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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>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20261005123555.253995-1-yhlee@isslab.korea.ac.kr>
This commit is contained in:
Yehyeong Lee 2026-10-05 21:35:54 +09:00 committed by Juergen Gross
parent 71d59fc2a5
commit 992df081bc
1 changed files with 1 additions and 0 deletions
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drivers/scsi/xen-scsifront.c
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@ -367,6 +367,7 @@ static void scsifront_do_response(struct vscsifrnt_info *info,
struct vscsifrnt_shadow *shadow;
if (ring_rsp->rqid >= VSCSIIF_MAX_REQS ||
!info->shadow[ring_rsp->rqid] ||
!info->shadow[ring_rsp->rqid]->inflight) {
scsifront_set_error(info, "illegal rqid returned by backend!");
return;