[PATCH net] eth: fbnic: validate the Rx completion buffer index

[PATCH net] eth: fbnic: validate the Rx completion buffer index

[Yehyeong Lee]

The buffer ID in an address/length Rx completion descriptor is a 16 bit
field, FBNIC_RCD_AL_BUFF_PAGE_MASK being DESC_GENMASK(15, 0) once the
fragment bits are removed, so the device can name any index from 0 to
65535.  fbnic_pkt_prepare() and fbnic_add_rx_frag() feed that value
straight into fbnic_page_pool_get_head() and fbnic_page_pool_get_data(),
which use it to subscript qt->sub0.rx_buf[] and qt->sub1.rx_buf[].

Those arrays only cover the buffers the driver posted.  They are
allocated with size_mask + 1 entries, and size_mask comes from
hpq_size / FBNIC_BD_FRAG_COUNT, so at the default FBNIC_HPQ_SIZE_DEFAULT
of 256 each array holds 256 entries of 16 bytes, that is 4096 bytes.  A
buffer ID of 65535 reaches 1048560 bytes past the start of the
allocation.

The fill side already masks every index it computes against
bdq->size_mask, and so does all of the ring head and tail arithmetic.
The completion side does not.  A descriptor naming a buffer that was
never posted makes fbnic_page_pool_get_head() decrement pagecnt_bias
through a pointer outside the array and hand the out-of-bounds
netmem_ref back as a struct page; page_pool_get_dma_addr() and
page_address() then dereference it and the skb is built on whatever it
points at.

Injecting an out-of-range header completion reports under KASAN:

  BUG: KASAN: slab-out-of-bounds in fbnic_page_pool_get_head
  Read of size 8 at addr ffff88800c719008
  The buggy address is located 8 bytes to the right of
   allocated 4096-byte region [ffff88800c718000, ffff88800c719000)

Check the ID against size_mask before it is used and drop the packet if
it is out of range.  Masking it instead would keep the access in bounds
but would silently attribute the completion to an unrelated buffer that
the driver does own, which is worse than a drop.  Set add_frag_failed so
the frame takes the existing error path.

Rejecting the header completion leaves pkt->buff uninitialized, and the
payload completions that follow are handled before the metadata
descriptor that looks at add_frag_failed, so fbnic_add_rx_frag() has to
bail out as well or xdp_buff_add_frag() writes through a shared info
pointer derived from a NULL data_hard_start.  Key that on
data_hard_start, the way fbnic_put_pkt_buff() already does, so the
buffer accounting on the normal path is untouched.

Fixes: a29b8eb6e533 ("eth: fbnic: Add basic Rx handling")
Cc: stable@vger.kernel.org
Signed-off-by: Yehyeong Lee <yhlee@isslab.korea.ac.kr>
---
 drivers/net/ethernet/meta/fbnic/fbnic_txrx.c | 27 ++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c b/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
index 10caacffee0f0..d85daab06c99c 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
+++ b/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
@@ -985,14 +985,24 @@ static void fbnic_pkt_prepare(struct fbnic_napi_vector *nv, u64 rcd,
 {
 	unsigned int hdr_pg_idx = FIELD_GET(FBNIC_RCD_AL_BUFF_PAGE_MASK, rcd);
 	unsigned int hdr_pg_off = FIELD_GET(FBNIC_RCD_AL_BUFF_OFF_MASK, rcd);
-	struct page *page = fbnic_page_pool_get_head(qt, hdr_pg_idx);
 	unsigned int len = FIELD_GET(FBNIC_RCD_AL_BUFF_LEN_MASK, rcd);
 	unsigned int frame_sz, hdr_pg_start, hdr_pg_end, headroom;
 	unsigned char *hdr_start;
+	struct page *page;
 
 	/* data_hard_start should always be NULL when this is called */
 	WARN_ON_ONCE(pkt->buff.data_hard_start);
 
+	/* A buffer ID the device never got from us cannot name a buffer we
+	 * own, so drop the packet instead of running off the end of rx_buf[].
+	 */
+	if (unlikely(hdr_pg_idx > qt->sub0.size_mask)) {
+		pkt->add_frag_failed = true;
+		return;
+	}
+
+	page = fbnic_page_pool_get_head(qt, hdr_pg_idx);
+
 	/* Short-cut the end calculation if we know page is fully consumed */
 	hdr_pg_end = FIELD_GET(FBNIC_RCD_AL_PAGE_FIN, rcd) ?
 		     FBNIC_BD_FRAG_SIZE : fbnic_hdr_pg_end(hdr_pg_off, len);
@@ -1026,10 +1036,23 @@ static void fbnic_add_rx_frag(struct fbnic_napi_vector *nv, u64 rcd,
 	unsigned int pg_idx = FIELD_GET(FBNIC_RCD_AL_BUFF_PAGE_MASK, rcd);
 	unsigned int pg_off = FIELD_GET(FBNIC_RCD_AL_BUFF_OFF_MASK, rcd);
 	unsigned int len = FIELD_GET(FBNIC_RCD_AL_BUFF_LEN_MASK, rcd);
-	netmem_ref netmem = fbnic_page_pool_get_data(qt, pg_idx);
 	unsigned int truesize;
+	netmem_ref netmem;
 	bool added;
 
