[PATCH 5.4 069/154] ARM: 9350/1: fault: Implement copy_from_kernel_nofault_allowed()

Greg Kroah-Hartman
2025-04-08T12:05:37Z

From: Kees Cook <keescook@chromium.org>

commit 169f9102f9198b04afffa6164372a4ba4070f412 upstream.

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.

Closes: https://lava.sirena.org.uk/scheduler/job/497571
Link: https://lore.kernel.org/all/202401181125.D48DCB4C@keescook/ [1]

Reported-by: Mark Brown <broonie@kernel.org>
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Cc: Wang Kefeng <wangkefeng.wang@huawei.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm/mm/fault.c | 7 +++++++
1 file changed, 7 insertions(+)

--- 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

/*