[PATCH v2] 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.

Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by:  Michael Zhivich <mzhivich@xxxxxxxxxx>
Fixes: 7a0395f6607a5 ("x86/bugs: Add a Transient Scheduler Attacks mitigation")
---

Changes in v2:
- Rework patch per feedback
- Add Cc: 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 efd42ee9d1cc..289ff197b1b3 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -378,6 +378,8 @@ 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 (cpu_has(c, X86_FEATURE_ZEN3) ||
 	    cpu_has(c, X86_FEATURE_ZEN4)) {
-- 
2.34.1