linux-kernel - Re: [PATCH] x86/microcode: Refactor Intel microcode loading

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Date:   Thu, 4 Apr 2019 18:27:53 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jann Horn <jannh@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH] x86/microcode: Refactor Intel microcode loading

On Thu, Apr 04, 2019 at 01:11:28PM +0200, Jann Horn wrote:
> This changes generic_load_microcode() to use the iov_iter API instead of
> an open-coded version. This allows us to avoid explicitly casting between
> user and kernel pointers.
> 
> Because the iov_iter API makes it hard to read the same location twice, as
> a side effect, this also fixes a double-read of the microcode header (which
> could e.g. lead to out-of-bounds reads in microcode_sanity_check()).
> Not that it matters much, only root can do this anyway...
> 
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
> I have tested that with this patch applied, microcode loading still works
> both via "iucode-tool -k" and via
> /sys/devices/system/cpu/microcode/reload.

Yeah, this cannot have worked because I think I broke it recently and
you'd need this:

---
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 5260185cbf7b..8a4a7823451a 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -418,8 +418,9 @@ static int do_microcode_update(const void __user *buf, size_t size)
                if (ustate == UCODE_ERROR) {
                        error = -1;
                        break;
-               } else if (ustate == UCODE_OK)
+               } else if (ustate == UCODE_NEW) {
                        apply_microcode_on_target(cpu);
+               }
        }
 
        return error;
---

Regardless, I'll take care of it. Thanks a lot for doing this cleanup,
it looks really cool and nicely clean - exactly how I envisioned it. :-)

I'll test it more later with the above fix and apply it.

Thx.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.