x86/hyperv: allocate the hypercall page with only read and execute bits

Patch series "fix a hyperv W^X violation and remove vmalloc_exec"

Dexuan reported a W^X violation due to the fact that the hyper hypercall
page due switching it to be allocated using vmalloc_exec.

The problem is that PAGE_KERNEL_EXEC as used by vmalloc_exec actually
sets writable permissions in the pte.  This series fixes the issue by
switching to the low-level __vmalloc_node_range interface that allows
specifing more detailed permissions instead.  It then also open codes
the other two callers and removes the somewhat confusing vmalloc_exec
interface.

Peter noted that the hyper hypercall page allocation also has another
long standing issue in that it shouldn't use the full vmalloc but just
the module space.  This issue is so far theoretical as the allocation is
done early in the boot process.  I plan to fix it with another bigger
series for 5.9.

This patch (of 3):

Avoid a W^X violation cause by the fact that PAGE_KERNEL_EXEC includes
the writable bit.

For this resurrect the removed PAGE_KERNEL_RX definition, but as
PAGE_KERNEL_ROX to match arm64 and powerpc.

Link: http://lkml.kernel.org/r/20200618064307.32739-2-hch@lst.de
Fixes: 78bb17f7 ("x86/hyperv: use vmalloc_exec for the hypercall page")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Dexuan Cui <decui@microsoft.com>
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/hyperv/hv_init.c

@@ -375,7 +375,9 @@ 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_exec(PAGE_SIZE);
+	hv_hypercall_pg = __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START,
+			VMALLOC_END, GFP_KERNEL, PAGE_KERNEL_ROX,
+			VM_FLUSH_RESET_PERMS, NUMA_NO_NODE, __func__);
 	if (hv_hypercall_pg == NULL) {
 		wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
 		goto remove_cpuhp_state;

arch/x86/include/asm/pgtable_types.h

@@ -194,6 +194,7 @@ enum page_cache_mode {
 #define _PAGE_TABLE_NOENC	 (__PP|__RW|_USR|___A|   0|___D|   0|   0)
 #define _PAGE_TABLE		 (__PP|__RW|_USR|___A|   0|___D|   0|   0| _ENC)
 #define __PAGE_KERNEL_RO	 (__PP|   0|   0|___A|__NX|___D|   0|___G)
+#define __PAGE_KERNEL_ROX	 (__PP|   0|   0|___A|   0|___D|   0|___G)
 #define __PAGE_KERNEL_NOCACHE	 (__PP|__RW|   0|___A|__NX|___D|   0|___G| __NC)
 #define __PAGE_KERNEL_VVAR	 (__PP|   0|_USR|___A|__NX|___D|   0|___G)
 #define __PAGE_KERNEL_LARGE	 (__PP|__RW|   0|___A|__NX|___D|_PSE|___G)
@@ -219,6 +220,7 @@ enum page_cache_mode {
 #define PAGE_KERNEL_RO		__pgprot_mask(__PAGE_KERNEL_RO         | _ENC)
 #define PAGE_KERNEL_EXEC	__pgprot_mask(__PAGE_KERNEL_EXEC       | _ENC)
 #define PAGE_KERNEL_EXEC_NOENC	__pgprot_mask(__PAGE_KERNEL_EXEC       |    0)
+#define PAGE_KERNEL_ROX		__pgprot_mask(__PAGE_KERNEL_ROX        | _ENC)
 #define PAGE_KERNEL_NOCACHE	__pgprot_mask(__PAGE_KERNEL_NOCACHE    | _ENC)
 #define PAGE_KERNEL_LARGE	__pgprot_mask(__PAGE_KERNEL_LARGE      | _ENC)
 #define PAGE_KERNEL_LARGE_EXEC	__pgprot_mask(__PAGE_KERNEL_LARGE_EXEC | _ENC)