On Wed, Jan 17, 2024 at 3:32 PM Yang Shi <shy828301@xxxxxxxxx> wrote: > > On Wed, Jan 17, 2024 at 9:40 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > On Tue, Jan 16, 2024 at 02:30:36PM -0800, Suren Baghdasaryan wrote: > > > On Tue, Jan 16, 2024 at 2:25 PM Yang Shi <shy828301@xxxxxxxxx> wrote: > > > > > > > > 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. > > > > > > How is making a system move vulnerabile not 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. > > > > > > This is not depending on luck. This is checking for possible > > > vulnerabilities in the system. > > > I admit I'm not a security expert, so I'm looping in Jeff and Kees to chime in. > > > > Hi! > > > > Just to chime in, but reduction in ASLR entropy is absolutely a > > regression. This is userspace visible (via /proc/sys/kernel/randomize_va_space, > > /proc/sys/vm/mmap_rnd*_bits) with expectations that it work as > > advertised. So, while 32-bit might be already ASLR-weak, we don't want > > to make things super bad nor break ASLR in compat mode under 64-bit > > systems. > > > > Having an opt-in sounds reasonable, but we need to wire it to the ASLR > > sysctls in some way so nothing lying about the ASLR entropy. > > Thanks for the explanation. IIUC the randomiza_va_space and > mmap_rnd_bits randomize the mmap_base and start_brk for each exec() > call. So the heap allocation is randomized. But it seems the formula > doesn't take into account huge page. ARM64 adjusts the mmap_rnd_bits > based on page size. > > I did a simple test, which conceptually does: > 1. call mmap to allocate 8M heap > 2. print out the allocated address > 3. run the program 1000 times (launch/exit/re-launch) > 4. check how many unique addresses > > With the default config on my arm64 VM (mmap_rnd_bits is 18), I saw > 134 unique addresses. Without the patch, I saw 945 unique addresses. > So I think the test could replicate what your test does. > > When I increased the mmap_rnd_bits to 24, I saw 988 unique addresses > with the patch. x86_64 should have 28 bits by default, it should > randomize the address quite well. I don't know why you still saw this, > or you have a different setting for mmap_rnd_bits? I checked the configuration on our test harness where the test failed: /proc/sys/vm/mmap_rnd_bits = 32 /proc/sys/vm/mmap_rnd_compat_bits = 16 The failure logs are: 10-20 14:37:52.123 7029 7029 V AslrMallocTest: 7 bits of entropy for allocation size 8388608 (minimum 8) 10-20 14:37:52.123 7029 7029 E AslrMallocTest: insufficient entropy for malloc(8388608) which come from here: https://cs.android.com/android/platform/superproject/main/+/main:cts/tests/aslr/src/AslrMallocTest.cpp;l=127 So, the allocation size for which this test failed was 2^23. > I'm wondering whether we should take into account huge page alignment > for mmap_rnd_bits. And I think this is a huge page common problem, we > have file mapping huge page aligned as well. > > 32 bit is easy, I think I can just make thp_get_unmapped_area() a > no-op on 32 bit system. > > > > > -Kees > > > > -- > > Kees Cook
