Re: [PATCH] staging: axis-fifo: fix integer overflow in write()

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Oct 07, 2025 at 03:58:13PM +0400, Murad Sadigov wrote:
> Fix integer overflow in axis_fifo_write() that allows local users
> to bypass buffer validation, potentially causing hardware FIFO
> buffer overflow and system denial of service.
> 
> The axis_fifo_write() function converts user-controlled size_t 'len'
> (64-bit) to unsigned int 'words_to_write' (32-bit) without overflow
> checking at line 322:
> 
>     words_to_write = len / sizeof(u32);
> 
> On 64-bit systems, when len equals 0x400000000 (16 GiB):
>   - Division: 0x400000000 / 4 = 0x100000000 (requires 33 bits)
>   - Truncation: Result stored in 32-bit variable = 0 (overflow)
>   - Validation bypass: if (0 > fifo_depth) evaluates to false
>   - Impact: Hardware FIFO overflow, system crash
> 
> This allows unprivileged local users with access to /dev/axis_fifo*
> to trigger denial of service.
> 
> The fix adds overflow check before type conversion to ensure len
> does not exceed the maximum safe value (UINT_MAX * sizeof(u32)).
> 
> Affected systems include embedded devices using Xilinx FPGA with
> AXI-Stream FIFO IP cores.
> 
> Signed-off-by: Murad Sadigov <sdgvmrd@gmail.com>
> ---
>  drivers/staging/axis-fifo/axis-fifo.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/staging/axis-fifo/axis-fifo.c
> b/drivers/staging/axis-fifo/axis-fifo.c
> index 1234567890ab..abcdef123456 100644
> --- a/drivers/staging/axis-fifo/axis-fifo.c
> +++ b/drivers/staging/axis-fifo/axis-fifo.c
> @@ -319,6 +319,13 @@ static ssize_t axis_fifo_write(struct file *f,
> const char __user *buf,
>   return -EINVAL;
>   }
> 
> + /* Prevent integer overflow in words calculation */
> + if (len > (size_t)UINT_MAX * sizeof(u32)) {
> + dev_err(fifo->dt_device,
> + "write length %zu exceeds maximum %zu bytes\n",
> + len, (size_t)UINT_MAX * sizeof(u32));
> + return -EINVAL;
> + }
> +

Something went wrong here, your email client dropped all of the leading
spaces :(

Also, you do not want to allow userspace to cause a DoS on the kernel
log, so don't log this information, just return an error, no need to
print anything.

thanks,

greg k-h