[PATCH v2 1/2] scsi: sg: validate scatter_elem_sz module parameter

[Yang Erkun <yangerkun@huawei.com>]

echo -1 or 0 > /sys/module/sg/parameters/scatter_elem_sz
exec 4<> /dev/sg0

The above triggers the following UBSAN warning:

UBSAN: shift-out-of-bounds in drivers/scsi/sg.c:1888:13
shift exponent 64 is too large for 32-bit type 'int'
.....
Call Trace:
 <TASK>
 dump_stack_lvl+0x64/0x80
 __ubsan_handle_shift_out_of_bounds+0x1d1/0x380
 sg_build_indirect.cold+0x38/0x4b
 sg_build_reserve+0x59/0x90
 sg_add_sfp+0x151/0x240
 sg_open+0x169/0x310
 chrdev_open+0xbe/0x240
 do_dentry_open+0x121/0x480
 vfs_open+0x2e/0xf0
 do_open+0x265/0x400
 path_openat+0x110/0x2b0
 do_file_open+0xe4/0x1a0
 do_sys_openat2+0x7f/0xe0
 __x64_sys_openat+0x56/0xa0
 do_syscall_64+0xf5/0x640
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

The scatter_elem_sz module parameter currently lacks validation. Setting
it to -1/0 causes a left shift overflow in sg_build_indirect. Although
this overflow does not currently cause other problem, allowing
scatter_elem_sz to be set to -1/0 is not appropriate given its intended
purpose. Therefore, this patch uses module_param_call to add validation
checks: reject non-positive values and values whose order exceeds
MAX_PAGE_ORDER, and round the accepted value up to a power-of-two
multiple of PAGE_SIZE.

Fixes: 6460e75a104d ("[SCSI] sg: fixes for large page_size")
Signed-off-by: Yang Erkun <yangerkun@huawei.com>
---
 drivers/scsi/sg.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 74cd4e8a61c2..c56f8460c03f 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -1622,7 +1622,33 @@ sg_remove_device(struct device *cl_dev)
 	kref_put(&sdp->d_ref, sg_device_destroy);
 }
 
-module_param_named(scatter_elem_sz, scatter_elem_sz, int, S_IRUGO | S_IWUSR);
+static int scatter_elem_sz_set(const char *val, const struct kernel_param *kp)
+{
+	int ret, new_val, order;
+
+	ret = kstrtoint(val, 0, &new_val);
+	if (ret)
+		return ret;
+
+	if (new_val <= 0) {
+		pr_err("sg: scatter_elem_sz must be positive, got %d\n", new_val);
+		return -EINVAL;
+	}
+
+	order = get_order(new_val);
+	if (order > MAX_PAGE_ORDER) {
+		pr_err("sg: scatter_elem_sz too large (order %d > MAX_PAGE_ORDER %d)\n",
+			order, MAX_PAGE_ORDER);
+		return -EINVAL;
+	}
+
+	scatter_elem_sz = 1 << (PAGE_SHIFT + order);
+	return 0;
+}
+
+module_param_call(scatter_elem_sz, scatter_elem_sz_set, param_get_int,
+		  &scatter_elem_sz, 0644);
+
 module_param_named(allow_dio, sg_allow_dio, int, S_IRUGO | S_IWUSR);
 
 static int def_reserved_size_set(const char *val, const struct kernel_param *kp)
-- 
2.52.0