Re: [PATCH v2] mm: align larger anonymous mappings on THP boundaries

[Yang Shi]

On Tue, Jan 16, 2024 at 11:16 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Tue, Jan 16, 2024 at 4:09 AM Jiri Slaby <jirislaby@xxxxxxxxxx> wrote:
> >
> > On 16. 01. 24, 12:53, Jiri Slaby wrote:
> > > Hi,
> > >
> > > On 09. 08. 22, 20:24, Rik van Riel wrote:
> > >> Align larger anonymous memory mappings on THP boundaries by
> > >> going through thp_get_unmapped_area if THPs are enabled for
> > >> the current process.
> > >>
> > >> With this patch, larger anonymous mappings are now THP aligned.
> > >> When a malloc library allocates a 2MB or larger arena, that
> > >> arena can now be mapped with THPs right from the start, which
> > >> can result in better TLB hit rates and execution time.
> > >
> > > This appears to break 32bit processes on x86_64 (at least). In
> > > particular, 32bit kernel or firefox builds in our build system.
> > >
> > > Reverting this on top of 6.7 makes it work again.
> > >
> > > Downstream report:
> > >   https://bugzilla.suse.com/show_bug.cgi?id=1218841
> > >
> > > So running:
> > > pahole -J --btf_gen_floats -j --lang_exclude=rust
> > > --skip_encoding_btf_inconsistent_proto --btf_gen_optimized .tmp_vmlinux.btf
> > >
> > > crashes or errors out with some random errors:
> > > [182671] STRUCT idr's field 'idr_next' offset=128 bit_size=0 type=181346
> > > Error emitting field
> > >
> > > strace shows mmap() fails with ENOMEM right before the errors:
> > > 1223  mmap2(NULL, 5783552, PROT_READ|PROT_WRITE,
> > > MAP_PRIVATE|MAP_ANONYMOUS, -1, 0 <unfinished ...>
> > > ...
> > > 1223  <... mmap2 resumed>)              = -1 ENOMEM (Cannot allocate
> > > memory)
> > >
> > > Note the .tmp_vmlinux.btf above can be arbitrary, but likely large
> > > enough. For reference, one is available at:
> > > https://decibel.fi.muni.cz/~xslaby/n/btf
> > >
> > > Any ideas?
> >
> > This works around the problem, of course (but is a band-aid, not a fix):
> >
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1829,7 +1829,7 @@ get_unmapped_area(struct file *file, unsigned long
> > addr, unsigned long len,
> >                   */
> >                  pgoff = 0;
> >                  get_area = shmem_get_unmapped_area;
> > -       } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> > +       } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> > !in_32bit_syscall()) {
> >                  /* Ensures that larger anonymous mappings are THP
> > aligned. */
> >                  get_area = thp_get_unmapped_area;
> >          }
> >
> >
> > thp_get_unmapped_area() does not take care of the legacy stuff...
>
> This change also affects the entropy of allocations. With this patch
> Android test [1] started failing and it requires only 8 bits of
> entropy. The feedback from our security team:
>
> 8 bits of entropy is already embarrassingly low, but was necessary for
> 32 bit ARM targets with low RAM at the time. It's definitely not
> acceptable for 64 bit targets.

Thanks for the report. Is it 32 bit only or 64 bit is also impacted?
If I understand the code correctly, it expects the address allocated
by malloc() is kind of randomized, right?

>
> Could this change be either reverted or made optional (opt-in/opt-out)?
> Thanks,
> Suren.
>
> [1] https://cs.android.com/android/platform/superproject/main/+/main:cts/tests/aslr/src/AslrMallocTest.cpp;l=130
>
> >
> > regards,
> > --
> > js
> > suse labs
> >
> >