[PATCH] x86/msr: Filter msr write by X86_IOC_WRMSR_REGS ioctl

[PATCH] x86/msr: Filter msr write by X86_IOC_WRMSR_REGS ioctl

[Misono Tomohiro]

commit a7e1f67ed29f ("x86/msr: Filter MSR writes") introduces a
module parameter to disable writing to msr device file (and add_taint()
upon writing). As msr register can be written by X86_IOC_WRMSR_REGS
ioctl too, they should be applied to the ioctl as well.

Fixes: a7e1f67ed29f ("x86/msr: Filter MSR writes")
Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
---
Hello,

I just noticed this when I read the code.
I'm not sure if anyone uses ioctl interface now, but I tested this
by resetting IA32_MPERF by X86_IOC_WRMSR_REGS ioctl.

Thanks,

 arch/x86/kernel/msr.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 8a67d1fa8dc5..d774618e75b8 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -182,6 +182,10 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 		err = security_locked_down(LOCKDOWN_MSR);
 		if (err)
 			break;
+		err = filter_write(regs[1]);
+		if (err)
+			return err;
+		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
 		err = wrmsr_safe_regs_on_cpu(cpu, regs);
 		if (err)
 			break;
-- 
2.26.2

Re: [PATCH] x86/msr: Filter msr write by X86_IOC_WRMSR_REGS ioctl

[Borislav Petkov]

On Wed, Jan 27, 2021 at 09:24:56PM +0900, Misono Tomohiro wrote:
> commit a7e1f67ed29f ("x86/msr: Filter MSR writes") introduces a
> module parameter to disable writing to msr device file (and add_taint()
> upon writing). As msr register can be written by X86_IOC_WRMSR_REGS
> ioctl too, they should be applied to the ioctl as well.

Blergh, I missed that. :-\

> Fixes: a7e1f67ed29f ("x86/msr: Filter MSR writes")
> Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
> ---
> Hello,
> 
> I just noticed this when I read the code.
> I'm not sure if anyone uses ioctl interface now, but I tested this
> by resetting IA32_MPERF by X86_IOC_WRMSR_REGS ioctl.

Yeah, I'm sure once this hits people's kernels, someone will complain.

In any case, good catch, thanks!

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

[tip: x86/misc] x86/MSR: Filter MSR writes through X86_IOC_WRMSR_REGS ioctl too

[tip-bot2 for Misono Tomohiro]

The following commit has been merged into the x86/misc branch of tip:

Commit-ID:     02a16aa13574c8526beadfc9ae8cc9b66315fa2d
Gitweb:        https://git.kernel.org/tip/02a16aa13574c8526beadfc9ae8cc9b66315fa2d
Author:        Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
AuthorDate:    Wed, 27 Jan 2021 21:24:56 +09:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 27 Jan 2021 19:06:47 +01:00

x86/MSR: Filter MSR writes through X86_IOC_WRMSR_REGS ioctl too

Commit

  a7e1f67ed29f ("x86/msr: Filter MSR writes")

introduced a module parameter to disable writing to the MSR device file
and tainted the kernel upon writing. As MSR registers can be written by
the X86_IOC_WRMSR_REGS ioctl too, the same filtering and tainting should
be applied to the ioctl as well.

 [ bp: Massage commit message and space out statements. ]

Fixes: a7e1f67ed29f ("x86/msr: Filter MSR writes")
Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210127122456.13939-1-misono.tomohiro@jp.fujitsu.com
---
 arch/x86/kernel/msr.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 8a67d1f..ed8ac6b 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -182,6 +182,13 @@ static long msr_ioctl(struct file *file, unsigned int ioc, unsigned long arg)
 		err = security_locked_down(LOCKDOWN_MSR);
 		if (err)
 			break;
+
+		err = filter_write(regs[1]);
+		if (err)
+			return err;
+
+		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+
 		err = wrmsr_safe_regs_on_cpu(cpu, regs);
 		if (err)
 			break;