On Sun, Jan 10, 2021 at 02:21:05PM +0000, Alexander Lobakin wrote:

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@vger.kernel.org # 4.7+ Signed-off-by: Alexander Lobakin alobakin@pm.me

Apologies for not being familiar enough with the issue to give a review but I did reproduce the hang that the commit mentions with malta_kvm_guest_defconfig + CONFIG_RELOCATABLE=y + CONFIG_RANDOMIZE_BASE=y and this patch does resolve it so:

Tested-by: Nathan Chancellor natechancellor@gmail.com

---

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

diff --git a/arch/mips/kernel/relocate.c b/arch/mips/kernel/relocate.c index 47aeb3350a76..0e365b7c742d 100644 --- a/arch/mips/kernel/relocate.c +++ b/arch/mips/kernel/relocate.c @@ -187,8 +187,14 @@ static int __init relocate_exception_table(long offset) 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. */ -- 2.30.0