Re: [PATCH] mm: Stop PMD alignment for PIE shared objects

[Matthew Wilcox]

On Thu, Dec 19, 2024 at 04:15:52PM -0500, Audra Mitchell wrote:
> After commit 1854bc6e2420 ("mm/readahead: Align file mappings for non-DAX")
> any request through thp_get_unmapped_area would align to a PMD_SIZE,
> causing shared objects to have less randomization than previously (9 less
> bits for 2MB PMDs). As these lower 9 bits are the most impactful for
> ASLR, this change could be argued to have an impact on security.

Yes, very tiresome people have been making that argument for a long
time.  Do you have anything further to add to the discussion that
happened here:

https://lore.kernel.org/linux-mm/20240118133504.2910955-1-shy828301@xxxxxxxxx/

particularly in light of 3afb76a66b55 existing.

> Fix this issue by checking that the request is aligned to the PMD_SIZE,
> otherwise fall back to mm_get_unmapped_area_vmflags().

NAK this version anyway.  Even if the executable is, say, 2.1MB in size,
we still want the first 2MB of the file to be covered with a PMD
mapping.