linux-kernel - Re: hv_hypercall

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Message-ID: <87v9mblpq6.fsf@vitty.brq.redhat.com>
Date:   Tue, 07 Apr 2020 09:28:01 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     x86@...nel.org, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: hv_hypercall_pg page permissios

Christoph Hellwig <hch@....de> writes:

> Hi all,
>
> The x86 Hyper-V hypercall page (hv_hypercall_pg) is the only allocation
> in the kernel using __vmalloc with exectutable persmissions, and the
> only user of PAGE_KERNEL_RX.  Is there any good reason it needs to
> be readable?  Otherwise we could use vmalloc_exec and kill off
> PAGE_KERNEL_RX.  Note that before 372b1e91343e6 ("drivers: hv: Turn off
> write permission on the hypercall page") it was even mapped writable..

[There is nothing secret in the hypercall page, by reading it you can
figure out if you're running on Intel or AMD (VMCALL/VMMCALL) but it's
likely not the only possible way :-)]

I see no reason for hv_hypercall_pg to remain readable. I just
smoke-tested

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 7581cab74acb..17845db67fe2 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -382,7 +382,7 @@ void __init hyperv_init(void)
        guest_id = generate_guest_id(0, LINUX_VERSION_CODE, 0);
        wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id);
 
-       hv_hypercall_pg  = __vmalloc(PAGE_SIZE, GFP_KERNEL, PAGE_KERNEL_RX);
+       hv_hypercall_pg  = vmalloc_exec(PAGE_SIZE);
        if (hv_hypercall_pg == NULL) {
                wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
                goto remove_cpuhp_state;

on a Hyper-V 2016 guest and nothing broke, feel free to go ahead and
kill PAGE_KERNEL_RX.

-- 
Vitaly