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

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Message-ID: <20160312135932.GA3649@x1.redhat.com>
Date:	Sat, 12 Mar 2016 21:59:32 +0800
From:	Baoquan He <bhe@...hat.com>
To:	xlpang@...hat.com
Cc:	Dave Young <dyoung@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, d.hatayama@...fujitsu.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

On 03/12/16 at 08:43pm, Xunlei Pang wrote:
> On 2016/03/12 at 12:49, Dave Young wrote:
> > 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.
> 
> *
> Can we use "tsz = min(m->offset + m->size - *fpos, buflen)" instead?
> I think it's ok for this case (both have positive values), nothing will go wrong,
> also can make the code cleaner.

We can't. Macro min() has a type checking. min_t is necessary here.

> 
> *
> >>
> >>>  			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
>