Re: [PATCH 09/10] kthread: Ensure struct kthread is present for all kthreads

Nathan Chancellor
2021-12-23T00:37:29Z

On Wed, Dec 22, 2021 at 05:22:45PM -0600, Eric W. Biederman wrote:
> Nathan Chancellor <nathan@kernel.org> writes:
>
> > On Wed, Dec 22, 2021 at 12:30:57PM -0600, Eric W. Biederman wrote:
> >> Nathan Chancellor <nathan@kernel.org> writes:
> >>
> >> > Hi Eric,
> >> >
> >> > On Wed, Dec 08, 2021 at 02:25:31PM -0600, Eric W. Biederman wrote:
> >> >> Today the rules are a bit iffy and arbitrary about which kernel
> >> >> threads have struct kthread present. Both idle threads and thread
> >> >> started with create_kthread want struct kthread present so that is
> >> >> effectively all kernel threads. Make the rule that if PF_KTHREAD
> >> >> and the task is running then struct kthread is present.
> >> >>
> >> >> This will allow the kernel thread code to using tsk->exit_code
> >> >> with different semantics from ordinary processes.
> >> >>
> >> >> To make ensure that struct kthread is present for all
> >> >> kernel threads move it's allocation into copy_process.
> >> >>
> >> >> Add a deallocation of struct kthread in exec for processes
> >> >> that were kernel threads.
> >> >>
> >> >> Move the allocation of struct kthread for the initial thread
> >> >> earlier so that it is not repeated for each additional idle
> >> >> thread.
> >> >>
> >> >> Move the initialization of struct kthread into set_kthread_struct
> >> >> so that the structure is always and reliably initailized.
> >> >>
> >> >> Clear set_child_tid in free_kthread_struct to ensure the kthread
> >> >> struct is reliably freed during exec. The function
> >> >> free_kthread_struct does not need to clear vfork_done during exec as
> >> >> exec_mm_release called from exec_mmap has already cleared vfork_done.
> >> >>
> >> >> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> >> >
> >> > This patch as commit 40966e316f86 ("kthread: Ensure struct kthread is
> >> > present for all kthreads") in -next causes an ARCH=arm
> >> > multi_v5_defconfig kernel to fail to boot in QEMU. I had to apply commit
> >> > 6692c98c7df5 ("fork: Stop protecting back_fork_cleanup_cgroup_lock with
> >> > CONFIG_NUMA") to get it to build and I applied commit dd621ee0cf8e
> >> > ("kthread: Warn about failed allocations for the init kthread") to avoid
> >> > the known runtime warning.
> >> >
> >> > $ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- distclean multi_v5_defconfig all
> >> >
> >> > $ qemu-system-arm \
> >> > -initrd rootfs.cpio \
> >> > -append earlycon \
> >> > -machine palmetto-bmc \
> >> > -no-reboot \
> >> > -dtb arch/arm/boot/dts/aspeed-bmc-opp-palmetto.dtb \
> >> > -display none \
> >> > -kernel arch/arm/boot/zImage \
> >> > -m 512m \
> >> > -nodefaults \
> >> > -serial mon:stdio
> >> > qemu-system-arm: warning: nic ftgmac100.0 has no peer
> >> > qemu-system-arm: warning: nic ftgmac100.1 has no peer
> >> > Booting Linux on physical CPU 0x0
> >> > Linux version 5.16.0-rc1-00016-g40966e316f86-dirty (nathan@archlinux-ax161) (arm-linux-gnueabi-gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2) #1 PREEMPT Wed Dec 22 18:08:53 UTC 2021
> >> > CPU: ARM926EJ-S [41069265] revision 5 (ARMv5TEJ), cr=00093177
> >> > CPU: VIVT data cache, VIVT instruction cache
> >> > OF: fdt: Machine model: Palmetto BMC
> >> > earlycon: ns16550a0 at MMIO 0x1e784000 (options '')
> >> > printk: bootconsole [ns16550a0] enabled
> >> > Memory policy: Data cache writethrough
> >> > cma: Reserved 16 MiB at 0x5b000000
> >> > Zone ranges:
> >> > DMA [mem 0x0000000040000000-0x000000005edfffff]
