Messages in this thread | | | Date | Wed, 3 Aug 2022 11:52:49 -0700 | From | Yury Norov <> | Subject | Re: [PATCH] sched, cpumask: don't leak impossible cpus via for_each_cpu_wrap(). |
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On Wed, Aug 03, 2022 at 10:49:57AM -0700, Neel Natu wrote: > On Tue, Aug 2, 2022 at 6:22 PM Yury Norov <yury.norov@gmail.com> wrote: > > > > On Tue, Aug 2, 2022 at 2:41 PM Neel Natu <neelnatu@google.com> wrote: > > > > > > The value of 'nr_cpumask_bits' is dependent on CONFIG_CPUMASK_OFFSTACK. > > > This in turn can change the set of cpus visited by for_each_cpu_wrap() > > > with a mask that has bits set in the range [nr_cpu_ids, NR_CPUS). > > > > > > Specifically on !CONFIG_CPUMASK_OFFSTACK kernels the API can iterate > > > over cpus outside the 'cpu_possible_mask'. > > > > > > Fix this to make its behavior match for_each_cpu() which always limits > > > the iteration to the range [0, nr_cpu_ids). > > > > > > Signed-off-by: Neel Natu <neelnatu@google.com> > > > > The patch itself doesn't look correct because it randomly switches a piece > > of cpumask API from nr_cpumask_bits to nr_cpu_ids, and doesn't touch > > others. > > > > However... > > > > I don't know the story behind having 2 variables holding the max possible > > number of cpus, and it looks like it dates back to prehistoric times. In > > modern kernel, there are 2 cases where nr_cpumask_bits == nr_cpu_ids > > for sure: it's CONFIG_CPUMASK_OFFSTACK=y and > > CONFIG_HOTPLUG_CPU=y. At least one of those is enabled in defconfig > > of every popular architecture. > > > > Hmm, in a kernel with CONFIG_HOTPLUG_CPU=y but not CONFIG_CPUMASK_OFFSTACK > I see "nr_cpu_ids = 20, nr_cpumask_bits = 512". FYI since it doesn't > match the observation > above that nr_cpumask_bits == nr_cpu_ids when CONFIG_HOTPLUG_CPU=y.
I said this because the comment in include/linux/cpumaks.h says: * If HOTPLUG is enabled, then cpu_possible_mask is forced to have * all NR_CPUS bits set, otherwise it is just the set of CPUs that * ACPI reports present at boot.
And because of the code that sets nr_cpu_ids:
void __init setup_nr_cpu_ids(void) { nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1; }
Some arches override it, so it may be an issue. Are you running x86, or maybe you have "nr_cpus" boot parameter?
But anyways, I feel like this should be investigated for more... Can you please share the config of your system and boot params? > > In case of HOTPLUG is off, I don't understand why we should have nr_cpu_ids > > and nr_cpumask_bits different - what case should it cover?... Interestingly, in > > comments to cpumask functions and in the code those two are referred > > interchangeably. > > > > Even more interestingly, we have a function bitmap_setall() that sets all bits > > up to nr_cpumask_bits, and it could trigger the problem that you described, > > I think you mean cpumask_setall() that in turn calls > bitmap_fill(nr_cpumask_bits)?
Yes, sorry.
> > so that someone would complain. (Are there any other valid reasons to set > > bits behind nr_cpu_ids intentionally?) > > > > I don't know of any although this wasn't the case that trigger in my case. > > > Can you share more details about how you triggered that? If you observe > > those bits set, something else is probably already wrong... > > The non-intuitive behavior of for_each_cpu_wrap() was triggered when iterating > over a cpumask passed by userspace that set a bit in the [nr_cpu_ids, > nr_cpumask_bits) > range.
If you receive bitmap from userspace, you need to copy it with bitmap_copy_clear_tail(), or bitmap_from_arr{32,64}, as appropriate. That will clear unneeded bits.
For user-to-kernel communications, I'd recommend passing bitmaps in a human-readable formats - hex string or bitmap list. Check the functions cpumask_parse_user() and cpumask_parselist_user(). This would help to avoid all possible issues related to endianness and 32/64 word length mismatch.
> The kernel code is out of tree but open source so happy to provide a > pointer if needed.
Yes please
> I considered ANDing the user supplied mask with 'cpu_possible_mask' > but that felt > like working around an inconsistency in the kernel API (hence the proposed fix). > > > So, if there is no real case in modern kernel to have nr_cpumask_bits and > > nr_cpu_ids different, the proper fix would be to just drop the first. > > > > I'll let other people more knowledgable than me in this area chime in. > I'll be happy either > way if that fixes the problem at hand. > > best > Neel > > > If there is such a case, this is probably your case, and we'd know more > > details to understand how to deal with that. > > > > Thanks, > > Yury
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