Subject: Re: [PATCH 02/64] mac80211: Use flex-array for radiotap header bitmap

From: Kees Cook <[email-masked]>
Date: Wed, 28 Jul 2021 16:33:18 -0700

On Wed, Jul 28, 2021 at 10:35:56AM +0300, Dan Carpenter wrote:
> On Tue, Jul 27, 2021 at 01:57:53PM -0700, Kees Cook wrote:
> > [...]
> > -	/**
> > -	 * @it_present: (first) present word
> > -	 */
> > -	__le32 it_present;
> > +	union {
> > +		/**
> > +		 * @it_present: (first) present word
> > +		 */
> > +		__le32 it_present;
> > +
> > +		struct {
> > +			/* The compiler makes it difficult to overlap
> > +			 * a flex-array with an existing singleton,
> > +			 * so we're forced to add an empty named
> > +			 * variable here.
> > +			 */
> > +			struct { } __unused;
> > +
> > +			/**
> > +			 * @bitmap: all presence bitmaps
> > +			 */
> > +			__le32 bitmap[];
> > +		};
> > +	};
> >  } __packed;
> 
> This patch is so confusing...
> 
> Btw, after the end of the __le32 data there is a bunch of other le64,
> u8 and le16 data so the struct is not accurate or complete.
> 
> It might be better to re-write this as something like this:
> 
> diff --git a/include/net/ieee80211_radiotap.h b/include/net/ieee80211_radiotap.h
> index c0854933e24f..0cb5719e9668 100644
> --- a/include/net/ieee80211_radiotap.h
> +++ b/include/net/ieee80211_radiotap.h
> @@ -42,7 +42,10 @@ struct ieee80211_radiotap_header {
>  	/**
>  	 * @it_present: (first) present word
>  	 */
> -	__le32 it_present;
> +	struct {
> +		__le32 it_present;
> +		char buff[];
> +	} data;
>  } __packed;

Ah-ha, got it:

diff --git a/include/net/ieee80211_radiotap.h b/include/net/ieee80211_radiotap.h
index c0854933e24f..6b7274edb3c6 100644
--- a/include/net/ieee80211_radiotap.h
+++ b/include/net/ieee80211_radiotap.h
@@ -43,6 +43,10 @@ struct ieee80211_radiotap_header {
 	 * @it_present: (first) present word
 	 */
 	__le32 it_present;
+	/**
+	 * @it_optional: all remaining presence bitmaps
+	 */
+	__le32 it_optional[];
 } __packed;
 
 /* version is always 0 */
diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
index 2563473b5cf1..b6a960d37278 100644
--- a/net/mac80211/rx.c
+++ b/net/mac80211/rx.c
@@ -359,7 +359,13 @@ ieee80211_add_rx_radiotap_header(struct ieee80211_local *local,
 
 	put_unaligned_le32(it_present_val, it_present);
 
-	pos = (void *)(it_present + 1);
+	/*
+	 * This references through an offset into it_optional[] rather
+	 * than via it_present otherwise later uses of pos will cause
+	 * the compiler to think we have walked past the end of the
+	 * struct member.
+	 */
+	pos = (void *)&rthdr->it_optional[it_present - rthdr->it_optional];
 
 	/* the order of the following fields is important */
 
diff --git a/net/wireless/radiotap.c b/net/wireless/radiotap.c
index 36f1b59a78bf..081f0a3bdfe1 100644
--- a/net/wireless/radiotap.c
+++ b/net/wireless/radiotap.c
@@ -115,10 +115,9 @@ int ieee80211_radiotap_iterator_init(
 	iterator->_max_length = get_unaligned_le16(&radiotap_header->it_len);
 	iterator->_arg_index = 0;
 	iterator->_bitmap_shifter = get_unaligned_le32(&radiotap_header->it_present);
-	iterator->_arg = (uint8_t *)radiotap_header + sizeof(*radiotap_header);
+	iterator->_arg = (uint8_t *)radiotap_header->it_optional;
 	iterator->_reset_on_ext = 0;
-	iterator->_next_bitmap = &radiotap_header->it_present;
-	iterator->_next_bitmap++;
+	iterator->_next_bitmap = radiotap_header->it_optional;
 	iterator->_vns = vns;
 	iterator->current_namespace = &radiotap_ns;
 	iterator->is_radiotap_ns = 1;

-- 
Kees Cook