Re: [PATCH] x86/msr: Block writes to certain MSRs unconditionally

[Andy Lutomirski]

> On Apr 10, 2021, at 5:11 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> 
> From: Borislav Petkov <bp@xxxxxxx>
> Date: Sat, 10 Apr 2021 14:08:13 +0200
> 
> There are a bunch of MSRs which luserspace has no business poking at,
> whatsoever. Add a ban list and put the TSC-related MSRs in there. Issue
> a big juicy splat to catch offenders.

Ack!

Can you add STAR, CSTAR, LSTAR, SYSENTER*, SYSCALL*, and EFER please? 

> 
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> ---
> arch/x86/kernel/msr.c | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
> 
> diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
> index ed8ac6bcbafb..574bd2c6d4f8 100644
> --- a/arch/x86/kernel/msr.c
> +++ b/arch/x86/kernel/msr.c
> @@ -78,6 +78,13 @@ static ssize_t msr_read(struct file *file, char __user *buf,
>    return bytes ? bytes : err;
> }
> 
> +static const u32 msr_ban_list[] = {
> +    MSR_IA32_TSC,
> +    MSR_TSC_AUX,
> +    MSR_IA32_TSC_ADJUST,
> +    MSR_IA32_TSC_DEADLINE,
> +};
> +
> static int filter_write(u32 reg)
> {
>    /*
> @@ -89,6 +96,16 @@ static int filter_write(u32 reg)
>     * avoid saturating the ring buffer.
>     */
>    static DEFINE_RATELIMIT_STATE(fw_rs, 30 * HZ, 1);
> +    int i;
> +
> +    for (i = 0; i < ARRAY_SIZE(msr_ban_list); i++) {
> +        if (msr_ban_list[i] != reg)
> +            continue;
> +
> +        WARN_ONCE(1, "Blocked write to MSR 0x%x\n", reg);
> +
> +        return -EINVAL;
> +    }
> 
>    switch (allow_writes) {
>    case MSR_WRITES_ON:  return 0;
> -- 
> 2.29.2
> 
> 
> -- 
> Regards/Gruss,
>    Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette