Re: [PATCH v9] mm: Optional full ASLR for mmap(), mremap(), vdso, stack and heap

[Topi Miettinen]

On 4.1.2021 17.53, Topi Miettinen wrote:
> Writing a new value of 3 to /proc/sys/kernel/randomize_va_space
> enables full randomization of memory mappings. With 2, the base of the
> VMA used for such mappings is random, but the mappings are created in
> predictable places within the VMA and in sequential order. With 3, new
> VMAs are created to fully randomize the mappings.
>
> Mappings created with mmap(NULL, ...) are randomized and mremap(...,
> MREMAP_MAYMOVE) will move the mappings even if not necessary. The
> locations of heap (memory allocated with brk()), stack and vdso are
> also randomized.

It seems that always moving the mappings in mremap() can expose bugs 
where the callers don't actually expect the mappings to move, even 
though MREMAP_MAYMOVE is specified (Debian libapt-pkg6.0):
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=980037

Perhaps this part should be a separate option to avoid tripping such 
bugs when compatibility is more important than improved ASLR and 
debugging. The option could be also used without randomize_va_space==3. 
How about something like sysctl.kernel.mremap_always_move, 
sysctl.vm.mremap_always_move or CONFIG_MREMAP_ALWAYS_MOVE?

-Topi