linux-kernel - Re: [PATCH] uaccess: Comment that copy to/from inatomic requires page fault disabled

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Message-Id: <20250912100723.4fce2b29c4d77f3a89c0b636@kernel.org>
Date: Fri, 12 Sep 2025 10:07:23 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Linus Torvalds
 <torvalds@...ux-foundation.org>, linux-mm@...ck.org, Kees Cook
 <keescook@...omium.org>, Aleksa Sarai <cyphar@...har.com>, Al Viro
 <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] uaccess: Comment that copy to/from inatomic requires
 page fault disabled

On Wed, 10 Sep 2025 16:18:20 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> The functions __copy_from_user_inatomic() and __copy_to_user_inatomic()
> both require that either the user space memory is pinned, or that page
> faults are disabled when they are called. If page faults are not disabled,
> and the memory is not present, the fault handling of reading or writing to
> that memory may cause the kernel to schedule. That would be bad in an
> atomic context.
> 
> Link: https://lore.kernel.org/all/20250819105152.2766363-1-luogengkun@huaweicloud.com/
> 

Looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thanks,


> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  include/linux/uaccess.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 1beb5b395d81..add99fa9b656 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -86,6 +86,12 @@
>   * as usual) and both source and destination can trigger faults.
>   */
>  
> +/*
> + * __copy_from_user_inatomic() is safe to use in an atomic context but
> + * the user space memory must either be pinned in memory, or page faults
> + * must be disabled, otherwise the page fault handling may cause the function
> + * to schedule.
> + */
>  static __always_inline __must_check unsigned long
>  __copy_from_user_inatomic(void *to, const void __user *from, unsigned long n)
>  {
> @@ -124,7 +130,8 @@ __copy_from_user(void *to, const void __user *from, unsigned long n)
>   * Copy data from kernel space to user space.  Caller must check
>   * the specified block with access_ok() before calling this function.
>   * The caller should also make sure he pins the user space address
> - * so that we don't result in page fault and sleep.
> + * or call page_fault_disable() so that we don't result in a page fault
> + * and sleep.
>   */
>  static __always_inline __must_check unsigned long
>  __copy_to_user_inatomic(void __user *to, const void *from, unsigned long n)
> -- 
> 2.50.1
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>