[PATCH 30/82] s390/kexec_file: Refactor intentional wrap-around calculation

[Kees Cook]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded unsigned wrap-around addition test to use
check_add_overflow(), retaining the result for later usage (which removes
the redundant open-coded addition). This paves the way to enabling the
unsigned wrap-around sanitizer[2] in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Cc: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
Cc: Sven Schnelle <svens@xxxxxxxxxxxxx>
Cc: Nico Boehr <nrb@xxxxxxxxxxxxx>
Cc: Philipp Rudo <prudo@xxxxxxxxxx>
Cc: Baoquan He <bhe@xxxxxxxxxx>
Cc: Tao Liu <ltao@xxxxxxxxxx>
Cc: Alexander Egorenkov <egorenar@xxxxxxxxxxxxx>
Cc: linux-s390@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/s390/include/asm/stacktrace.h    | 6 ++++--
 arch/s390/kernel/machine_kexec_file.c | 5 +++--
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/s390/include/asm/stacktrace.h b/arch/s390/include/asm/stacktrace.h
index 31ec4f545e03..3ce08d32a8ad 100644
--- a/arch/s390/include/asm/stacktrace.h
+++ b/arch/s390/include/asm/stacktrace.h
@@ -34,11 +34,13 @@ int get_stack_info(unsigned long sp, struct task_struct *task,
 static inline bool on_stack(struct stack_info *info,
 			    unsigned long addr, size_t len)
 {
+	unsigned long sum;
+
 	if (info->type == STACK_TYPE_UNKNOWN)
 		return false;
-	if (addr + len < addr)
+	if (check_add_overflow(addr, len, &sum))
 		return false;
-	return addr >= info->begin && addr + len <= info->end;
+	return addr >= info->begin && sum <= info->end;
 }
 
 /*
diff --git a/arch/s390/kernel/machine_kexec_file.c b/arch/s390/kernel/machine_kexec_file.c
index 8d207b82d9fe..e5e925423061 100644
--- a/arch/s390/kernel/machine_kexec_file.c
+++ b/arch/s390/kernel/machine_kexec_file.c
@@ -238,6 +238,7 @@ void *kexec_file_add_components(struct kimage *image,
 	unsigned long max_command_line_size = LEGACY_COMMAND_LINE_SIZE;
 	struct s390_load_data data = {0};
 	unsigned long minsize;
+	unsigned long sum;
 	int ret;
 
 	data.report = ipl_report_init(&ipl_block);
@@ -256,10 +257,10 @@ void *kexec_file_add_components(struct kimage *image,
 	if (data.parm->max_command_line_size)
 		max_command_line_size = data.parm->max_command_line_size;
 
-	if (minsize + max_command_line_size < minsize)
+	if (check_add_overflow(minsize, max_command_line_size, &sum))
 		goto out;
 
-	if (image->kernel_buf_len < minsize + max_command_line_size)
+	if (image->kernel_buf_len < sum)
 		goto out;
 
 	if (image->cmdline_buf_len >= max_command_line_size)
-- 
2.34.1