Messages in this thread | | | From | (Eric W. Biederman) | Date | Thu, 18 Nov 2021 14:57:17 -0600 | Subject | Re: BUG: KASAN: use-after-free in dec_rlimit_ucounts |
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Qian Cai <quic_qiancai@quicinc.com> writes:
> On Thu, Nov 18, 2021 at 01:46:05PM -0600, Eric W. Biederman wrote: >> Is it possible? Yes it is possible. That is one place where >> a use-after-free has shown up and I expect would show up in the >> future. >> >> That said it is hard to believe there is still a user-after-free in the >> code. We spent the last kernel development cycle pouring through and >> correcting everything we saw until we ultimately found one very subtle >> use-after-free. >> >> If you have a reliable reproducer that you can share, we can look into >> this and see if we can track down where the reference count is going >> bad. >> >> It tends to take instrumenting the entire life cycle every increment and >> every decrement and then pouring through the logs to track down a >> use-after-free. Which is not something we can really do without a >> reproducer. > > The reproducer is just to run trinity by an unprivileged user on defconfig > with KASAN enabled (On linux-next, you can do "make defconfig debug.conf" > [1], but dont think other debugging options are relevent here.) > > $ trinity -C 31 -N 10000000 > > It is always reproduced on an arm64 server here within 5-minute so far. > Some debugging progress so far. BTW, this could happen on user_shm_unlock() > path as well.
Does this only happen on a single architecture? If so I wonder if perhaps some of the architectures atomic primitives are implemented improperly.
Unfortunately I don't have any arm64 machines where I can easily test this.
The call path you posted from user_shm_unlock is another path where a use-after-free has show up in the past.
My blind guess would be that I made an implementation mistake in inc_rlimit_get_ucounts or dec_rlimit_put_ucounts but I can't see it right now.
Eric
> Call trace: > dec_rlimit_ucounts > user_shm_unlock > (inlined by) user_shm_unlock at mm/mlock.c:854 > shmem_lock > shmctl_do_lock > ksys_shmctl.constprop.0 > __arm64_sys_shmctl > invoke_syscall > el0_svc_common.constprop.0 > do_el0_svc > el0_svc > el0t_64_sync_handler > el0t_64_sync > > I noticed in dec_rlimit_ucounts(), dec == 0 and type == > UCOUNT_RLIMIT_MEMLOCK. > > [1] https://lore.kernel.org/lkml/20211115134754.7334-1-quic_qiancai@quicinc.com/
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