Messages in this thread | | | Date | Wed, 24 Jun 2020 22:51:40 -0700 | From | Sean Christopherson <> | Subject | Re: [PATCH -v2.1] x86/msr: Filter MSR writes |
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On Mon, Jun 15, 2020 at 08:38:37AM +0200, Borislav Petkov wrote: > Here's an improved v2 with sparse warnings fixed: > > --- > Disable writing to MSRs from userspace by default. Writes can still be > allowed by supplying the allow_writes=1 module parameter and the kernel > will be tainted so that it shows in oopses. > > Having unfettered access to all MSRs on a system is and has always been > a disaster waiting to happen. Think performance counter MSRs, MSRs with > sticky or locked bits, MSRs making major system changes like loading > microcode, MTRRs, PAT configuration, TSC counter, security mitigations > MSRs, you name it. > > This also destroys all the kernel's caching of MSR values for > performance, as the recent case with MSR_AMD64_LS_CFG showed. > > Another example is writing MSRs by mistake by simply typing the wrong > MSR address. System freezes have been experienced that way. > > In general, poking at MSRs under the kernel's feet is a bad bad idea. > > So disable poking directly at the MSRs by default. If userspace still > wants to do that, then proper interfaces should be defined which > are under the kernel's control and accesses to those MSRs can be > synchronized and sanitized properly. > > Changelog: > - taint before WRMSR, all > - make param 0600, Sean. > - do not deny but log writes by default, Linus. > > [ Fix sparse warnings ] > Reported-by: kernel test robot <lkp@intel.com> > Signed-off-by: Borislav Petkov <bp@suse.de>
A few non-functional nits below.
Tested-by: Sean Christopherson <sean.j.christopherson@intel.com>
> --- > arch/x86/kernel/msr.c | 69 +++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 69 insertions(+) > > diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c > index 1547be359d7f..576c43e39247 100644 > --- a/arch/x86/kernel/msr.c > +++ b/arch/x86/kernel/msr.c > @@ -42,6 +42,14 @@ > static struct class *msr_class; > static enum cpuhp_state cpuhp_msr_state; > > +enum allow_write_msrs { > + MSR_WRITES_ON, > + MSR_WRITES_OFF, > + MSR_WRITES_DEFAULT, > +}; > + > +static enum allow_write_msrs allow_writes = MSR_WRITES_DEFAULT; > + > static ssize_t msr_read(struct file *file, char __user *buf, > size_t count, loff_t *ppos) > { > @@ -70,6 +78,24 @@ static ssize_t msr_read(struct file *file, char __user *buf, > return bytes ? bytes : err; > } > > +static int filter_write(u32 reg) > +{ > + switch (allow_writes) { > + case MSR_WRITES_ON: return 0; break; > + case MSR_WRITES_OFF: return -EPERM; break;
The breaks after the returns are unnecessary.
> + default: break; > + } > + > + if (reg == MSR_IA32_ENERGY_PERF_BIAS) > + return 0; > + > + pr_err_ratelimited("Write to unrecognized MSR 0x%x by %s\n" > + "Please report to x86@kernel.org\n", > + reg, current->comm);
Maybe s/unrecognized/unauthorized? Unrecognized implies the kernel doesn't know anything about the MSR being written, which may not hold true.
> + return 0; > +} > + > static ssize_t msr_write(struct file *file, const char __user *buf, > size_t count, loff_t *ppos) > { > @@ -84,6 +110,10 @@ static ssize_t msr_write(struct file *file, const char __user *buf, > if (err) > return err; > > + err = filter_write(reg); > + if (err) > + return err; > + > if (count % 8) > return -EINVAL; /* Invalid chunk size */ > > @@ -92,9 +122,13 @@ static ssize_t msr_write(struct file *file, const char __user *buf, > err = -EFAULT; > break; > } > + > + add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK); > + > err = wrmsr_safe_on_cpu(cpu, reg, data[0], data[1]); > if (err) > break; > +
Random leftover whitespace change.
> tmp += 2; > bytes += 8; > }
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