Re: [PATCH RFC] wifi: ath12k: workaround fortify warnings in ath12k_wow_convert_8023_to_80211()

[Kees Cook <kees@kernel.org>]

On Thu, Jul 04, 2024 at 05:43:41PM +0300, Kalle Valo wrote:
> From: Kalle Valo <quic_kvalo@quicinc.com>
> 
> Johannes reported with GCC 11.4 there's a fortify warning below. The warning is
> not seen with GCC 12.1 nor 13.2. Weirdly moving the other operand of sum to the
> other side the warning goes away. This is safe to do as the value of the
> operand is check earlier. But the code looks worse with this so I'm not sure
> what to do.

FWIW, this isn't fortify, but -Wrestrict. I would expect the same
warnings even with CONFIG_FORTIFY_SOURCE disabled. Regardless, it's
worth figuring out what's going on. It looks like this is GCC's value
range tracker deciding it sees a way for things to go weird.

I suspect they fixed -Wrestrict in later GCC versions. It might need to
be version-limited...

> In file included from ./include/linux/string.h:374,
>                  from ./include/linux/bitmap.h:13,
>                  from ./include/linux/cpumask.h:13,
>                  from ./include/linux/sched.h:16,
>                  from ./include/linux/delay.h:23,
>                  from drivers/net/wireless/ath/ath12k/wow.c:7:
> drivers/net/wireless/ath/ath12k/wow.c: In function ‘ath12k_wow_convert_8023_to_80211.constprop’:
> ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ accessing 18446744073709551611 or more bytes at offsets 0 and 0 overlaps 9223372036854775799 bytes at offset -9223372036854775804 [-Werror=restrict]

These huge negative values imply to me that GCC is looking at some
signed values somewhere.

> [...]
> diff --git a/drivers/net/wireless/ath/ath12k/wow.c b/drivers/net/wireless/ath/ath12k/wow.c
> index c5cba825a84a..e9588bb7561c 100644
> --- a/drivers/net/wireless/ath/ath12k/wow.c
> +++ b/drivers/net/wireless/ath/ath12k/wow.c
> @@ -186,7 +186,7 @@ ath12k_wow_convert_8023_to_80211(struct ath12k *ar,
>  	if (eth_pkt_ofs < ETH_ALEN) {
>  		pkt_ofs = eth_pkt_ofs + a1_ofs;
>  
> -		if (eth_pkt_ofs + eth_pat_len < ETH_ALEN) {
> +		if (eth_pat_len < ETH_ALEN - eth_pkt_ofs) {
>  			memcpy(pat, eth_pat, eth_pat_len);
>  			memcpy(bytemask, eth_bytemask, eth_pat_len);

Both eth_pkt_ofs and eth_pat_len are size_t. ETH_ALEN isn't, but it
would be promoted to size_t here. The value tracker should see that
eth_pkt_ofs could be [0..ETH_ALEN). eth_pat_len is coming from an "int",
though, so that might be the confusion. It may think eth_pat_len could
be [0..UINT_MAX] (i.e. the full range of int within size_t).

So [0..ETH_ALEN) + [0..UINT_MAX] < 6 might be doing something wrong in
GCC 11.x, and it's not actually doing the size_t promotion correctly,
or deciding something has wrapped and then thinking eth_pat_len could
span a giant region of the address space, which freaks out -Wrestrict.
i.e. it's seeing that for the "if" to be true, eth_pat_len could be large
enough to wrap around the addition (though this shouldn't be possible
for 64-bit size_t).

So I could see how [0..UINT_MAX] < 6 - [0..ETH_ALEN) would make it
happier: the right side is now [1..6], so eth_pat_len becomes [1..6).

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook