[PATCH 1/1] x86: sanitize argument of clearcpuid command-line option

[Lukasz Odzioba <lukasz.odzioba@intel.com>]

A negative number can be specified in the cmdline which will be used as
setup_clear_cpu_cap() argument. With that we can clear/set some bit in
memory predceeding boot_cpu_data/cpu_caps_cleared which may cause kernel
to misbehave. This patch adds lower bound check to setup_disablecpuid().

Fixes: ac72e7888a61 ("x86: add generic clearcpuid=... option")

Signed-off-by: Lukasz Odzioba <lukasz.odzioba@intel.com>
---
As an example let's change definition of one_hundred variable:
ffffffff81c4eeec d one_hundred
ffffffff81d69720 D boot_cpu_data (0x14 is x86_capability offset)

8*(0xffffffff81d69734-0xffffffff81c4eeec) => 9257536 -2 because we
want to clear the second bit. With clearcpuid=-9257534 we change the
definition of one_hundread to 96 which is used among other things
as sysfs' max value for swappiness, so we can check the effect like so:
# echo 96 >  /proc/sys/vm/swappiness
# echo 97 >  /proc/sys/vm/swappiness
-bash: echo: write error: Invalid argument
---
 arch/x86/kernel/cpu/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index dc1697c..9bab7a8 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1221,7 +1221,7 @@ static __init int setup_disablecpuid(char *arg)
 {
 	int bit;
 
-	if (get_option(&arg, &bit) && bit < NCAPINTS*32)
+	if (get_option(&arg, &bit) && bit >= 0 && bit < NCAPINTS * 32)
 		setup_clear_cpu_cap(bit);
 	else
 		return 0;
-- 
1.8.3.1