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

[Audra Mitchell <audra@redhat.com>]

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.
Running the pie-so program [1] multiple times we can see the randomization
loss as the lower address bits get aligned to a 2MB size on x86 (pie-so):

    # ./all-gather && ./all-bits
    ---------[SNIP]---------
    aslr heap       19 bits
    aslr exec       00 bits
    aslr mmap       29 bits
    aslr so         00 bits
    aslr stack      31 bits
    aslr pie-exec   30 bits
    aslr pie-heap   30 bits
    aslr pie-so     20 bits
    aslr pie-mmap   29 bits
    aslr pie-stack  30 bits

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

[1]  https://github.com/stevegrubb/distro-elf-inspector
Fixes: 1854bc6e2420 ("mm/readahead: Align file mappings for non-DAX")

Signed-off-by: Audra Mitchell <audra@redhat.com>
---
 mm/huge_memory.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ee335d96fc39..696caf6cbf4a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1101,6 +1101,9 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
 	if (len_pad < len || (off + len_pad) < off)
 		return 0;
 
+	if (!IS_ALIGNED(len, PMD_SIZE))
+		return 0;
+
 	ret = mm_get_unmapped_area_vmflags(current->mm, filp, addr, len_pad,
 					   off >> PAGE_SHIFT, flags, vm_flags);
 
-- 
2.45.0