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Date:   Mon, 16 Jan 2023 16:08:33 +0300
From:   Dan Carpenter <error27@gmail.com>
To:     Baoquan He <bhe@redhat.com>
Cc:     oe-kbuild@lists.linux.dev, linux-mm@kvack.org, lkp@intel.com,
        oe-kbuild-all@lists.linux.dev, linux-kernel@vger.kernel.org,
        urezki@gmail.com, lstoakes@gmail.com, stephen.s.brennan@oracle.com,
        willy@infradead.org, akpm@linux-foundation.org, hch@infradead.org
Subject: Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out
 vm_map_ram areas
Message-ID: <Y8VMUUOlkwuu5xn6@kadam>
References: <20230113031921.64716-4-bhe@redhat.com>
 <202301132345.KVjvHMFq-lkp@intel.com>
 <Y8QI977QBDbuuGW5@fedora>
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On Sun, Jan 15, 2023 at 10:08:55PM +0800, Baoquan He wrote:
> > f181234a5a21fd0 Chen Wandun             2021-09-02  3650  	if ((unsigned long)addr + count <= va->va_start)
> > f181234a5a21fd0 Chen Wandun             2021-09-02  3651  		goto finished;
> > f181234a5a21fd0 Chen Wandun             2021-09-02  3652  
> > f608788cd2d6cae Serapheim Dimitropoulos 2021-04-29  3653  	list_for_each_entry_from(va, &vmap_area_list, list) {
> > e81ce85f960c2e2 Joonsoo Kim             2013-04-29  3654  		if (!count)
> > e81ce85f960c2e2 Joonsoo Kim             2013-04-29  3655  			break;
> > e81ce85f960c2e2 Joonsoo Kim             2013-04-29  3656  
> > 129dbdf298d7383 Baoquan He              2023-01-13  3657  		vm = va->vm;
> > 129dbdf298d7383 Baoquan He              2023-01-13  3658  		flags = va->flags & VMAP_FLAGS_MASK;
> > 129dbdf298d7383 Baoquan He              2023-01-13  3659  
> > 129dbdf298d7383 Baoquan He              2023-01-13  3660  		if (!vm && !flags)
> >                                                                             ^^^
> > vm can be NULL if a flag in VMAP_FLAGS_MASK is set.
> > 
> > e81ce85f960c2e2 Joonsoo Kim             2013-04-29  3661  			continue;
> 
> Right, after the 'continue;' line, only two cases could happen when it
> comes here. (vm != null) or (vm->flags & VMAP_RAM) is true.
>

You're saying VMAP_RAM, but strictly speaking the code is checking
VMAP_FLAGS_MASK and not VMAP_RAM.

+#define VMAP_RAM               0x1 /* indicates vm_map_ram area*/
+#define VMAP_BLOCK             0x2 /* mark out the vmap_block sub-type*/
+#define VMAP_FLAGS_MASK                0x3

If we assume that vm is NULL, VMAP_BLOCK is set and VMAP_RAM is clear
then it would lead to a NULL dereference.  There might be reasons why
that combination is impossible outside the function but we can't tell
from the information we have here.

Which is fine, outside information is a common reason for false
positives with this check.  But I was just concerned about the mix of
VMAP_FLAGS_MASK and VMAP_RAM.

regards,
dan carpenter

 
> > e81ce85f960c2e2 Joonsoo Kim             2013-04-29  3662  
> > 129dbdf298d7383 Baoquan He              2023-01-13  3663  		vaddr = (char *) va->va_start;
> > 129dbdf298d7383 Baoquan He              2023-01-13 @3664  		size = vm ? get_vm_area_size(vm) : va_size(va);
> >                                                                                ^^
> > 
> > 129dbdf298d7383 Baoquan He              2023-01-13  3665  
> > 129dbdf298d7383 Baoquan He              2023-01-13  3666  		if (addr >= vaddr + size)
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3667  			continue;
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3668  		while (addr < vaddr) {
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3669  			if (count == 0)
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3670  				goto finished;
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3671  			*buf = '\0';
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3672  			buf++;
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3673  			addr++;
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3674  			count--;
> > ^1da177e4c3f415 Linus Torvalds          2005-04-16  3675  		}
> > 129dbdf298d7383 Baoquan He              2023-01-13  3676  		n = vaddr + size - addr;
> > d0107eb07320b5d KAMEZAWA Hiroyuki       2009-09-21  3677  		if (n > count)
> > d0107eb07320b5d KAMEZAWA Hiroyuki       2009-09-21  3678  			n = count;
> > 129dbdf298d7383 Baoquan He              2023-01-13  3679  
> > 129dbdf298d7383 Baoquan He              2023-01-13  3680  		if (flags & VMAP_RAM)
> > 
> > assume VMAP_RAM is not set
> 
> OK, then vm is not null.
> > 
> > 129dbdf298d7383 Baoquan He              2023-01-13  3681  			vmap_ram_vread(buf, addr, n, flags);
> > 129dbdf298d7383 Baoquan He              2023-01-13 @3682  		else if (!(vm->flags & VM_IOREMAP))
> >                                                                                    ^^^^^^^^^
> > Unchecked dereference.  Should this be "flags" instead of "vm->flags"?
> 
> Thus, here, in 'else if', vm is not null. And in this 'else if', we are
> intending to check vm->flags. I don't see issue or risk here. Please
> help check if I miss anything.
> 
> Thanks
> Baoquan
>