scsi: xen/scsifront: Check for a NULL shadow entry on a backend response

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scsifront_do_response() validates the rqid in a backend response against
VSCSIIF_MAX_REQS before using it to index info->shadow[], but then
dereferences the entry to test ->inflight without checking that the slot
is populated.  Shadow slots are NULL before a command is submitted (the
host private area is zeroed at allocation) and are reset to NULL in
_scsifront_put_rqid() once a request completes, so an in-range rqid that
does not correspond to an outstanding request makes the frontend
dereference a NULL pointer.

A malicious or buggy backend can thus crash the guest by returning a
response whose rqid is in range but not in flight -- for example a
spurious response before any command has been issued, or a duplicate of
one already completed.  The ring has only VSCSIIF_MAX_REQS (16) slots,
so before the first command every in-range rqid selects a NULL slot.

Reject a response whose shadow slot is not populated.

Fixes: 6d1c2f48f3 ("xen/scsifront: harden driver against malicious backend")
Cc: stable@vger.kernel.org
Signed-off-by: Yehyeong Lee <yhlee@isslab.korea.ac.kr>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://patch.msgid.link/20261005123555.253995-1-yhlee@isslab.korea.ac.kr
Signed-off-by: Martin K. Petersen (Oracle) <mkp@kernel.org>

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Yehyeong Lee 2026-10-05 21:35:54 +09:00 • committed by Martin K. Petersen (Oracle)

parent 469fa05566

commit 8c8222c255

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drivers/scsi/xen-scsifront.c

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