Date: Fri, 11 Mar 2016 12:27:11 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Dave Young <dyoung@redhat.com>
Cc: linux-kernel@vger.kernel.org, d.hatayama@jp.fujitsu.com, bhe@redhat.com,
        vgoyal@redhat.com, kexec@lists.infradead.org, nasa4836@gmail.com,
        mhuang@redhat.com
Subject: Re: [PATCH V2] proc-vmcore: wrong data type casting fix
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References: <20160311084248.GA9417@dhcp-128-65.nay.redhat.com>
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On Fri, 11 Mar 2016 16:42:48 +0800 Dave Young <dyoung@redhat.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.

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@redhat.com>
> Signed-off-by: Dave Young <dyoung@redhat.com>

I think we'll need a cc:stable here.

> --- 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?


>  			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,