> >> > Normal empty
> >> > HighMem [mem 0x000000005ee00000-0x000000005fffffff]
> >> > Movable zone start for each node
> >> > Early memory node ranges
> >> > node 0: [mem 0x0000000040000000-0x000000005bffffff]
> >> > node 0: [mem 0x000000005c000000-0x000000005dffffff]
> >> > node 0: [mem 0x000000005e000000-0x000000005edfffff]
> >> > node 0: [mem 0x000000005ee00000-0x000000005fffffff]
> >> > Initmem setup node 0 [mem 0x0000000040000000-0x000000005fffffff]
> >> > Built 1 zonelists, mobility grouping on. Total pages: 130084
> >> > Kernel command line: earlycon
> >> > Dentry cache hash table entries: 65536 (order: 6, 262144 bytes, linear)
> >> > Inode-cache hash table entries: 32768 (order: 5, 131072 bytes, linear)
> >> > mem auto-init: stack:off, heap alloc:off, heap free:off
> >> > Memory: 433140K/524288K available (9628K kernel code, 2019K rwdata, 2368K rodata, 340K init, 661K bss, 74764K reserved, 16384K cma-reserved, 0K highmem)
> >> > SLUB: HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
> >> > rcu: Preemptible hierarchical RCU implementation.
> >> > rcu: RCU event tracing is enabled.
> >> > Trampoline variant of Tasks RCU enabled.
> >> > rcu: RCU calculated value of scheduler-enlistment delay is 10 jiffies.
> >> > NR_IRQS: 16, nr_irqs: 16, preallocated irqs: 16
> >> > i2c controller registered, irq 16
> >> > random: get_random_bytes called from start_kernel+0x408/0x624 with crng_init=0
> >> > clocksource: FTTMR010-TIMER2: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 79635851949 ns
> >> > sched_clock: 32 bits at 24MHz, resolution 41ns, wraps every 89478484971ns
> >> > Switching to timer-based delay loop, resolution 41ns
> >> > Console: colour dummy device 80x30
> >> > printk: console [tty0] enabled
> >> > printk: bootconsole [ns16550a0] disabled
> >> >
> >> > After that, it just hangs.
> >> >
> >> > The rootfs is available at https://github.com/ClangBuiltLinux/boot-utils
> >> > in the images/arm folder.
> >> >
> >> > If there is any more information that I can provide or changes to test,
> >> > please let me know.
>
> I have managed to reproduce, fix and verify my fix, please
> see below.
>
>
> Subject: [PATCH] kthread: Never put_user the set_child_tid address
>
> Kernel threads abuse set_child_tid. Historically that has been fine
> as set_child_tid was initialized after the kernel thread had been
> forked. Unfortunately storing struct kthread in set_child_tid after
> the thread is running makes struct kthread being unusable for storing
> result codes of the thread.
>
> When set_child_tid is set to struct kthread during fork that results
> in schedule_tail writing the thread id to the beggining of struct
> kthread (if put_user does not realize it is a kernel address).
>
> Solve this by skipping the put_user for all kthreads.
>
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Link: https://lkml.kernel.org/r/YcNsG0Lp94V13whH@archlinux-ax161
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Thanks a lot for the quick fix. I can confirm that it resolves the
failure on my side.

Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
> kernel/sched/core.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ee222b89c692..d8adbea77be1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4908,7 +4908,7 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
> finish_task_switch(prev);
> preempt_enable();
>
> - if (current->set_child_tid)
> + if (!(current->flags & PF_KTHREAD) && current->set_child_tid)
> put_user(task_pid_vnr(current), current->set_child_tid);
>
> calculate_sigpending();
> --
> 2.29.2
>
>
> Eric