN data_size,
 	if (data_size < (sizeof(UINT32) + sizeof(UINT16) + 2 + 4))
 		return EFI_INVALID_PARAMETER;
 
-	UINT8 *cur = data + sizeof(UINT32);
+	UINT8 *start = (UINT8 *)data;
+	UINT8 *cur = start + sizeof(UINT32);
 	UINT16 fplistlen = *(UINT16 *)cur;
 	/*
 	 * If there's not enough space for the file path list and the
@@ -1263,8 +1268,8 @@ get_load_option_optional_data(UINT8 *data, UINTN data_size,
 		return EFI_INVALID_PARAMETER;
 
 	cur += sizeof(UINT16);
-	UINTN limit = data_size - (cur - data) - fplistlen;
-	UINTN i;
+	UINT32 limit = data_size - (cur - start) - fplistlen;
+	UINT32 i;
 	for (i = 0; i < limit ; i++) {
 		/* If the description isn't valid UCS2-LE, it's not valid. */
 		if (i % 2 != 0) {
@@ -1380,6 +1385,57 @@ done:
 	return ret;
 }
 
+/*
+ * Split the supplied load options in to a NULL terminated
+ * string representing the path of the second stage loader,
+ * and return a pointer to the remaining load options data
+ * and its remaining size.
+ *
+ * This expects the supplied load options to begin with a
+ * string that is either NULL terminated or terminated with
+ * a space and some optional data. It will return NULL if
+ * the supplied load options contains no spaces or NULL
+ * terminators.
+ */
+static CHAR16 *
+split_load_options(VOID *in, UINT32 in_size,
+		   VOID **remaining,
+		   UINT32 *remaining_size) {
+	UINTN i;
+	CHAR16 *arg0 = NULL;
+	CHAR16 *start = (CHAR16 *)in;
+
+	/* Skip spaces */
+	for (i = 0; i < in_size / sizeof(CHAR16); i++) {
+		if (*start != L' ')
+			break;
+
+		start++;
+	}
+
+	in_size -= ((VOID *)start - in);
+
+	/*
+	 * Ensure that the first argument is NULL terminated by
+	 * replacing L' ' with L'\0'.
+	 */
+	for (i = 0; i < in_size / sizeof(CHAR16); i++) {
+		if (start[i] == L' ' || start[i] == L'\0') {
+			start[i] = L'\0';
+			arg0 = (CHAR16 *)start;
+			break;
+		}
+	}
+
+	if (arg0) {
+		UINTN skip = i + 1;
+		*remaining_size = in_size - (skip * sizeof(CHAR16));
+		*remaining = *remaining_size > 0 ? start + skip : NULL;
+	}
+
+	return arg0;
+}
+
 /*
  * Check the load options to specify the second stage loader
  */
@@ -1387,20 +1443,11 @@ EFI_STATUS set_second_stage (EFI_HANDLE image_handle)
 {
 	EFI_STATUS efi_status;
 	EFI_LOADED_IMAGE *li = NULL;
-	CHAR16 *start = NULL;
-	UINTN remaining_size = 0;
+	VOID *remaining = NULL;
+	UINT32 remaining_size;
 	CHAR16 *loader_str = NULL;
-	UINTN loader_len = 0;
-	unsigned int i;
-	UINTN second_stage_len;
 
-	second_stage_len = (StrLen(DEFAULT_LOADER) + 1) * sizeof(CHAR16);
-	second_stage = AllocatePool(second_stage_len);
-	if (!second_stage) {
-		perror(L"Could not allocate %lu bytes\n", second_stage_len);
-		return EFI_OUT_OF_RESOURCES;
-	}
-	StrCpy(second_stage, DEFAULT_LOADER);
+	second_stage = DEFAULT_LOADER;
 	load_options = NULL;
 	load_options_size = 0;
 
@@ -1499,105 +1546,44 @@ EFI_STATUS set_second_stage (EFI_HANDLE image_handle)
 		return EFI_SUCCESS;
 
