[PATCH v3] scsi: scsi_debug: fix REPORT ZONES alloc_len underflow OOB write

[Ibrahim Hashimov <security@auditcode.ai>]

resp_report_zones() derives the number of zone descriptors that fit in
the reply buffer from the command allocation length:

	rep_max_zones = (alloc_len - 64) >> ilog2(RZONES_DESC_HD);

	arr = kzalloc(alloc_len, GFP_ATOMIC | __GFP_NOWARN);

alloc_len is taken directly from the CDB and is fully controlled by the
initiator. When alloc_len is smaller than the 64-byte report header
(RZONES_DESC_HD), the subtraction underflows and rep_max_zones becomes a
huge value. The buffer is then allocated with only alloc_len bytes, which
is smaller than the 64-byte header the code unconditionally writes, and
the descriptor loop is bounded by the bogus rep_max_zones. Both the header
store and the following zone descriptors are then written past the end of
the undersized allocation, corrupting adjacent slab memory.

Fix it by sizing the buffer to a whole number of 64-byte blocks that
cover the requested allocation length:

	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);

	arr = kzalloc(arr_len, GFP_ATOMIC | __GFP_NOWARN);

RZONES_DESC_HD is a power of two, so ALIGN() rounds alloc_len up to the
next multiple of 64 and rep_max_zones can no longer underflow: for any
alloc_len of 1 to 64 it is 0, so only the header is built. arr_len is
always RZONES_DESC_HD * (rep_max_zones + 1), which is exactly large enough
for the header plus every descriptor the loop may write, so the report is
always assembled within bounds, including a possibly partial trailing
zone descriptor. The existing copy-out still transfers only what the host
asked for:

	fill_from_dev_buffer(scp, arr, min_t(u32, alloc_len, rep_len));

so an allocation length that ends in the middle of a zone descriptor
returns the correctly truncated partial descriptor, as permitted by the
SCSI/ZBC specifications, while never reading past arr_len.

The aligned length and the buffer size are computed in 64-bit (alloc_len
is cast to u64 before ALIGN and the size product uses a u64 block size) so
a crafted allocation length near U32_MAX cannot wrap them to a small value;
such a request simply fails the large allocation and returns a check
condition instead of overflowing the buffer.

This was found by static analysis. A KASAN slab-out-of-bounds runtime
reproduction of the original underflow is being re-run against the ALIGN
based fix and will be reported separately.

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
---
v3: adopt Damien Le Moal's ALIGN-based buffer sizing (Suggested-by) so a
partial trailing zone descriptor is filled and returned per the SCSI/ZBC
specs; v2 emitted only the report header for allocation lengths of 65..127
bytes. The buffer is sized to a whole number of 64-byte blocks covering
alloc_len; the existing min(alloc_len, rep_len) copy-out still truncates the
transfer to the requested length.
Computed in 64-bit to avoid a u32 wrap of the aligned length/size for
allocation lengths near U32_MAX (which would otherwise reintroduce the
overflow).
v2: https://lore.kernel.org/linux-scsi/20260709194824.50777-1-security@auditcode.ai/
 drivers/scsi/scsi_debug.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 9d1c9c41d0f9..12e5a8624511 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 +5912,11 @@ 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 = (ALIGN((u64)alloc_len, RZONES_DESC_HD) - RZONES_DESC_HD) >>
+			ilog2(RZONES_DESC_HD);
+	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)