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. -Kees -- Kees Cook
