[PATCH] scsi: core: Validate MODE SENSE lengths in scsi_cdl_enable()

[Alberto Carboneri <acarboneri@drivesec.com>]

scsi_cdl_enable() uses length fields returned by MODE SENSE to locate the
ATA feature mode page in a 64-byte stack buffer. A target can report a
total length shorter than its mode header and block descriptors. The
unsigned subtraction used for the MODE SELECT length can wrap, and the
separately computed buf_data can point beyond buf.

During automatic scan, enable is false, so the read-modify-write of
buf_data[4] can clear the low two bits of a target-selected out-of-bounds
stack byte. scsi_mode_select() can then copy up to 64 bytes from outside
the buffer into the outgoing MODE SELECT payload, disclosing stack contents
to the target.

This is reachable while scanning a USB storage device that identifies as
an ATA device and advertises CDL support. No filesystem mount or userspace
access to the block device is required.

On upstream commit cee9395acd80 ("Linux 7.3-rc1"), a build-specific,
one-vCPU QEMU/Raw Gadget proof using QEMU-only multi-UDC allocator
sampling executed a fixed proof command inside the guest and created a
UID-0-owned marker during automatic enumeration, with KASLR and NX
enabled.

The issue was independently found during security research at Drivesec
S.r.l.

Cap the available length to the buffer size. Validate and consume the mode
header and block descriptor lengths before using the page, and require the
five bytes needed to access the CDL field.

Fixes: 1b22cfb14142 ("scsi: core: Allow enabling and disabling command duration limits")
Reported-by: Sashiko AI Review <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/linux-scsi/20260717192313.93D791F000E9@smtp.kernel.org/
Link: https://lore.kernel.org/linux-scsi/20260717222931.AC4EE1F000E9@smtp.kernel.org/
Link: https://lore.kernel.org/linux-scsi/df13ec87ac9b28e3b0a2d9eb26477e276ff0278a.camel@HansenPartnership.com/
Cc: stable@vger.kernel.org
Assisted-by: LLM
Co-developed-by: Pimen Flavian Dei (Drivesec S.r.l.) <fdei@drivesec.com>
Signed-off-by: Pimen Flavian Dei (Drivesec S.r.l.) <fdei@drivesec.com>
Signed-off-by: Alberto Carboneri (Drivesec S.r.l.) <acarboneri@drivesec.com>
---
Tested on x86-64:
- GCC 13.3.0, W=1 drivers/scsi/scsi.o
- Clang 18.1.3, W=1 drivers/scsi/scsi.o
- Malformed BDL92 MODE SENSE regression, 3/3 fresh QEMU boots:
  scan continued with no MODE SELECT, disclosure, Oops, or panic
- Applies cleanly to v6.12.107

 drivers/scsi/scsi.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 76cdad063f7b..f285521d9de6 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -727,6 +727,7 @@ int scsi_cdl_enable(struct scsi_device *sdev, bool enable)
 		struct scsi_mode_data data;
 		struct scsi_sense_hdr sshdr;
 		char *buf_data;
+		size_t avail, offset;
 		int len;
 
 		ret = scsi_mode_sense(sdev, 0x08, 0x0a, 0xf2, buf, sizeof(buf),
@@ -735,11 +736,24 @@ int scsi_cdl_enable(struct scsi_device *sdev, bool enable)
 			return -EINVAL;
 
 		/* Enable or disable CDL using the ATA feature page */
-		len = min_t(size_t, sizeof(buf),
-			    data.length - data.header_length -
-			    data.block_descriptor_length);
-		buf_data = buf + data.header_length +
-			data.block_descriptor_length;
+		avail = min_t(size_t, data.length, sizeof(buf));
+		if (data.header_length > avail)
+			return -EINVAL;
+
+		offset = data.header_length;
+		avail -= data.header_length;
+
+		if (data.block_descriptor_length > avail)
+			return -EINVAL;
+
+		offset += data.block_descriptor_length;
+		avail -= data.block_descriptor_length;
+
+		if (avail < 5)
+			return -EINVAL;
+
+		buf_data = buf + offset;
+		len = avail;
 
 		/*
 		 * If we want to enable CDL and CDL is already enabled on the

base-commit: af8c27375733fb6a6df9fa484cda77cc3dd0cb80
-- 
2.43.0