CVE-2026-31412 — usb: gadget: f_mass_storage: Fix potential integer overflow in check_command_size_in_blocks()

Affected: Linux

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

usb: gadget: f_mass_storage: Fix potential integer overflow in check_command_size_in_blocks()

The `check_command_size_in_blocks()` function calculates the data size in bytes by left shifting `common->data_size_from_cmnd` by the block size (`common->curlun->blkbits`). However, it does not validate whether this shift operation will cause an integer overflow.

Initially, the block size is set up in `fsg_lun_open()` , and the `common->data_size_from_cmnd` is set up in `do_scsi_command()`. During initialization, there is no integer overflow check for the interaction between two variables.

So if a malicious USB host sends a SCSI READ or WRITE command requesting a large amount of data (`common->data_size_from_cmnd`), the left shift operation can wrap around. This results in a truncated data size, which can bypass boundary checks and potentially lead to memory corruption or out-of-bounds accesses.

Fix this by using the check_shl_overflow() macro to safely perform the shift and catch any overflows.

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CVE-2026-31412 — usb: gadget: f_mass_storage: Fix potential integer overflow in check_command_size_in_blocks()