[PATCH 4.19 06/43] MIPS: relocatable: fix possible boot hangup with KASLR enabled

From: Greg Kroah-Hartman
Date: Mon Jan 18 2021 - 14:56:24 EST

From: Alexander Lobakin <alobakin@xxxxx>

commit 69e976831cd53f9ba304fd20305b2025ecc78eab upstream.

LLVM-built Linux triggered a boot hangup with KASLR enabled.

arch/mips/kernel/relocate.c:get_random_boot() uses linux_banner,
which is a string constant, as a random seed, but accesses it
as an array of unsigned long (in rotate_xor()).
When the address of linux_banner is not aligned to sizeof(long),
such access emits unaligned access exception and hangs the kernel.

Use PTR_ALIGN() to align input address to sizeof(long) and also
align down the input length to prevent possible access-beyond-end.

Fixes: 405bc8fd12f5 ("MIPS: Kernel: Implement KASLR using CONFIG_RELOCATABLE")
Cc: stable@xxxxxxxxxxxxxxx # 4.7+
Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

arch/mips/kernel/relocate.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

--- a/arch/mips/kernel/relocate.c
+++ b/arch/mips/kernel/relocate.c
@@ -187,8 +187,14 @@ static int __init relocate_exception_tab
static inline __init unsigned long rotate_xor(unsigned long hash,
const void *area, size_t size)
- size_t i;
- unsigned long *ptr = (unsigned long *)area;
+ const typeof(hash) *ptr = PTR_ALIGN(area, sizeof(hash));
+ size_t diff, i;
+ diff = (void *)ptr - area;
+ if (unlikely(size < diff + sizeof(hash)))
+ return hash;
+ size = ALIGN_DOWN(size - diff, sizeof(hash));

for (i = 0; i < size / sizeof(hash); i++) {
/* Rotate by odd number of bits and XOR. */