Re: [PATCH] net: Allow mac_pton() to work on non-NULL terminated strings

[Andrew Lunn <andrew@lunn.ch>]

On Fri, Feb 23, 2018 at 09:17:48PM +0100, Stefan Hellermann wrote:
> Commit 4904dbda41c8 ("ARM: orion5x: use mac_pton() helper") crashes my
> QNAP TS-209 NAS early on boot.
> 
> The boot code for the TS-209 is looping through an ext2 filesystem on a
> 384kB mtd partition (factory configuration put there by QNAP). There it
> looks on every 1kB boundary if there is a valid MAC address. The
> filesystem has a 1kB block size, so this seems to work.
> 
> On my device the MAC address is on the 37th 1kB block. But: On the 27th
> block is a large file (1,5kB) without 0 bytes inside. The code in
> qnap_tsx09_find_mac_addr() maps 1kB into memory (not a whole file or the
> whole 384kB) and then calls qnap_tsx09_check_mac_addr() -> mac_pton() ->
> strlen() on this memory block. as there is no 0 byte in the file on the
> 27th block, strlen() runs into bad memory and the machine panics. The old
> code had no strlen().
> 
> Actually mac_pton() doesn't need to call strlen(), the following loop
> catches short strings quite nicely. The strlen() seems to be an
> optimization for calls to mac_pton with empty string. But this is rarely
> the case and this is not a hot path. Remove it to reduce code size and
> speed up calls with an not empty string.
> 
> Besides fixing the crash there is are other users interested in
> this change, see https://patchwork.ozlabs.org/patch/851008/
> 
> Fixes: 4904dbda41c8 ("ARM: orion5x: use mac_pton() helper")
> Signed-off-by: Stefan Hellermann <stefan@the2masters.de>
> Cc: <stable@vger.kernel.org> [4.4+]

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew