linux-kernel - Re: [PATCH v2] mm/vmalloc: fix data race in show_numa

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Message-ID: <aBn4uqA7hnLBH2kL@pc636>
Date: Tue, 6 May 2025 13:55:38 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: Jeongjun Park <aha310510@...il.com>
Cc: akpm@...ux-foundation.org, urezki@...il.com, edumazet@...gle.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/vmalloc: fix data race in show_numa_info()

On Tue, May 06, 2025 at 05:25:19PM +0900, Jeongjun Park wrote:
> The following data-race was found in show_numa_info():
> 
> ==================================================================
> BUG: KCSAN: data-race in vmalloc_info_show / vmalloc_info_show
> 
> read to 0xffff88800971fe30 of 4 bytes by task 8289 on cpu 0:
>  show_numa_info mm/vmalloc.c:4936 [inline]
>  vmalloc_info_show+0x5a8/0x7e0 mm/vmalloc.c:5016
>  seq_read_iter+0x373/0xb40 fs/seq_file.c:230
>  proc_reg_read_iter+0x11e/0x170 fs/proc/inode.c:299
>  new_sync_read fs/read_write.c:489 [inline]
>  vfs_read+0x5b4/0x740 fs/read_write.c:570
>  ksys_read+0xbe/0x190 fs/read_write.c:713
>  __do_sys_read fs/read_write.c:722 [inline]
>  __se_sys_read fs/read_write.c:720 [inline]
>  __x64_sys_read+0x41/0x50 fs/read_write.c:720
>  x64_sys_call+0x1729/0x1fd0 arch/x86/include/generated/asm/syscalls_64.h:1
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xa6/0x1b0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> write to 0xffff88800971fe30 of 4 bytes by task 8287 on cpu 1:
>  show_numa_info mm/vmalloc.c:4934 [inline]
>  vmalloc_info_show+0x38f/0x7e0 mm/vmalloc.c:5016
>  seq_read_iter+0x373/0xb40 fs/seq_file.c:230
>  proc_reg_read_iter+0x11e/0x170 fs/proc/inode.c:299
>  new_sync_read fs/read_write.c:489 [inline]
>  vfs_read+0x5b4/0x740 fs/read_write.c:570
>  ksys_read+0xbe/0x190 fs/read_write.c:713
>  __do_sys_read fs/read_write.c:722 [inline]
>  __se_sys_read fs/read_write.c:720 [inline]
>  __x64_sys_read+0x41/0x50 fs/read_write.c:720
>  x64_sys_call+0x1729/0x1fd0 arch/x86/include/generated/asm/syscalls_64.h:1
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xa6/0x1b0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> value changed: 0x0000008f -> 0x00000000
> ==================================================================
> 
> According to this report, there is a read/write data-race because m->private
> is accessible to multiple CPUs. To fix this, instead of allocating the heap
> in proc_vmalloc_init() and passing the heap address to m->private,
> show_numa_info() should allocate the heap.
> 
> One thing to note is that show_numa_info() is called in a critical section
> of a spinlock, so it must be allocated on the heap with GFP_ATOMIC flag.
> 
> Fixes: a47a126ad5ea ("vmallocinfo: add NUMA information")
> Suggested-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Jeongjun Park <aha310510@...il.com>
> ---
> v2: Refactoring some functions and fix patch as per Eric Dumazet suggestion
> - Link to v1: https://lore.kernel.org/all/20250505171948.24410-1-aha310510@gmail.com/
> ---
>  mm/vmalloc.c | 50 +++++++++++++++++++++++++-------------------------
>  1 file changed, 25 insertions(+), 25 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 3ed720a787ec..a5e795346141 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4914,28 +4914,32 @@ bool vmalloc_dump_obj(void *object)
>  #endif
>  
>  #ifdef CONFIG_PROC_FS
> +
> +/*
> + * Print number of pages allocated on each memory node.
> + *
> + * This function can only be called if CONFIG_NUMA is enabled
> + * and VM_UNINITIALIZED bit in v->flags is disabled.
> + */
>  static void show_numa_info(struct seq_file *m, struct vm_struct *v)
>  {
> -	if (IS_ENABLED(CONFIG_NUMA)) {
> -		unsigned int nr, *counters = m->private;
> -		unsigned int step = 1U << vm_area_page_order(v);
> +	unsigned int nr, *counters;
> +	unsigned int step = 1U << vm_area_page_order(v);
>  
> -		if (!counters)
> -			return;
> +	counters = kcalloc(nr_node_ids, sizeof(unsigned int), GFP_ATOMIC);
> +	if (!counters)
> +		return;
>  
> -		if (v->flags & VM_UNINITIALIZED)
> -			return;
> -		/* Pair with smp_wmb() in clear_vm_uninitialized_flag() */
> -		smp_rmb();
> +	/* Pair with smp_wmb() in clear_vm_uninitialized_flag() */
> +	smp_rmb();
>  
> -		memset(counters, 0, nr_node_ids * sizeof(unsigned int));
> +	for (nr = 0; nr < v->nr_pages; nr += step)
> +		counters[page_to_nid(v->pages[nr])] += step;
> +	for_each_node_state(nr, N_HIGH_MEMORY)
> +		if (counters[nr])
> +			seq_printf(m, " N%u=%u", nr, counters[nr]);
>  
> -		for (nr = 0; nr < v->nr_pages; nr += step)
> -			counters[page_to_nid(v->pages[nr])] += step;
> -		for_each_node_state(nr, N_HIGH_MEMORY)
> -			if (counters[nr])
> -				seq_printf(m, " N%u=%u", nr, counters[nr]);
> -	}
> +	kfree(counters);
>  }
>  
>  static void show_purge_info(struct seq_file *m)
> @@ -5013,7 +5017,10 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
>  			if (is_vmalloc_addr(v->pages))
>  				seq_puts(m, " vpages");
>  
> -			show_numa_info(m, v);
> +			if (!(v->flags & VM_UNINITIALIZED) &&
>
I think it makes sense to move the VM_UNINITIALIZED check before:

<snip>
			v = va->vm;

			seq_printf(m, "0x%pK-0x%pK %7ld",
				v->addr, v->addr + v->size, v->size);
<snip>

because it can be still un-initialized, thus flags like "nr_pages", etc
will not be valid.

Any thoughts? It has nothing to do with a patch, because it fixes other
race issue and what i propose might well be a separate patch if we agree.

Thanks!

--
Uladzislau Rezki