Request for guidance on GRUB boot params init_on_free, page_poison

ArchEr9 · 2025-03-18 13:42:06

I was going through the GRUB2 parameters where I came across the following 4 parameters to harden page allocation in the kernel
1) page_poison: Page poisoning flag.
2) slub_debug: Slub poisoning flag.
3) init_on_free
4) init_on_alloc

After doing some research I found out that init_on_alloc and init_on_free were introduced in 2019 after some persistent badgering by Linux users. According to the documentation of both of these parameters

Both init_on_alloc=1 and init_on_free=1 guarantee that the allocator returns zeroed memory. The two exceptions are slab caches with constructors and SLAB_TYPESAFE_BY_RCU flag. Those are never zero-initialized to preserve their semantics.
....
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If either SLUB poisoning or page poisoning is enabled, we disable init_on_alloc and init_on_free so that initialization doesn't interfere with debugging.

From the documentation it appears that the choice is between using init_on_alloc/init_on_free or slub_debug/page_poison. Going by the documentation it is an either-or option. We cannot mix and match. So for example if we were to use slub_debug alone along with init_on_* then it would still disable the init_on_* parameters. The same thing happens if we use only page_poison along with the init_on_* parameters.

This had led to various distros, llike VoidLinux, Tails and others to prioritize init_on_free and init_on_alloc. On the other hand some other independent standards, as recent as 2022, that prioritize setting the page_poison parameter over init_on_* parameters. So guidance is not clear cut or uniform

I have the following queries on this

1) Does having all 4 of the above mentioned parameters in GRUB actually lead to init_on_alloc and init_on_free being ignored with only page_poison and slub_debug being used by the kernel?
2) What would be preferred mechanism? Going with init_on_alloc and init_on_free OR going with slub_debug and page_poison?
3) Are there some scenarios where page_poison/slub_debug would be preferred over init_on_free/init_on_alloc?
4) According to documentation the parameters init_on_* do not touch slab caches. Is there some way in which slab caches, when they are released, can be poisoned too or values zeroed out, especially if we are using init_on_* parameters?

ArchEr9 · 2025-04-10 05:58:21

Any help guys? Is this the correct category for this topic? Should I be moving this to say Kernel & Hardware category or to some other category?

Lone_Wolf · 2025-04-10 08:53:29

You did notice (like from where you found the documentation) those are not grub parameters but kernel parameters ?
Please change the title accordingly .

Read https://wiki.archlinux.org/title/Securi … _hardening .

If you feel moving the thread will help, use the report button (bottom right) to request it.

Arch Linux

#1 2025-03-18 13:42:06

Request for guidance on GRUB boot params init_on_free, page_poison

#2 2025-04-10 05:58:21

Re: Request for guidance on GRUB boot params init_on_free, page_poison

#3 2025-04-10 08:53:29

Re: Request for guidance on GRUB boot params init_on_free, page_poison

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