Security things in Linux v5.3


In the continuing work to remove “uninitialized” variables from the kernel, Alexander Potapenko added new “init_on_alloc” and “init_on_free” boot parameters (with associated Kconfig defaults) to perform zeroing of heap memory either at allocation time (i.e. all kmalloc()s effectively become kzalloc()s), at free time (i.e. all kfree()s effectively become kzfree()s), or both. The performance impact of the former under most workloads appears to be under 1%, if it’s measurable at all. The “init_on_free” option, however, is more costly but adds the benefit of reducing the lifetime of heap contents after they have been freed (which might be useful for some use-after-free attacks or side-channel attacks). Everyone should enable CONFIG_INIT_ON_ALLOC_DEFAULT_ON=1 (or boot with “init_on_alloc=1“), and the more paranoid system builders should add CONFIG_INIT_ON_FREE_DEFAULT_ON=1 (or “init_on_free=1” at boot). As workloads are found that cause performance concerns, tweaks to the initialization coverage can be added.
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