Re: [PATCH] wifi: mwifiex: Fix memcpy() field-spanning write warning in mwifiex_config_scan()

[Brian Norris]

On Tue, Sep 17, 2024 at 06:08:41PM +0300, Alper Nebi Yasak wrote:
> Replace one-element array with a flexible-array member in `struct
> mwifiex_ie_types_wildcard_ssid_params` to fix the following warning
> on a MT8173 Chromebook (mt8173-elm-hana):
> 
> [  356.775250] ------------[ cut here ]------------
> [  356.784543] memcpy: detected field-spanning write (size 6) of single field "wildcard_ssid_tlv->ssid" at drivers/net/wireless/marvell/mwifiex/scan.c:904 (size 1)
> [  356.813403] WARNING: CPU: 3 PID: 742 at drivers/net/wireless/marvell/mwifiex/scan.c:904 mwifiex_scan_networks+0x4fc/0xf28 [mwifiex]
> 
> The "(size 6)" above is exactly the length of the SSID of the network
> this device was connected to. The source of the warning looks like:
> 
>     ssid_len = user_scan_in->ssid_list[i].ssid_len;
>     [...]
>     memcpy(wildcard_ssid_tlv->ssid,
>            user_scan_in->ssid_list[i].ssid, ssid_len);
> 
> Also adjust a #define that uses sizeof() on this struct to keep the
> value same as before.
> 
> Signed-off-by: Alper Nebi Yasak <alpernebiyasak@xxxxxxxxx>
> ---
> I found these relevant patches that modify other such arrays, where the
> second one removes a -1 from some sizeof() calculation:
> 
> https://lore.kernel.org/lkml/Y9xkECG3uTZ6T1dN@work/T/#u
> https://lore.kernel.org/lkml/ZsZa5xRcsLq9D+RX@elsanto/T/#u
> 
> So I think we need the +1 to keep things same. But it appears to work
> fine without it, so I'm not sure. Maybe it should've had a -1 before
> that I would remove with this?

Thanks for the investigation and patch! I believe I agree with the other
comments, that then "+ 1" isn't necessary. It's just a wasteful extra
byte of allocation. Can you send a v2? (Bonus: with the suggested Fixes
tag. Double bonus if you test KASAN with __counted_by, for a second
patch.)

Thanks,
Brian