io_uring/kbuf: fix unintentional sign extension on shift of reg.bgid

Shifting reg.bgid << IORING_OFF_PBUF_SHIFT results in a promotion
from __u16 to a 32 bit signed integer, this is then sign extended
to a 64 bit unsigned long on 64 bit architectures. If reg.bgid is
greater than 0x7fff then this leads to a sign extended result where
all the upper 32 bits of mmap_offset are set to 1. Fix this by
casting reg.bgid to the same type as mmap_offset before performing
the shift.

Fixes: ef62de3 ("io_uring/kbuf: use region api for pbuf rings")
Signed-off-by: Colin Ian King <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: ColinIanKing

io_uring/kbuf.c

@@ -640,7 +640,7 @@ int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 			return -ENOMEM;
 	}
 
-	mmap_offset = reg.bgid << IORING_OFF_PBUF_SHIFT;
+	mmap_offset = (unsigned long)reg.bgid << IORING_OFF_PBUF_SHIFT;
 	ring_size = flex_array_size(br, bufs, reg.ring_entries);
 
 	memset(&rd, 0, sizeof(rd));