[PATCH] RDMA/rxe: reject non-8-byte ATOMIC_WRITE payloads

[Michael Bommarito <michael.bommarito@gmail.com>]

atomic_write_reply() at drivers/infiniband/sw/rxe/rxe_resp.c
unconditionally dereferences 8 bytes at payload_addr(pkt):

    value = *(u64 *)payload_addr(pkt);

check_rkey() previously accepted an ATOMIC_WRITE request with
pktlen == resid == 0 because the length validation only compared
pktlen against resid. A remote initiator that sets the RETH
length to 0 therefore reaches atomic_write_reply() with a
zero-byte logical payload, and the responder reads sizeof(u64)
bytes from past the logical end of the packet into skb->head
tailroom, then writes those 8 bytes into the attacker's MR via
rxe_mr_do_atomic_write(). That is a remote disclosure of 4 bytes
of kernel tailroom per probe (the other 4 bytes are the packet's
own trailing ICRC).

IBA oA19-28 defines ATOMIC_WRITE as exactly 8 bytes. Anything
else is protocol-invalid. Hoist a strict length check into
check_rkey() so the responder never reaches the unchecked
dereference, and keep the existing WRITE-family length logic for
the normal RDMA WRITE path.

Reproduced on mainline with an unmodified rxe driver: a
sustained zero-length ATOMIC_WRITE probe repeatedly leaks
adjacent skb head-buffer bytes into the attacker's MR,
including recognisable kernel strings and partial
kernel-direct-map pointer words.  With this patch applied the
responder rejects the PDU and the MR stays all-zero.

Fixes: 034e285f8b99 ("RDMA/rxe: Make responder support atomic write on RC service")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
Previously reported to security@ (2026-04-18); reposting
publicly at the maintainer's request.

Per-probe evidence from a 100K-packet run on the clean
unpatched tree at 9ca18fc915c6 (single attacker QP against a
hairpin target QP over a veth pair; each probe one crafted
zero-length ATOMIC_WRITE PDU):

    transmitted packets:          100,000
    observed MR writes:            48,575
    non-zero leaked tails:         33,297  (68.55% of observed writes)
    mostly-printable tails:         3,796  (7.81%)
    fully-printable tails:          2,241  (4.61%)
    unique non-zero tails:         22,220

Each probe is a fresh skb head-buffer allocation, so the 4
attacker-visible bytes after the ICRC are an independent
sample of slab-adjacent memory.  Content distribution across
the 48,575 observed writes: 31.45% zero, 4.61% fully
printable, 3.20% mostly printable, 12.06% header/sentinel-
looking (08004500, 08004508, ffffffff, ...), 48.68% other
binary.  80.9% of unique non-zero tails were singletons, so
the leak is not dominated by one repeated value.

Representative printable fragments observed on the attacker
side:

    74 6f 70 2e   "top."
    66 72 65 65   "free"
    45 78 65 63   "Exec"
    2f 73 79 73   "/sys"
    72 6f 6f 74   "root"
    45 56 50 41   "EVPA"
    43 4f 44 45   "CODE"

Partial pointer-like recoveries (4-byte words ending in the
kernel-direct-map prefix 0xffff....):

    3,361 observations ending in ffff
    1,364 unique ....ffff tails
    most common:
      81 88 ff ff   LE 0xffff8881   1.68% of observed writes
      80 88 ff ff   LE 0xffff8880   0.22%

The run did not recover full 64-bit kernel virtual addresses
(only 4 bytes per probe are attacker-observable), but the
partial pointer material is consistent with a KASLR-weakening
primitive under sustained probing.  With the fix applied, the
same harness leaves the attacker MR all-zero.
---
 drivers/infiniband/sw/rxe/rxe_resp.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/infiniband/sw/rxe/rxe_resp.c b/drivers/infiniband/sw/rxe/rxe_resp.c
index 711f73e0bbb1..09ba21d0f3c4 100644
--- a/drivers/infiniband/sw/rxe/rxe_resp.c
+++ b/drivers/infiniband/sw/rxe/rxe_resp.c
@@ -526,7 +526,19 @@ static enum resp_states check_rkey(struct rxe_qp *qp,
 	}
 
 skip_check_range:
-	if (pkt->mask & (RXE_WRITE_MASK | RXE_ATOMIC_WRITE_MASK)) {
+	if (pkt->mask & RXE_ATOMIC_WRITE_MASK) {
+		/* IBA oA19-28: ATOMIC_WRITE payload is exactly 8 bytes.
+		 * Reject any other length before the responder reads
+		 * sizeof(u64) bytes from payload_addr(pkt); a shorter
+		 * payload would read past the logical end of the packet
+		 * into skb->head tailroom.
+		 */
+		if (resid != sizeof(u64) || pktlen != sizeof(u64) ||
+		    bth_pad(pkt)) {
+			state = RESPST_ERR_LENGTH;
+			goto err;
+		}
+	} else if (pkt->mask & RXE_WRITE_MASK) {
 		if (resid > mtu) {
 			if (pktlen != mtu || bth_pad(pkt)) {
 				state = RESPST_ERR_LENGTH;
-- 
2.53.0