[PATCH v4] scsi: scsi_debug: fix REPORT ZONES alloc_len underflow OOB write

[Ibrahim Hashimov <security@auditcode.ai>]

resp_report_zones() sizes the reply buffer from the CDB allocation
length. The v3 fix rounds alloc_len up with ALIGN() before deriving the
descriptor count:

	rep_max_zones = (ALIGN((u64)alloc_len, RZONES_DESC_HD) -
			 RZONES_DESC_HD) >> ilog2(RZONES_DESC_HD);
	arr_len = (u64)RZONES_DESC_HD * (rep_max_zones + 1);

For alloc_len in 0xFFFFFFC1..0xFFFFFFFF, ALIGN() rounds up to
0x100000000, so arr_len is 4 GB. On 32-bit, kzalloc()'s size_t is
32-bit and truncates 0x100000000 to 0; kzalloc(0) returns
ZERO_SIZE_PTR, which passes the !arr check, and desc = arr + 64 is then
dereferenced in the loop -> out-of-bounds write / panic.

Clamp rep_max_zones to devip->nr_zones. The loop already stops at
sdebug_capacity (after nr_zones zones), so a report can never hold more
than nr_zones descriptors; the clamp does not change the report, it
only bounds arr_len to (nr_zones + 1) * RZONES_DESC_HD, a real device
property that can never reach 0x100000000.

Fixes: 7db0e0c8190a ("scsi: scsi_debug: Fix buffer size of REPORT ZONES command")
Suggested-by: Damien Le Moal <dlemoal@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Ibrahim Hashimov <security@auditcode.ai>
Assisted-by: AuditCode-AI:2026.07
---
v4: clamp rep_max_zones to the device zone count (devip->nr_zones) so arr_len
cannot reach 0x100000000 and be truncated by kzalloc()'s 32-bit size_t on
32-bit platforms (which returns ZERO_SIZE_PTR and bypasses the NULL check),
as reported by sashiko-bot / Bart Van Assche on v3. This restores a bound
that pre-dated the loop refactor: the original REPORT ZONES code already
capped rep_max_zones at the device zone count. The clamp is additive and does
not change any report (the loop already stops at sdebug_capacity), so it only
bounds the allocation. Dropped Damien's v3 Reviewed-by since v4 adds a
functional line he has not reviewed; his ALIGN sizing is otherwise unchanged.
v3: https://lore.kernel.org/linux-scsi/20260710055755.53830-1-security@auditcode.ai/
 drivers/scsi/scsi_debug.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 9d1c9c41d0f9..643051332132 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -5890,6 +5890,7 @@ static int resp_report_zones(struct scsi_cmnd *scp,
 	u32 alloc_len, rep_opts, rep_len;
 	bool partial;
 	u64 lba, zs_lba;
+	u64 arr_len;
 	u8 *arr = NULL, *desc;
 	u8 *cmd = scp->cmnd;
 	struct sdeb_zone_state *zsp = NULL;
@@ -5911,9 +5912,12 @@ static int resp_report_zones(struct scsi_cmnd *scp,
 		return check_condition_result;
 	}
 
-	rep_max_zones = (alloc_len - 64) >> ilog2(RZONES_DESC_HD);
+	rep_max_zones = (ALIGN((u64)alloc_len, RZONES_DESC_HD) - RZONES_DESC_HD) >>
+			ilog2(RZONES_DESC_HD);
+	rep_max_zones = min_t(unsigned int, rep_max_zones, devip->nr_zones);
+	arr_len = (u64)RZONES_DESC_HD * (rep_max_zones + 1);
 
-	arr = kzalloc(alloc_len, GFP_ATOMIC | __GFP_NOWARN);
+	arr = kzalloc(arr_len, GFP_ATOMIC | __GFP_NOWARN);
 	if (!arr) {
 		mk_sense_buffer(scp, ILLEGAL_REQUEST, INSUFF_RES_ASC,
 				INSUFF_RES_ASCQ);
-- 
2.50.1 (Apple Git-155)