The C standard doesn't always guarantee that struct and union padding
bits are zero initialized, even if the code initializes a struct.
For QEMU, this is potentially problematic, because we often have
structs that match data structures in guest memory, where we
initialize them and then bulk copy them into the guest. If the
compiler didn't zero init the whole of the memory containing the
struct, we could potentially leak random data from the host into the
guest via the padding bytes.
We already use -ftrivial-auto-var-init=zero, which will zero out
padding in many of these cases, but -fzero-init-padding-bits=all
closes some gaps, for example cases where we initialize a
variable with a struct initializer, and cases involving unions.
Follow the Linux kernel in using both options. Compare kernel
commit dce4aab8441 ("kbuild: Use -fzero-init-padding-bits=all").
This option exists in gcc-15 and above; it's not supported
by clang, but clang documents that it guarantees zero init
of these cases always:
https://clang.llvm.org/docs/LanguageExtensions.html#union-and-aggregate-initialization-in-c
Older gcc which don't have the option behave as if it were set.
(These options are passed through the cc.get_supported_arguments()
filter, so we don't need to do anything extra to avoid passing it to
a compiler that doesn't recognize it.)
Cc: [email protected]
Signed-off-by: Peter Maydell <[email protected]>
---
CC stable just as a precautionary thing; it's safe and we
might as well make sure the hardening options are set there.
---
meson.build | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/meson.build b/meson.build
index 5fbdc75a0f..d3df10eeef 100644
--- a/meson.build
+++ b/meson.build
@@ -684,6 +684,12 @@ hardening_flags = [
# it harder to take advantage of uninitialized stack
# data to drive exploits
'-ftrivial-auto-var-init=zero',
+ # Ensure GCC zero-initializes padding bits and trailing fields in
+ # unions. This avoids potentially leaking host data into the guest
+ # when we init a struct and copy it into guest memory. GCC 15 and
+ # clang don't have this, but they zero the padding and trailing
+ # portions of a union by default.
+ '-fzero-init-padding-bits=all',
]
# Zero out registers used during a function call
--
2.43.0