+	/* The header completion was rejected, so buff was never initialized
+	 * and add_frag_failed is already set.  There is nothing to add to.
+	 */
+	if (unlikely(!pkt->buff.data_hard_start))
+		return;
+
+	if (unlikely(pg_idx > qt->sub1.size_mask)) {
+		pkt->add_frag_failed = true;
+		return;
+	}
+
+	netmem = fbnic_page_pool_get_data(qt, pg_idx);
+
 	truesize = FIELD_GET(FBNIC_RCD_AL_PAGE_FIN, rcd) ?
 		   FBNIC_BD_FRAG_SIZE - pg_off : ALIGN(len, 128);
 
-- 
2.43.0

Re: [PATCH net] eth: fbnic: validate the Rx completion buffer index

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

 - What hardware the change was tested on. For driver fixes please
   mention the device (and if relevant firmware version) used for
   testing, or say that the change was not tested on real hardware.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.

Re: [PATCH net] eth: fbnic: validate the Rx completion buffer index

[Daniel Zahka]

On Fri Oct 9, 2026 at 1:53 PM EDT, Yehyeong Lee wrote:
> The buffer ID in an address/length Rx completion descriptor is a 16 bit
> field, FBNIC_RCD_AL_BUFF_PAGE_MASK being DESC_GENMASK(15, 0) once the
> fragment bits are removed, so the device can name any index from 0 to
> 65535.  fbnic_pkt_prepare() and fbnic_add_rx_frag() feed that value
> straight into fbnic_page_pool_get_head() and fbnic_page_pool_get_data(),
> which use it to subscript qt->sub0.rx_buf[] and qt->sub1.rx_buf[].
>
> Those arrays only cover the buffers the driver posted.  They are
> allocated with size_mask + 1 entries, and size_mask comes from
> hpq_size / FBNIC_BD_FRAG_COUNT, so at the default FBNIC_HPQ_SIZE_DEFAULT
> of 256 each array holds 256 entries of 16 bytes, that is 4096 bytes.  A
> buffer ID of 65535 reaches 1048560 bytes past the start of the
> allocation.
>
> The fill side already masks every index it computes against
> bdq->size_mask, and so does all of the ring head and tail arithmetic.
> The completion side does not.  A descriptor naming a buffer that was
> never posted makes fbnic_page_pool_get_head() decrement pagecnt_bias
> through a pointer outside the array and hand the out-of-bounds
> netmem_ref back as a struct page; page_pool_get_dma_addr() and
> page_address() then dereference it and the skb is built on whatever it
> points at.
>
> Injecting an out-of-range header completion reports under KASAN:

How exactly are you injecting the completion? Just overwriting the id
field in the Rx completion with an arbitrary value from the driver?

The buffer id is chosen by SW when the BD is submitted to the device,
and the device just echoes it back like a cookie when that buffer is
used.

