linux-kernel - Re: [PATCH V2] proc-vmcore: wrong data type casting fix

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Date:	Sat, 12 Mar 2016 12:49:34 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, d.hatayama@...fujitsu.com,
	bhe@...hat.com, vgoyal@...hat.com, kexec@...ts.infradead.org,
	nasa4836@...il.com, mhuang@...hat.com
Subject: Re: [PATCH V2] proc-vmcore: wrong data type casting fix

Hi, Andrew

On 03/11/16 at 12:27pm, Andrew Morton wrote:
> On Fri, 11 Mar 2016 16:42:48 +0800 Dave Young <dyoung@...hat.com> wrote:
> 
> > On i686 PAE enabled machine the contiguous physical area could be large
> > and it can cause trimming down variables in below calculation in
> > read_vmcore() and mmap_vmcore():
> > 
> > 	tsz = min_t(size_t, m->offset + m->size - *fpos, buflen);
> > 
> > Then the real size passed down is not correct any more.
> > Suppose m->offset + m->size - *fpos being truncated to 0, buflen >0 then
> > we will get tsz = 0. It is of course not an expected result.
> 
> I don't really understand this.
> 
> vmcore.offset if loff_t which is 64-bit
> vmcore.size is long long
> *fpos is loff_t
> 
> so the expression should all be done with 64-bit arithmetic anyway.

#define min_t(type, x, y) ({                    \
        type __min1 = (x);                      \
        type __min2 = (y);                      \
        __min1 < __min2 ? __min1: __min2; })

Here x = m->offset + m->size - *fpos; the expression is done with 64bit
arithmetic, it is true. But x will be cast to size_t then compare x with y
The casting will cause problem.

> 
> Maybe buflen (size_t) has the wrong type, but the result of the other
> expression should be in-range by the time we come to doing the
> comparison.
> 
> > During our tests there are two problems caused by it:
> > 1) read_vmcore will refuse to continue so makedumpfile fails.
> > 2) mmap_vmcore will trigger BUG_ON() in remap_pfn_range().
> > 
> > Use unsigned long long in min_t instead so that the variables are not
> > truncated.
> > 
> > Signed-off-by: Baoquan He <bhe@...hat.com>
> > Signed-off-by: Dave Young <dyoung@...hat.com>
> 
> I think we'll need a cc:stable here.

Agreed. Do you think I need repost for this?

> 
> > --- linux-x86.orig/fs/proc/vmcore.c
> > +++ linux-x86/fs/proc/vmcore.c
> > @@ -231,7 +231,9 @@ static ssize_t __read_vmcore(char *buffe
> >  
> >  	list_for_each_entry(m, &vmcore_list, list) {
> >  		if (*fpos < m->offset + m->size) {
> > -			tsz = min_t(size_t, m->offset + m->size - *fpos, buflen);
> > +			tsz = (size_t)min_t(unsigned long long,
> > +					    m->offset + m->size - *fpos,
> > +					    buflen);
> 
> This is rather a mess.  Can we please try to fix this bug by choosing
> appropriate types rather than all the typecasting?

file read/mmap buflen is size_t, so tsz is alwyas less then buflen unless
m->offset + m->size - *fpos < buflen. The only problem is we need avoid large
value of m->offset + m->size - *fpos being casted thus it will mistakenly be
less than buflen.

> 
> 
> >  			start = m->paddr + *fpos - m->offset;
> >  			tmp = read_from_oldmem(buffer, tsz, &start, userbuf);
> >  			if (tmp < 0)
> > @@ -461,7 +463,8 @@ static int mmap_vmcore(struct file *file
> >  		if (start < m->offset + m->size) {
> >  			u64 paddr = 0;
> >  
> > -			tsz = min_t(size_t, m->offset + m->size - start, size);
> > +			tsz = (size_t)min_t(unsigned long long,
> > +					    m->offset + m->size - start, size);
> >  			paddr = m->paddr + start - m->offset;
> >  			if (vmcore_remap_oldmem_pfn(vma, vma->vm_start + len,
> >  						    paddr >> PAGE_SHIFT, tsz,

Thanks
Dave