 	/*
-	 * Check and see if this is just a list of strings.  If it's an
-	 * EFI_LOAD_OPTION, it'll be 0, since we know EndEntire device path
-	 * won't pass muster as UCS2-LE.
-	 *
-	 * If there are 3 strings, we're launched from the shell most likely,
-	 * But we actually only care about the second one.
+	 * See if this is an EFI_LOAD_OPTION and extract the optional
+	 * data if it is. This will return an error if it is not a valid
+	 * EFI_LOAD_OPTION.
 	 */
-	UINTN strings = count_ucs2_strings(li->LoadOptions,
-					   li->LoadOptionsSize);
-
-	/*
-	 * In some cases we get strings == 1 because BDS is using L' ' as the
-	 * delimeter:
-	 * 0000:74 00 65 00 73 00 74 00 2E 00 65 00 66 00 69 00 t.e.s.t...e.f.i.
-	 * 0016:20 00 6F 00 6E 00 65 00 20 00 74 00 77 00 6F 00 ..o.n.e...t.w.o.
-	 * 0032:20 00 74 00 68 00 72 00 65 00 65 00 00 00       ..t.h.r.e.e...
-	 *
-	 * If so replace it with NULs since the code already handles that
-	 * case.
-	 */
-	if (strings == 1) {
-		UINT16 *cur = start = li->LoadOptions;
-
-		/* replace L' ' with L'\0' if we find any */
-		for (i = 0; i < li->LoadOptionsSize / 2; i++) {
-			if (cur[i] == L' ')
-				cur[i] = L'\0';
-		}
-
-		/* redo the string count */
-		strings = count_ucs2_strings(li->LoadOptions,
-					     li->LoadOptionsSize);
-	}
-
-	/*
-	 * If it's not string data, try it as an EFI_LOAD_OPTION.
-	 */
-	if (strings == 0) {
-		/*
-		 * We at least didn't find /enough/ strings.  See if it works
-		 * as an EFI_LOAD_OPTION.
-		 */
-		efi_status = get_load_option_optional_data(li->LoadOptions,
-							   li->LoadOptionsSize,
-							   (UINT8 **)&start,
-							   &loader_len);
-		if (EFI_ERROR(efi_status))
-			return EFI_SUCCESS;
-
-		remaining_size = 0;
-	} else if (strings >= 2) {
+	efi_status = get_load_option_optional_data(li->LoadOptions,
+						   li->LoadOptionsSize,
+						   &li->LoadOptions,
+						   &li->LoadOptionsSize);
+	if (EFI_ERROR(efi_status)) {
 		/*
+		 * it's not an EFI_LOAD_OPTION, so it's probably just a string
+		 * or list of strings.
+		 *
 		 * UEFI shell copies the whole line of the command into
-		 * LoadOptions.  We ignore the string before the first L'\0',
-		 * i.e. the name of this program.
+		 * LoadOptions. We ignore the first string, i.e. the name of this
+		 * program in this case.
 		 */
-		UINT16 *cur = li->LoadOptions;
-		for (i = 1; i < li->LoadOptionsSize / 2; i++) {
-			if (cur[i - 1] == L'\0') {
-				start = &cur[i];
-				remaining_size = li->LoadOptionsSize - (i * 2);
-				break;
-			}
+		CHAR16 *loader_str = split_load_options(li->LoadOptions,
+							li->LoadOptionsSize,
+							&remaining,
+							&remaining_size);
+
+		if (loader_str && is_our_path(li, loader_str)) {
+			li->LoadOptions = remaining;
+			li->LoadOptionsSize = remaining_size;
 		}
-
-		remaining_size -= i * 2 + 2;
-	} else if (strings == 1 && is_our_path(li, start)) {
-		/*
-		 * And then I found a version of BDS that gives us our own path
-		 * in LoadOptions:
-
-77162C58                           5c 00 45 00 46 00 49 00          |\.E.F.I.|
-77162C60  5c 00 42 00 4f 00 4f 00  54 00 5c 00 42 00 4f 00  |\.B.O.O.T.\.B.O.|
-77162C70  4f 00 54 00 58 00 36 00  34 00 2e 00 45 00 46 00  |O.T.X.6.4...E.F.|
-77162C80  49 00 00 00                                       |I...|
-
-		* which is just cruel... So yeah, just don't use it.
-		*/
-		return EFI_SUCCESS;
 	}
 
+	loader_str = split_load_options(li->LoadOptions, li->LoadOptionsSize,
+					&remaining, &remaining_size);
+
 	/*
 	 * Set up the name of the alternative loader and the LoadOptions for
 	 * the loader
 	 */
-	if (loader_len > 0) {
-		/* we might not always have a NULL at the end */
-		loader_str = AllocatePool(loader_len + 2);
-		if (!loader_str) {
-			perror(L"Failed to allocate loader string\n");
-			return EFI_OUT_OF_RESOURCES;
-		}
-
-		for (i = 0; i < loader_len / 2; i++)
-			loader_str[i] = start[i];
-		loader_str[loader_len/2] = L'\0';
-
+	if (loader_str) {
 		second_stage = loader_str;
-		load_options = remaining_size ? start + (loader_len/2) : NULL;
+		load_options = remaining;
 		load_options_size = remaining_size;
 	}
 
@@ -1777,12 +1763,6 @@ shim_fini(void)
 
 	unhook_exit();
 
-	/*
-	 * Free the space allocated for the alternative 2nd stage loader
-	 */
-	if (load_options_size > 0 && second_stage)
-		FreePool(second_stage);
-
 	console_fini();
 }
 
-- 
2.32.0