[PATCH 1/1] efi_loader: fix get_last_capsule()

[Heinrich Schuchardt]

fix get_last_capsule() leads to writes beyond the stack allocated buffer.
This was indicated when enabling the stack protector.

utf16_utf8_strcpy() only stops copying when reaching '\0'. The current
invocation always writes beyond the end of value[].

The output length of utf16_utf8_strcpy() may be longer than the number of
UTF-16 tokens. E.g has "CapsuleКиев" has 11 UTF-16 tokens but 15 UTF-8
tokens. Hence, using utf16_utf8_strcpy() without checking the input may
lead to further writes beyond value[].

The current invocation of strict_strtoul() reads beyond the end of value[].

A non-hexadecimal value after "Capsule" (e.g. "CapsuleZZZZ") must result in
an error. We cat catch this by checking the return value of strict_strtoul().

A value that is too short after "Capsule" (e.g. "Capsule0") must result in
an error. We must check the string length of value[].

Signed-off-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
---
 lib/efi_loader/efi_capsule.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/lib/efi_loader/efi_capsule.c b/lib/efi_loader/efi_capsule.c
index d39d731080..0017f0c0db 100644
--- a/lib/efi_loader/efi_capsule.c
+++ b/lib/efi_loader/efi_capsule.c
@@ -42,20 +42,28 @@ static struct efi_file_handle *bootdev_root;
 static __maybe_unused unsigned int get_last_capsule(void)
 {
 	u16 value16[11]; /* "CapsuleXXXX": non-null-terminated */
-	char value[11], *p;
+	char value[5];
 	efi_uintn_t size;
 	unsigned long index = 0xffff;
 	efi_status_t ret;
+	int i;

 	size = sizeof(value16);
 	ret = efi_get_variable_int(L"CapsuleLast", &efi_guid_capsule_report,
 				   NULL, &size, value16, NULL);
-	if (ret != EFI_SUCCESS || u16_strncmp(value16, L"Capsule", 7))
+	if (ret != EFI_SUCCESS || size != 22 ||
+	    u16_strncmp(value16, L"Capsule", 7))
 		goto err;
+	for (i = 0; i < 4; ++i) {
+		u16 c = value16[i + 7];

-	p = value;
-	utf16_utf8_strcpy(&p, value16);
-	strict_strtoul(&value[7], 16, &index);
+		if (!c)
+			goto err;
+		value[i] = c;
+	}
+	value[4] = 0;
+	if (strict_strtoul(value, 16, &index))
+		index = 0xffff;
 err:
 	return index;
 }
--
2.30.0