On 18.06.24 12:21, Ard Biesheuvel wrote:

On Mon, 17 Jun 2024 at 23:01, Alexander Graf [off-list ref] wrote:

quoted

On 17.06.24 22:40, Steven Rostedt wrote:

quoted

On Mon, 17 Jun 2024 09:07:29 +0200
Alexander Graf[off-list ref]  wrote:

quoted

Hey Steve,


I believe we're talking about 2 different things :). Let me rephrase a
bit and make a concrete example.

Imagine you have passed the "reserve_mem=12M:4096:trace" kernel command
line option. The kernel now comes up and allocates a random chunk of
memory that - by (admittedly good) chance - may be at the same physical
location as before. Let's assume it deemed 0x1000000 as a good offset.

Note, it's not random. It picks from the top of available memory every
time. But things can mess with it (see below).

quoted

Let's now assume you're running on a UEFI system. There, you always have
non-volatile storage available to you even in the pre-boot phase. That
means the kernel could create a UEFI variable that says "12M:4096:trace
-> 0x1000000". The pre-boot phase takes all these UEFI variables and
marks them as reserved. When you finally reach your command line parsing
logic for reserve_mem=, you can flip all reservations that were not on
the command line back to normal memory.

That way you have pretty much guaranteed persistent memory regions, even
with KASLR changing your memory layout across boots.

The nice thing is that the above is an extension of what you've already
built: Systems with UEFI simply get better guarantees that their regions
persist.

This could be an added feature, but it is very architecture specific,
and would likely need architecture specific updates.

It definitely would be an added feature, yes. But one that allows you to
ensure persistence a lot more safely :).

quoted

quoted

quoted

quoted

quoted

Requirement:

Need a way to reserve memory that will be at a consistent location for
every boot, if the kernel and system are the same. Does not need to work
if rebooting to a different kernel, or if the system can change the
memory layout between boots.

The reserved memory can not be an hard coded address, as the same kernel /
command line needs to run on several different machines. The picked memory
reservation just needs to be the same for a given machine, but may be

With KASLR is enabled, doesn't this approach break too often to be
reliable enough for the data you want to extract?

Picking up the idea above, with a persistent variable we could even make
KASLR avoid that reserved pstore region in its search for a viable KASLR
offset.

I think I was hit by it once in all my testing. For our use case, the
few times it fails to map is not going to affect what we need this for
at all.

Once is pretty good. Do you know why? Also once out of how many runs? Is
the randomness source not as random as it should be or are the number of
bits for KASLR maybe so few on your target architecture that the odds of
hitting anything become low? Do these same constraints hold true outside
of your testing environment?

So I just ran it a hundred times in a loop. I added a patch to print
the location of "_text". The loop was this:

    for i in `seq 100`; do
          ssh root@debiantesting-x86-64 "dmesg | grep -e 'text starts' -e 'mapped boot'  >> text; grub-reboot '1>0'; sleep 0.5; reboot"
          sleep 25
    done

It searches dmesg for my added printk as well as the print of were the
ring buffer was loaded in physical memory.

It takes about 15 seconds to reboot, so I waited 25. The results are
attached. I found that it was consistent 76 times, which means 1 out of
4 it's not. Funny enough, it broke whenever it loaded the kernel below
0x100000000. And then it would be off by a little.

It was consistently at:

    0x27d000000

And when it failed, it was at 0x27ce00000.

Note, when I used the e820 tables to do this, I never saw a failure. My
assumption is that when it is below 0x100000000, something else gets
allocated causing this to get pushed down.

Thinking about it again: What if you run the allocation super early (see
arch/x86/boot/compressed/kaslr.c:handle_mem_options())?

That code is not used by EFI boot anymore.

In general, I would recommend (and have recommended) against these
kinds of hacks in mainline, because -especially on x86- there is
always someone that turns up with some kind of convoluted use case
that gets broken if we try to change anything in the boot code.

I spent considerable time over the past year making the EFI/x86 boot
code compatible with the new MS imposed requirements on PC boot
firmware (related to secure boot and NX restrictions on memory
mappings). This involved some radical refactoring of the boot
sequence, including the KASLR logic. Adding fragile code there that
will result in regressions observable to end users when it gets broken
is really not what I'd like to see.

So I would personally prefer for this code not to go in at all. But if
it does go in (and Steven has already agreed to this), it needs a
giant disclaimer that it is best effort and may get broken
inadvertently by changes that may seem unrelated.

Alright, happy to rest my case about making it more reliable for now 
then :).

IMHO the big fat disclaimer should be in the argument name. 
"reserve_mem" to me sounds like it actually guarantees a reservation - 
which it doesn't. Can we name it more along the lines of "debug" (to 
indicate it's not for production data) or "phoenix" (usually gets reborn 
out of ashes, but you can never know for sure): "debug_mem", / 
"phoenix_mem"?

quoted

If you stick to
allocating only from top, you're effectively kernel version independent
for your allocations because none of the kernel code ran yet and
definitely KASLR independent because you're running deterministically
before KASLR even gets allocated.

Allocating top down under EFI is almost guaranteed to result in
problems, because that is how the EFI page allocator works as well.
This means that allocations will move around depending on, e.g.,
whether some USB stick was inserted on the first boot and removed on
the second, or whether your external display was on or off.

I believe most UEFI implementations only allocate top down in the lower 
32bits. But yes, it's fragile, I hear you. Let's embrace the flaky 
nature of the beast then :).


Alex




Amazon Web Services Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B
Sitz: Berlin
Ust-ID: DE 365 538 597