Re: [uaccess] 780464aed0: WARNING:at_arch/x86/include/asm/uaccess.h:#strnlen_user/0x

[Masami Hiramatsu <mhiramat@kernel.org>]

On Tue, 5 Mar 2019 10:07:29 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Mar 05, 2019 at 11:36:35AM +0900, Masami Hiramatsu wrote:
> > I think the better way to do this is allowing strncpy_from_user()
> O
> > if some conditions are match, like
> > 
> >  - strncpy_from_user() will be able to copy user memory with set_fs(USER_DS)
> >  - strncpy_from_user() can copy kernel memory with set_fs(KERNEL_DS)
> >  - strncpy_from_user() can access unsafe memory in IRQ context if
> >    pagefault is disabled.
> > 
> > This is almost done, except for CONFIG_DEBUG_ATOMIC_SLEEP=y on x86.
> > 
> > So, what about adding a condition to WARN_ON_IN_IRQ() like below
> > instead of introducing user_access_ok() ?
> > 
> > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > index 780f2b42c8ef..ec0f0b74c9ab 100644
> > --- a/arch/x86/include/asm/uaccess.h
> > +++ b/arch/x86/include/asm/uaccess.h
> > @@ -70,7 +70,7 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, un
> >  })
> >  
> >  #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> > -# define WARN_ON_IN_IRQ()      WARN_ON_ONCE(!in_task()) 
> > +# define WARN_ON_IN_IRQ()      WARN_ON_ONCE(pagefault_disabled() && !in_task())
> 
> That doesn't make any kind of sense to me; see faulthandler_disabled().
> IOW. interrupt (and any atomic context really) won't take faults anyway.

Hmm, I thought CONFIG_DEBUG_ATOMIC_SLEEP=y tries to detect that some operations
which can sleep in atomic, like IRQ context, doesn't it?
(note that above should be !pagefault_disabled() anyway) 

So I guessed WARN_ON_IN_IRQ() intended to detect the access_ok() was used
in atomic, because it might follow some copy_from_user() like operation
which can sleep when it hits a pagefault. Is my guess wrong?

If correct, I think if pagefault is disabled, the caller never sleep,
so we don't need to take care of that.

Could you tell me why WARN_ON_ONCE(!in_task()) is needed in access_ok()?

> 
> I dislike that whole KERNEL_DS thing, but obviously that's not something
> that's going away.
> 
> Would something like:
> 
> 	WARN_ON_ONCE(!(in_task || segment_eq(get_fs(), USER_DS)))
> 
> Work? Then we allow KERNEL_DS in task context, but for interrupt and
> others require USER_DS.

But what would this mean? I can't understand why we limit using
access_ok() so strictly and narrow the cases.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>