[PATCH] ARM: fault: Implement copy_from_kernel_nofault_allowed()

[Kees Cook]

Under PAN emulation when dumping backtraces from things like the
LKDTM EXEC_USERSPACE test[1], a double fault (which would hang a CPU)
would happen because of dump_instr() attempting to read a userspace
address. Make sure copy_from_kernel_nofault() does not attempt this
any more.

Reported-by: Mark Brown <broonie at kernel.org>
Link: https://lore.kernel.org/all/202401181125.D48DCB4C@keescook/ [1]
Suggested-by: "Russell King (Oracle)" <linux at armlinux.org.uk>
Cc: Russell King <linux at armlinux.org.uk>
Cc: Ard Biesheuvel <ardb at kernel.org>
Cc: Wang Kefeng <wangkefeng.wang at huawei.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Ben Hutchings <ben at decadent.org.uk>
Cc: linux-arm-kernel at lists.infradead.org
Signed-off-by: Kees Cook <keescook at chromium.org>
---
 arch/arm/mm/fault.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index e804432e905e..bc5b959b6f90 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -25,6 +25,13 @@
 
 #include "fault.h"
 
+bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
+{
+	unsigned long addr = (unsigned long)unsafe_src;
+
+	return addr >= TASK_SIZE && ULONG_MAX - addr >= size;
+}
+
 #ifdef CONFIG_MMU
 
 /*
-- 
2.34.1