Messages in this thread Patch in this message | | | From | Vitaly Kuznetsov <> | Subject | Re: hv_hypercall_pg page permissios | Date | Tue, 07 Apr 2020 09:28:01 +0200 |
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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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