arm64: compat: Poison the compat sigpage

Commit 9c698bff ("ARM: ensure the signal page contains defined contents")
poisoned the unused portions of the signal page for 32-bit Arm.

Implement the same poisoning for the compat signal page on arm64 rather
than using __GFP_ZERO.
Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210318170738.7756-6-will@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/vdso.c

@@ -318,17 +318,20 @@ static int aarch32_alloc_kuser_vdso_page(void)
 	return 0;
 }
 
+#define COMPAT_SIGPAGE_POISON_WORD	0xe7fddef1
 static int aarch32_alloc_sigpage(void)
 {
 	extern char __aarch32_sigret_code_start[], __aarch32_sigret_code_end[];
 	int sigret_sz = __aarch32_sigret_code_end - __aarch32_sigret_code_start;
-	unsigned long sigpage;
+	__le32 poison = cpu_to_le32(COMPAT_SIGPAGE_POISON_WORD);
+	void *sigpage;
 
-	sigpage = get_zeroed_page(GFP_KERNEL);
+	sigpage = (void *)__get_free_page(GFP_KERNEL);
 	if (!sigpage)
 		return -ENOMEM;
 
-	memcpy((void *)sigpage, __aarch32_sigret_code_start, sigret_sz);
+	memset32(sigpage, (__force u32)poison, PAGE_SIZE / sizeof(poison));
+	memcpy(sigpage, __aarch32_sigret_code_start, sigret_sz);
 	aarch32_sig_page = virt_to_page(sigpage);
 	return 0;
 }