Re: Bad virt_to_phys since commit 54c7a8916a887f35

From: Steven Price
Date: Thu May 16 2019 - 10:07:33 EST

On 16/05/2019 14:41, Mark Rutland wrote:
> On Thu, May 16, 2019 at 02:38:20PM +0100, Mark Rutland wrote:
>> Hi,
>> Since commit:
>> 54c7a8916a887f35 ("initramfs: free initrd memory if opening /initrd.image fails")
> Ugh, I dropped a paragarph here.
> Since that commit, I'm seeing a boot-time splat on arm64 when using
> CONFIG_DEBUG_VIRTUAL. I'm running an arm64 syzkaller instance, and this
> kills the VM, preventing further testing, which is unfortunate.
> Mark.
>> IIUC prior to that commit, we'd only attempt to free an intird if we had
>> one, whereas now we do so unconditionally. AFAICT, in this case
>> initrd_start has not been initialized (I'm not using an initrd or
>> initramfs on my system), so we end up trying virt_to_phys() on a bogus
>> VA in free_initrd_mem().
>> Any ideas on the right way to fix this?

Your analysis looks right to me. In my review I'd managed to spot the
change in behaviour when CONFIG_INITRAMFS_FORCE is set (the initrd is
freed), but I'd overlooked what happens if initrd_start == 0 (the
non-existent initrd is attempted to be freed).

I suspect the following is sufficient to fix the problem:

diff --git a/init/initramfs.c b/init/initramfs.c
index 435a428c2af1..178130fd61c2 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -669,7 +669,7 @@ static int __init populate_rootfs(void)
* If the initrd region is overlapped with crashkernel reserved region,
* free only memory that is not part of crashkernel region.
- if (!do_retain_initrd && !kexec_free_initrd())
+ if (!do_retain_initrd && initrd_start && !kexec_free_initrd())
free_initrd_mem(initrd_start, initrd_end);
initrd_start = 0;
initrd_end = 0;