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SubjectRe: hv_hypercall_pg page permissios
Date
Christoph Hellwig <hch@lst.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

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