[PATCH v3 6.6] x86/bugs: Fix use of possibly uninit value in amd_check_tsa_microcode()

[Michael Zhivich]

For kernels compiled with CONFIG_INIT_STACK_NONE=y, the value of __reserved
field in zen_patch_rev union on the stack may be garbage.  If so, it will
prevent correct microcode check when consulting p.ucode_rev, resulting in
incorrect mitigation selection.

This is a stable-only fix.

Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Michael Zhivich <mzhivich@xxxxxxxxxx>
Fixes: 78192f511f40 ("x86/bugs: Add a Transient Scheduler Attacks mitigation")
---

Changes in v3:
- separate "fixes" tag for each stable

 arch/x86/kernel/cpu/amd.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index e67d7603449b..bf07b2c5418a 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -599,6 +599,8 @@ static bool amd_check_tsa_microcode(void)
 	p.model		= c->x86_model;
 	p.ext_model	= c->x86_model >> 4;
 	p.stepping	= c->x86_stepping;
+	/* reserved bits are expected to be 0 in test below */
+	p.__reserved	= 0;
 
 	if (c->x86 == 0x19) {
 		switch (p.ucode_rev >> 8) {
-- 
2.34.1