[PATCH] cdrom: bound the device-supplied offset in cdrom_mrw_set_lba_space

[Jay Vadayath <jay@artiphishell.com>]

cdrom_mrw_set_lba_space() issues a MODE SENSE(10) into a 16-byte stack
buffer (char buffer[16]), then takes offset = be16_to_cpu(mph->desc_length)
straight from reply bytes 6..7 and does buffer[offset + 3] = space. The
desc_length field is fully device-controlled (0..65535) and nothing
checks that offset + 3 lies within the buffer, nor that size (derived
from mode_data_length) fits. A drive that returns desc_length = 24 turns
the assignment into buffer[27] = space, a one-byte write 12 bytes past
the end of the stack buffer.

This runs when an MRW-writable CD-ROM is opened O_RDWR, so a malicious or
buggy USB/ATAPI drive (or an emulated one) is enough: no privilege beyond
being able to open /dev/sr0 for writing on an attacker-supplied device.
The sink lives in the CD-ROM core, so it affects every cdrom driver
(sr, ide-cd, pcd, gdrom).

  BUG: KASAN: stack-out-of-bounds in cdrom_mrw_set_lba_space.constprop.0+0x25c/0x260
  Write of size 1 at addr ffff88800639f6db by task poc/133
  Call Trace:
   dump_stack_lvl+0x64/0x80
   print_report+0xd0/0x630
   kasan_report+0xe5/0x120
   cdrom_mrw_set_lba_space.constprop.0+0x25c/0x260
   cdrom_open+0x1169/0x2080
   sr_block_open+0x9e/0x140
   blkdev_get_whole+0x8c/0x200
   bdev_open+0x214/0xc40
   blkdev_open+0x1f8/0x370
   do_dentry_open+0x408/0x1290
   path_openat+0xb2f/0x2210
   do_sys_openat2+0x126/0x170
   ...

Both desc_length (offset) and mode_data_length (size) come straight from
the device reply, so reject anything that would not fit in buffer:
offset + 3 >= sizeof(buffer) or size > sizeof(buffer) returns -EIO before
the write. The sibling cdrom_mode_select consumer of size is covered by
the same guard.

This bug was discovered by Artiphishell's vTriage pipeline, which
generated an emulated raw-gadget USB BOT CD-ROM (INQUIRY TYPE_ROM
advertising MRW-write) plus an open(/dev/sr0, O_RDWR) client that drives
the capability-probe chain (page 0x2a -> GET CONFIGURATION CDF_MRW ->
MODE SENSE page 0x03 with desc_length = 24) and reliably triggers the
KASAN report on an unpatched kernel. The fix below was drafted with the
Claude coding assistant; a userspace reproducer is available on request.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jay Vadayath <jay@artiphishell.com>
---
 drivers/cdrom/cdrom.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/cdrom/cdrom.c b/drivers/cdrom/cdrom.c
index 4f1fd389260f..5e910d79045f 100644
--- a/drivers/cdrom/cdrom.c
+++ b/drivers/cdrom/cdrom.c
@@ -571,6 +571,10 @@ static int cdrom_mrw_set_lba_space(struct cdrom_device_info *cdi, int space)
 	offset = be16_to_cpu(mph->desc_length);
 	size = be16_to_cpu(mph->mode_data_length) + 2;
 
+	/* Both fields are device-supplied; reject values that overflow buffer[]. */
+	if (offset + 3 >= (int)sizeof(buffer) || size > (int)sizeof(buffer))
+		return -EIO;
+
 	buffer[offset + 3] = space;
 	cgc.buflen = size;
 
-- 
2.51.2