>
>   BUG: KASAN: slab-out-of-bounds in fbnic_page_pool_get_head
>   Read of size 8 at addr ffff88800c719008
>   The buggy address is located 8 bytes to the right of
>    allocated 4096-byte region [ffff88800c718000, ffff88800c719000)
>
> Check the ID against size_mask before it is used and drop the packet if
> it is out of range.  Masking it instead would keep the access in bounds
> but would silently attribute the completion to an unrelated buffer that
> the driver does own, which is worse than a drop.  Set add_frag_failed so
> the frame takes the existing error path.
>
> Rejecting the header completion leaves pkt->buff uninitialized, and the
> payload completions that follow are handled before the metadata
> descriptor that looks at add_frag_failed, so fbnic_add_rx_frag() has to
> bail out as well or xdp_buff_add_frag() writes through a shared info
> pointer derived from a NULL data_hard_start.  Key that on
> data_hard_start, the way fbnic_put_pkt_buff() already does, so the
> buffer accounting on the normal path is untouched.
>
> Fixes: a29b8eb6e533 ("eth: fbnic: Add basic Rx handling")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yehyeong Lee <yhlee@isslab.korea.ac.kr>
> ---
>  drivers/net/ethernet/meta/fbnic/fbnic_txrx.c | 27 ++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c b/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
> index 10caacffee0f0..d85daab06c99c 100644
> --- a/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
> +++ b/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
> @@ -985,14 +985,24 @@ static void fbnic_pkt_prepare(struct fbnic_napi_vector *nv, u64 rcd,
>  {
>  	unsigned int hdr_pg_idx = FIELD_GET(FBNIC_RCD_AL_BUFF_PAGE_MASK, rcd);
>  	unsigned int hdr_pg_off = FIELD_GET(FBNIC_RCD_AL_BUFF_OFF_MASK, rcd);
> -	struct page *page = fbnic_page_pool_get_head(qt, hdr_pg_idx);
>  	unsigned int len = FIELD_GET(FBNIC_RCD_AL_BUFF_LEN_MASK, rcd);
>  	unsigned int frame_sz, hdr_pg_start, hdr_pg_end, headroom;
>  	unsigned char *hdr_start;
> +	struct page *page;
>  
>  	/* data_hard_start should always be NULL when this is called */
>  	WARN_ON_ONCE(pkt->buff.data_hard_start);
>  
> +	/* A buffer ID the device never got from us cannot name a buffer we
> +	 * own, so drop the packet instead of running off the end of rx_buf[].
> +	 */
> +	if (unlikely(hdr_pg_idx > qt->sub0.size_mask)) {
> +		pkt->add_frag_failed = true;
> +		return;
> +	}
> +
> +	page = fbnic_page_pool_get_head(qt, hdr_pg_idx);
> +
>  	/* Short-cut the end calculation if we know page is fully consumed */
>  	hdr_pg_end = FIELD_GET(FBNIC_RCD_AL_PAGE_FIN, rcd) ?
>  		     FBNIC_BD_FRAG_SIZE : fbnic_hdr_pg_end(hdr_pg_off, len);
> @@ -1026,10 +1036,23 @@ static void fbnic_add_rx_frag(struct fbnic_napi_vector *nv, u64 rcd,
>  	unsigned int pg_idx = FIELD_GET(FBNIC_RCD_AL_BUFF_PAGE_MASK, rcd);
>  	unsigned int pg_off = FIELD_GET(FBNIC_RCD_AL_BUFF_OFF_MASK, rcd);
>  	unsigned int len = FIELD_GET(FBNIC_RCD_AL_BUFF_LEN_MASK, rcd);
> -	netmem_ref netmem = fbnic_page_pool_get_data(qt, pg_idx);
>  	unsigned int truesize;
> +	netmem_ref netmem;
>  	bool added;
>  
> +	/* The header completion was rejected, so buff was never initialized
> +	 * and add_frag_failed is already set.  There is nothing to add to.
> +	 */
> +	if (unlikely(!pkt->buff.data_hard_start))
> +		return;
> +
> +	if (unlikely(pg_idx > qt->sub1.size_mask)) {
> +		pkt->add_frag_failed = true;
> +		return;
> +	}
> +
> +	netmem = fbnic_page_pool_get_data(qt, pg_idx);
> +
>  	truesize = FIELD_GET(FBNIC_RCD_AL_PAGE_FIN, rcd) ?
>  		   FBNIC_BD_FRAG_SIZE - pg_off : ALIGN(len, 128);
>  

Re: [PATCH net] eth: fbnic: validate the Rx completion buffer index

[Yehyeong Lee]

On Fri Oct 9, 2026 at 2:14 PM EDT, Daniel Zahka wrote:
> How exactly are you injecting the completion? Just overwriting the id
> field in the Rx completion with an arbitrary value from the driver?

Yes.  No emulated fbnic, so this isn't from real hardware: on a KASAN
build I ran the completion handlers with FBNIC_RCD_AL_BUFF_PAGE_MASK set
out of range and saw the out-of-bounds access, which the patch removes.
Found by reading the code, not from a report.

> The buffer id is chosen by SW when the BD is submitted to the device,
> and the device just echoes it back like a cookie when that buffer is
> used.

Agreed, a conforming device stays in range: fbnic_bd_prep() puts the
ring slot i, already masked to size_mask in fbnic_fill_bdq(), into
FBNIC_BD_PAGE_ID_MASK, and rx_buf[i] is that slot, so every id the
driver hands out is in range by construction.

The completion path is the one place that takes the id back and uses it
as the rx_buf[] subscript without bounding it.  A device that returns an
id it was never given then gets an out-of-bounds pagecnt_bias decrement
and an out-of-bounds netmem dereference: a device-triggered memory
safety bug, just not one a conforming device reaches.  The patch bounds
the index and drops the frame for one comparison per completion.