Re: [PATCH net v3] nfc: nci: fix use of uninitialized memory in NFC-DEP general bytes

[David Heidelberg]

On 28/07/2026 15:28, Muhammad Bilal wrote:
> nci_store_general_bytes_nfc_dep() derives the length of the NFC-DEP
> general bytes by subtracting the fixed general-bytes offset from the ATR
> length:
>
>    atr_res_len - NFC_ATR_RES_GT_OFFSET   (poll, offset 15)
>    atr_req_len - NFC_ATR_REQ_GT_OFFSET   (listen, offset 14)
>
> It never checks that the ATR is at least that long. When a
> RF_INTF_ACTIVATED_NTF reports an ATR shorter than the offset the
> subtraction is negative; because min_t() casts its arguments to __u8 the
> negative value becomes large and is then capped at
> NFC_ATR_RES_GB_MAXSIZE / NFC_ATR_REQ_GB_MAXSIZE. remote_gb_len is thus
> set to up to 47/48 even though only atr_res_len/atr_req_len bytes of the
> on-stack atr_res/atr_req buffer were copied from the packet, and the
> following memcpy() reads the uninitialized remainder into
> ndev->remote_gb.
>
> Skip storing the general bytes when the ATR is shorter than the
> general-bytes offset. remote_gb_len is already zeroed unconditionally
> at the top of the function by commit 9c328f54741b ("net: nfc: nci: Add
> parameter validation for packet data"), so the short-ATR case needs no
> separate zeroing here anymore, unlike in the v2 posted upstream.
>
> Fixes: a99903ec4566 ("NFC: NCI: Handle Target mode activation")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Muhammad Bilal <meatuni001@xxxxxxxxx>
> ---
> v3: Rebased onto current net.git for-next, requested by David
>      Heidelberg. The function moved and gained an unconditional
>      "remote_gb_len = 0" at entry (from commit 9c328f54741b, landed
>      after v2 was posted), which made v2's explicit zeroing inside the
>      short-ATR branch redundant, so this version drops it and only adds
>      the missing length checks.
> v2: Also zero remote_gb_len explicitly in the short-ATR branch so a
>      stale value from a previous activation doesn't survive into the
>      new session.
>
>   net/nfc/nci/ntf.c | 6 ++++++
>   1 file changed, 6 insertions(+)
Hello Muhammad and Lekë.

I see you recently addressing same issues, such as this patch:

nfc: nci: fix use of uninitialized memory in NFC-DEP general bytes

and later Lekë's:

nfc: nci: fix u8 underflow in nci_store_general_bytes_nfc_dep [1].

As I see you finding similar bugs and addressing them, I would be more than 
happy if you would be interested to sync who's working on what and - that's huge 
value here - cross review your patches, as you working in the similar field. 
Sending good patch is great, but giving a good review can have a huge value and 
move things forwards [2] (as you can see Tested-by and Reviewed-by is important 
number too :) ).

Thank you both for your work
David

[1] https://lore.kernel.org/r/20260729011547.19191-1-snowwlake@xxxxxxxxxx
[2] https://lwn.net/Articles/1077425/