Re: [PATCH] drbd: reject data replies with an out-of-range payload size

["Christoph Böhmwalder" <christoph.boehmwalder@linbit.com>]

On Thu, Jul 09, 2026 at 10:28:37PM -0400, Michael Bommarito wrote:
>recv_dless_read() receives a P_DATA_REPLY from a peer into the bio of an
>outstanding read request. The peer-supplied payload length reaches it as
>the signed int data_size, and two peer-controlled inputs can make it
>negative. With a negotiated data-integrity-alg the digest length is
>subtracted first, so a reply whose payload is smaller than the digest
>underflows data_size. With no integrity algorithm (the default) data_size
>is assigned from the unsigned h95/h100 wire length and drbdd() never
>bounds it for a payload-carrying command, so a length above INT_MAX casts
>it negative; this path needs no non-default feature. The bio receive loop
>then computes expect = min_t(int, data_size, bv_len), which is negative,
>and drbd_recv_all_warn(mapped, expect) receives with a size_t of SIZE_MAX
>into the first mapped page.
>
>The sibling receive path read_in_block() is not affected: it uses an
>unsigned size and rejects it against DRBD_MAX_BIO_SIZE before receiving.
>Reject a data reply whose size is negative after the optional digest
>subtraction, covering both triggers.
>
>Impact: a malicious or man-in-the-middle DRBD peer copies attacker-chosen
>bytes past a bio page in the receiver, corrupting kernel memory. A node
>that reads from its peer (a diskless node, or read-balancing to the peer)
>is exposed in the default configuration; data-integrity-alg is not
>required.
>
>Fixes: b411b3637fa7 ("The DRBD driver")
>Cc: stable@vger.kernel.org
>Assisted-by: Codex:gpt-5-5-xhigh
>Assisted-by: Claude:claude-opus-4-8
>Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>

Looks correct, thanks.

Note that DRBD usually chooses to trusts its peers by design, but I
agree that memory corruption is a bad enough consequence that we
should put the guard there regardless.

Reviewed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>