[PATCH 3.16 79/87] cpu/speculation: Warn on unsupported mitigations= parameter

[Ben Hutchings]

3.16.75-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>

commit 1bf72720281770162c87990697eae1ba2f1d917a upstream.

Currently, if the user specifies an unsupported mitigation strategy on the
kernel command line, it will be ignored silently.  The code will fall back
to the default strategy, possibly leaving the system more vulnerable than
expected.

This may happen due to e.g. a simple typo, or, for a stable kernel release,
because not all mitigation strategies have been backported.

Inform the user by printing a message.

Fixes: 98af8452945c5565 ("cpu/speculation: Add 'mitigations=' cmdline option")
Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Jiri Kosina <jkosina@xxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190516070935.22546-1-geert@xxxxxxxxxxxxxx
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/cpu.c | 3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -804,6 +804,9 @@ static int __init mitigations_parse_cmdl
 		cpu_mitigations = CPU_MITIGATIONS_OFF;
 	else if (!strcmp(arg, "auto"))
 		cpu_mitigations = CPU_MITIGATIONS_AUTO;
+	else
+		pr_crit("Unsupported mitigations=%s, system may still be vulnerable\n",
+			arg);
 
 	return 0;
 }