[PATCH v2] scsi: scsi_debug: fix REPORT ZONES alloc_len underflow OOB write

[Ibrahim Hashimov <security@auditcode.ai>]

resp_report_zones() reads the REPORT ZONES(16) ALLOCATION LENGTH field
(cmd[10..13]) into the unsigned alloc_len and, apart from the
alloc_len == 0 fast path, uses it without flooring it against the
64-byte report header:

	rep_max_zones = (alloc_len - 64) >> ilog2(RZONES_DESC_HD);
	arr = kzalloc(alloc_len, GFP_ATOMIC | __GFP_NOWARN);
	...
	desc = arr + 64;

For any alloc_len in the range 1..63, alloc_len - 64 wraps around
(alloc_len and rep_max_zones are unsigned), so rep_max_zones becomes a
huge value instead of zero. At the same time arr is allocated with the
raw alloc_len, which is smaller than the 64-byte header the function
always builds, and desc is set to arr + 64, already past the end of the
allocation. The report header stores (put_unaligned_be32 at arr+0,
put_unaligned_be64 at arr+8 and arr+16) can then run past a sub-24-byte
buffer, and the per-zone descriptor loop, no longer bounded by the
inflated rep_max_zones, writes 64-byte descriptors from desc onward,
producing a slab out-of-bounds write.

Fix it the way ZBC and SPC require: allocation length truncation is not
an error, and a small alloc_len is a legitimate probe a host uses to
read the zone list length before allocating a full buffer. Clamp
rep_max_zones to zero when alloc_len is below the header size so no
descriptor is emitted, and size the allocation to at least the header
so the unconditional 64-byte header build cannot overflow. The existing
copy-out already truncates the result with
fill_from_dev_buffer(scp, arr, min_t(u32, alloc_len, rep_len)), so the
host still receives exactly the alloc_len bytes it asked for. There is
no functional change for alloc_len >= 64.

This supersedes the previous approach of rejecting a sub-header
allocation length with a check condition, which would have broken those
legitimate small-alloc_len probes.

Verified on a v6.19 KASAN build: with scsi_debug loaded as
zbc=host-managed, issuing REPORT ZONES(16) via SG_IO with alloc_len=32
triggers a KASAN slab-out-of-bounds write in resp_report_zones()
before this change, and the same command produces no report once the
clamp and allocation floor are applied. Reproduction requires
CAP_SYS_RAWIO to submit the raw CDB.

Fixes: 7db0e0c8190a ("scsi: scsi_debug: Fix buffer size of REPORT ZONES command")
Cc: stable@vger.kernel.org
Signed-off-by: Ibrahim Hashimov <security@auditcode.ai>
Assisted-by: AuditCode-AI:2026.07
---
v2: address sashiko-bot review of v1
(https://lore.kernel.org/linux-scsi/20260709150631.45018-1-security@auditcode.ai/):
rejecting a sub-header allocation length with a check condition violates the
ZBC/SPC rule that allocation-length truncation is not an error and breaks
legitimate small-alloc_len zone-list-length probes. Instead clamp rep_max_zones
to zero and floor the allocation at the 64-byte header, letting the existing
min(alloc_len, rep_len) copy-out return the truncated header. No functional
change for alloc_len >= 64.
 drivers/scsi/scsi_debug.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 9d1c9c41d0f9..a21d76fe35f6 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -5911,9 +5911,11 @@ 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 = (alloc_len < RZONES_DESC_HD) ? 0 :
+			(alloc_len - RZONES_DESC_HD) >> ilog2(RZONES_DESC_HD);
 
-	arr = kzalloc(alloc_len, GFP_ATOMIC | __GFP_NOWARN);
+	arr = kzalloc(max_t(u32, alloc_len, RZONES_DESC_HD),
+		      GFP_ATOMIC | __GFP_NOWARN);
 	if (!arr) {
 		mk_sense_buffer(scp, ILLEGAL_REQUEST, INSUFF_RES_ASC,
 				INSUFF_RES_ASCQ);
-- 
2.50.1 (Apple Git-155)