On Thu, Aug 16, 2012 at 3:14 AM, David Gibson
<da...@gibson.dropbear.id.au> wrote:
> On Wed, Aug 15, 2012 at 02:41:56PM +0100, Stefan Hajnoczi wrote:
>> On Sat, Aug 11, 2012 at 01:33:42PM +0100, Stefan Hajnoczi wrote:
>> > On Sat, Aug 11, 2012 at 12:11 AM, Peter Crosthwaite
>> > <peter.crosthwa...@petalogix.com> wrote:
>> > > On Fri, Aug 10, 2012 at 11:42 PM, Stefan Hajnoczi <stefa...@gmail.com>
>> > > wrote:
>> > >> On Fri, Aug 10, 2012 at 01:54:26PM +1000, Peter A. G. Crosthwaite wrote:
>> > >>> The sizep arg is populated with the size of the loaded device tree.
>> > >>> Since this
>> > >>> is one of those informational "please populate" type arguments it
>> > >>> should be
>> > >>> optional. Guarded writes to *sizep against NULL accordingly.
>> > >>>
>> > >>> Signed-off-by: Peter A. G. Crosthwaite
>> > >>> <peter.crosthwa...@petalogix.com>
>> > >>> Acked-by: Alexander Graf <ag...@suse.de>
>> > >>> ---
>> > >>> device_tree.c | 8 ++++++--
>> > >>> 1 files changed, 6 insertions(+), 2 deletions(-)
>> > >>>
>> > >>> diff --git a/device_tree.c b/device_tree.c
>> > >>> index d7a9b6b..641a48a 100644
>> > >>> --- a/device_tree.c
>> > >>> +++ b/device_tree.c
>> > >>> @@ -71,7 +71,9 @@ void *load_device_tree(const char *filename_path,
>> > >>> int *sizep)
>> > >>> int ret;
>> > >>> void *fdt = NULL;
>> > >>>
>> > >>> - *sizep = 0;
>> > >>> + if (sizep) {
>> > >>> + *sizep = 0;
>> > >>> + }
>> > >>> dt_size = get_image_size(filename_path);
>> > >>> if (dt_size < 0) {
>> > >>> printf("Unable to get size of device tree file '%s'\n",
>> > >>> @@ -104,7 +106,9 @@ void *load_device_tree(const char *filename_path,
>> > >>> int *sizep)
>> > >>> filename_path);
>> > >>> goto fail;
>> > >>> }
>> > >>> - *sizep = dt_size;
>> > >>> + if (sizep) {
>> > >>> + *sizep = dt_size;
>> > >>> + }
>> > >>
>> > >> What can the caller do with this void* buffer without knowing its size?
>> > >>
>> > >
>> > > Sanity check the machine:
>> > >
>> > > dtb = load_device_tree( ... ); //dont care how big it is
>> > > foo = fdt_gep_prop( dtb, ... );
>> > > if (foo != object_get_prop(foo_device, foo_prop, ... )) {
>> > > hw_error("your dtb is bad because ... !\n", ... );
>> > > }
>> >
>> > What happens if the fdt is corrupt or malicious? I guess we'll access
>> > memory beyond the end of blob.
>> >
>> > This seems to be libfdt's fault. I didn't see an API to validate the
>> > blob's size.
>> >
>> > I'm "happy" with this patch but if fdt's can ever come from untrusted
>> > sources then we're in trouble.
>>
>> Jon/David, can you confirm that libfdt has no way of check the size of
>> the fdt blob?
>
> That's not rentirely true.
>
>> For example, if I pass a corrupt or malicious blob to libfdt, is there a
>> way to detect that or will it access memory beyond the end of the blob
>> as we query the device tree?
>
> So, libfdt does trust the blob size that's given in the blob header,
> since libfdt itself doesn't really have any other source for the
> blob/buffer size. If you have another source for your buffer size
> though, you can check that quite easily against fdt_totalsize(blob)
> (which returns the header value). If you can think of a helper
> function that would make this easier, I'd be happy to add it to
> libfdt.
>
> Once the header size is validated, though, libfdt *is* supposed to be
> safe against a corrupt or malicious blob. I can't guarantee that we
> don't have bugs here, but any crash on malicious data I do consider a
> bug and will fix once I'm aware of it.
David:
fdt_check_header() does not check off_dt_struct, off_dt_strings,
off_mem_rsvmap, size_dt_strings, size_dt_struct against the blob size.
For example, fdt_get_mem_rsv() will access out-of-bounds memory if
off_mem_rsvmap is invalid. Or another example, fdt_offset_ptr() does
bounds checking on offset + len but only against the size_dt_struct
header field, which was never checked against the blob size.
Having the user check fdt_totalsize(blob) is not enough. libfdt
itself needs to use the blob's external size to validate the fdt
header. Something like:
/**
* fdt_check_header_size - sanity check a device tree's size
* @fdt: pointer to a flattened device tree
* @size: fdt size in bytes
*
* fdt_check_header_size() checks that the given flattened
* device tree header describes a data layout that fits within
* the given size limit. Use this to check untrusted fdt input
* immediately after calling fdt_check_header() and before calling
* other functions.
*
* returns:
* 0, if the fdt fits within the given size limit
* -FDT_ERR_NOSPACE, fdt would exceed given size
* -FDT_ERR_BADMAGIC,
* -FDT_ERR_BADVERSION, standard meanings
*/
int fdt_check_blob_size(const void *fdt, size_t size);
Also, fdt_string() documentation says the function returns NULL if
stroffset is out of bounds. The implementation does not check and
will return an out-of-bounds pointer.
Peter:
When libfdt adds the fdt_check_block_size() function then the QEMU
patch is no longer useful since the header size should be validate
(this requires a non-NULL size argument). I suggest we drop the
patch.
Stefan
