In the Linux kernel, the following vulnerability has been resolved: svcrdma: Fix pcl_for_each_segment for empty chunks When a parsed chunk

In the Linux kernel, the following vulnerability has been resolved:

svcrdma: Fix pcl_for_each_segment for empty chunks

When a parsed chunk list contains a chunk whose ch_segcount is zero,
pcl_for_each_segment computes its inclusive upper bound as
&chunk->ch_segments[ch_segcount - 1]. ch_segcount is u32, so the
subtraction wraps to 0xFFFFFFFF and the bound lands far past the
ch_segments flex array. The loop body then walks unrelated memory at
sizeof(struct svc_rdma_segment) stride until it faults.

A zero-segcount chunk is reachable from the wire:
xdr_check_write_chunk() only rejects segcount values greater than
rc_maxpages, and pcl_alloc_write() links a freshly allocated chunk
onto rc_write_pcl/rc_reply_pcl before its segment-fill loop runs,
so a Write or Reply chunk advertising zero segments leaves
ch_segcount == 0 on the list. When the transport has negotiated
Send-With-Invalidate, svc_rdma_get_inv_rkey() iterates all four
PCLs with pcl_for_each_segment and dereferences segment->rs_handle
on each iteration, turning the underflow into an out-of-bounds read
and a general protection fault.

 1xdr_check_write_list / xdr_check_reply_chunk
 2  pcl_alloc_write()
 3    chunk = pcl_alloc_chunk(...)  /* ch_segcount = 0 */
 4    list_add_tail(&chunk->ch_list, &pcl->cl_chunks)
 5    /* fill loop iterates zero times for wire segcount 0 */
 6 
 7svc_rdma_get_inv_rkey()
 8  pcl_for_each_chunk(rc_write_pcl)
 9    pcl_for_each_segment(segment, chunk)
10      pos <= &ch_segments[0u - 1u]  /* 0xFFFFFFFF */
11      segment->rs_handle            /* OOB read -> GPF */

Fix by switching the macro to a half-open upper bound that uses
ch_segcount directly. For ch_segcount == 0 the loop start equals the
loop end and the body is skipped; for ch_segcount > 0 the iteration
range is unchanged. All six existing call sites in
net/sunrpc/xprtrdma/svc_rdma_recvfrom.c and
net/sunrpc/xprtrdma/svc_rdma_rw.c remain correct under the new bound,
so no caller changes are needed.