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From: Suren Baghdasaryan <surenb@google.com>
Date: Tue, 16 Jan 2024 13:57:51 -0800
Message-ID: <CAJuCfpH5gwQc0mBzQ5LOMY9URCTh=58yUJd8pbzzynqAy8_yXw@mail.gmail.com>
Subject: Re: [PATCH v2] mm: align larger anonymous mappings on THP boundaries
To: Yang Shi <shy828301@gmail.com>
Cc: Jiri Slaby <jirislaby@kernel.org>, Rik van Riel <riel@surriel.com>, 
	Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, kernel-team@fb.com, 
	Matthew Wilcox <willy@infradead.org>, Christoph Lameter <cl@linux.com>
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On Tue, Jan 16, 2024 at 12:56=E2=80=AFPM Yang Shi <shy828301@gmail.com> wro=
te:
>
> On Tue, Jan 16, 2024 at 11:16=E2=80=AFAM Suren Baghdasaryan <surenb@googl=
e.com> wrote:
> >
> > On Tue, Jan 16, 2024 at 4:09=E2=80=AFAM Jiri Slaby <jirislaby@kernel.or=
g> 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=3D1218841
> > > >
> > > > So running:
> > > > pahole -J --btf_gen_floats -j --lang_exclude=3Drust
> > > > --skip_encoding_btf_inconsistent_proto --btf_gen_optimized .tmp_vml=
inux.btf
> > > >
> > > > crashes or errors out with some random errors:
> > > > [182671] STRUCT idr's field 'idr_next' offset=3D128 bit_size=3D0 ty=
pe=3D181346
> > > > 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>)              =3D -1 ENOMEM (Cannot alloc=
ate
> > > > 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 fi=
x):
> > >
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -1829,7 +1829,7 @@ get_unmapped_area(struct file *file, unsigned l=
ong
> > > addr, unsigned long len,
> > >                   */
> > >                  pgoff =3D 0;
> > >                  get_area =3D 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 =3D 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.

>
> >
> > 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:ct=
s/tests/aslr/src/AslrMallocTest.cpp;l=3D130
> >
> > >
> > > regards,
> > > --
> > > js
> > > suse labs
> > >
> > >