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

[Yang Shi]

On Tue, Jan 16, 2024 at 1:58 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Tue, Jan 16, 2024 at 12:56 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
> >
> > 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?
>
> Yes, correct, the test expects a certain level of address randomization.
> The test failure was reported while running kernel_virt_x86_64 target
> (Android emulator on x86), so it does impact 64bit targets.

IIUC this breaks the "expectation" for randomized addresses returned
by malloc(), but it doesn't break any real Android application, right?
So this is a security concern instead of a real regression.

I think we can make this opt-in with a knob. Anyone who outweighs
security could opt this feature out. However I'm wondering whether
Android should implement a general address randomization mechanism
instead of depending on "luck" if you do care about it.

>
> >
> > >
> > > 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
> > > >
> > > >