[PATCH v2] efi/libstub: Avoid CopyMem/SetMem EFI services after ExitBootServices

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

From: Ard Biesheuvel <ardb@kernel.org>

Given that memset/memcpy are intrinsics, the compiler might insert calls
to these routines unpredictably, including in code that executes after
ExitBootServices(). In this case, the respective boot services are no
longer accessible, and calling them will cause a crash.

So fall back to a bytewise copy/store if this happens to occur.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
v2: preserve memmove() semantics

 drivers/firmware/efi/libstub/intrinsics.c | 26 ++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/drivers/firmware/efi/libstub/intrinsics.c b/drivers/firmware/efi/libstub/intrinsics.c
index 965e734f6f98..418cd2e6dccc 100644
--- a/drivers/firmware/efi/libstub/intrinsics.c
+++ b/drivers/firmware/efi/libstub/intrinsics.c
@@ -15,8 +15,31 @@ void *__memmove(void *__dest, const void *__src, size_t count) __alias(memmove);
 void *__memset(void *s, int c, size_t count) __alias(memset);
 #endif
 
+static void *efistub_memmove(u8 *dst, const u8 *src, size_t len)
+{
+	if (src > dst || dst >= (src + len))
+		for (size_t i = 0; i < len; i++)
+			dst[i] = src[i];
+	else
+		for (ssize_t i = len - 1; i >= 0; i--)
+			dst[i] = src[i];
+
+	return dst;
+}
+
+static void *efistub_memset(void *dst, int c, size_t len)
+{
+	for (u8 *d = dst; len--; d++)
+		*d = c;
+
+	return dst;
+}
+
 void *memcpy(void *dst, const void *src, size_t len)
 {
+	if (efi_table_attr(efi_system_table, boottime) == NULL)
+		return efistub_memmove(dst, src, len);
+
 	efi_bs_call(copy_mem, dst, src, len);
 	return dst;
 }
@@ -25,6 +48,9 @@ extern void *memmove(void *dst, const void *src, size_t len) __alias(memcpy);
 
 void *memset(void *dst, int c, size_t len)
 {
+	if (efi_table_attr(efi_system_table, boottime) == NULL)
+		return efistub_memset(dst, c, len);
+
 	efi_bs_call(set_mem, dst, len, c & U8_MAX);
 	return dst;
 }
-- 
2.48.1.658.g4767266eb4-goog