linux-kernel - Re: [PATCH v2 1/2] printk: Fix panic caused by passing log_buf

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Date:   Fri, 21 Sep 2018 09:37:53 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     He Zhe <zhe.he@...driver.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        sergey.senozhatsky@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] printk: Fix panic caused by passing log_buf_len
 to command line

On Thu 2018-09-20 12:30:56, Steven Rostedt wrote:
> On Fri, 21 Sep 2018 00:16:50 +0800
> He Zhe <zhe.he@...driver.com> wrote:
> 
> > On 2018年09月19日 10:43, Steven Rostedt wrote:
> > > On Wed, 19 Sep 2018 11:39:32 +0900
> > > Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:
> > >  
> > >> On (09/19/18 10:27), He Zhe wrote:  
> > >>> On 2018年09月19日 09:50, Sergey Senozhatsky wrote:    
> > >>>> On (09/19/18 01:17), zhe.he@...driver.com wrote:    
> > >>>>> @@ -1048,7 +1048,14 @@ static void __init log_buf_len_update(unsigned size)
> > >>>>>  /* save requested log_buf_len since it's too early to process it */
> > >>>>>  static int __init log_buf_len_setup(char *str)
> > >>>>>  {
> > >>>>> -	unsigned size = memparse(str, &str);
> > >>>>> +	unsigned size;    
> > >>>> 	unsigned int size;    
> > >>> This is in v1 but then Steven suggested that it should be split out
> > >>> and only keep the pure fix part here.    
> > >> Ah, I see.
> > >>
> > >> Hmm... memparse() returns u64 value. A user *probably* can ask the kernel
> > >> to allocate log_buf larger than 'unsigned int'.
> > >>
> > >> So may be I'd do two fixes here:
> > >>
> > >>  First  - switch to u64 size.
> > >>  Second - check for NULL str.
> > >>
> > >>
> > >> Steven, Petr, what do you think?
> > >>  
> > > I think I would switch it around. Check for NULL first, and then switch
> > > to u64. It was always an int, do we need to backport converting it to
> > > u64 to stable? The NULL check is a definite, the overflow of int
> > > shouldn't crash anything.  
> > 
> 
> Hi Zhe,
> 
> > To switch to u64, several variables need to be adjusted to new type to aligned
> > with new_log_buf_len. And currently new_log_buf_len is passed to
> > memblock_virt_alloc(phys_addr_t, phys_addr_t). So we can't simply define
> > new_log_buf_len as u64. We need to define it as phys_addr_t tomake it work
> > well for both 32bit and 64bit arches, since a 32-bit architecture can set
> > ARCH_PHYS_ADDR_T_64BIT if it needs a 64-bit phys_addr_t.
> 
> The above explanation verifies more that the NULL pointer check needs
> to be first, and that the change in size should not be backported to
> stable because it has a high risk to doing the change as compared to it
> being a problem for older kernels.

I agree that the NULL check should go first.

I would personally keep the size as unsigned int. IMHO, a support
for a log buffer bigger than 4GB is not worth the complexity.

Best Regards,
Petr