io_uring/filetable: clamp alloc_hint to the configured alloc range

Explicit fixed file install/remove operations on slots outside the
configured alloc range can corrupt alloc_hint via io_file_bitmap_set()
and io_file_bitmap_clear(), which unconditionally update alloc_hint to
the bit position. This causes subsequent auto-allocations to fall
outside the configured range.

For example, if the alloc range is [10, 20) and a file is removed at
slot 2, alloc_hint gets set to 2. The next auto-alloc then starts
searching from slot 2, potentially returning a slot below the range.

Fix this by clamping alloc_hint to [file_alloc_start, file_alloc_end)
at the top of io_file_bitmap_get() before starting the search.

Cc: stable@vger.kernel.org
Fixes: 6e73dff ("io_uring: let to set a range for file slot allocation")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/filetable.c

@@ -22,6 +22,10 @@ static int io_file_bitmap_get(struct io_ring_ctx *ctx)
 	if (!table->bitmap)
 		return -ENFILE;
 
+	if (table->alloc_hint < ctx->file_alloc_start ||
+	    table->alloc_hint >= ctx->file_alloc_end)
+		table->alloc_hint = ctx->file_alloc_start;
+
 	do {
 		ret = find_next_zero_bit(table->bitmap, nr, table->alloc_hint);
 		if (ret != nr)