[PATCH v2 16/19] KVM: arm64: Add build-time sanity checks for flags

Marc Zyngier
2022-06-10T09:35:48Z

Flags are great, but flags can also be dangerous: it is easy
to encode a flag that is bigger than its container (unless the
container is a u64), and it is easy to construct a flag value
that doesn't fit in the mask that is associated with it.

Add a couple of build-time sanity checks that ensure we catch
these two cases.

Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/include/asm/kvm_host.h | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index ffbeb5f5692e..6a37018f40b7 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -433,8 +433,20 @@ struct kvm_vcpu_arch {
#define __unpack_flag(_set, _f, _m) _f
#define unpack_vcpu_flag(...) __unpack_flag(__VA_ARGS__)

+#define __build_check_flag(v, flagset, f, m) \
+ do { \
+ typeof(v->arch.flagset) *_fset; \
+ \
+ /* Check that the flags fit in the mask */ \
+ BUILD_BUG_ON(HWEIGHT(m) != HWEIGHT((f) | (m))); \
+ /* Check that the flags fit in the type */ \
+ BUILD_BUG_ON((sizeof(*_fset) * 8) <= __fls(m)); \
+ } while (0)
+
#define __vcpu_get_flag(v, flagset, f, m) \
({ \
+ __build_check_flag(v, flagset, f, m); \
+ \
v->arch.flagset & (m); \
})

@@ -442,6 +454,8 @@ struct kvm_vcpu_arch {
do { \
typeof(v->arch.flagset) *fset; \
\
+ __build_check_flag(v, flagset, f, m); \
+ \
fset = &v->arch.flagset; \
if (HWEIGHT(m) > 1) \
*fset &= ~(m); \
@@ -452,6 +466,8 @@ struct kvm_vcpu_arch {
do { \
typeof(v->arch.flagset) *fset; \
\
+ __build_check_flag(v, flagset, f, m); \
+ \
fset = &v->arch.flagset; \
*fset &= ~(m); \
} while (0)
--
2.34.1