CVE-2025-39901

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

i40e: remove read access to debugfs files

The 'command' and 'netdev_ops' debugfs files are a legacy debugging

interface supported by the i40e driver since its early days by commit

02e9c290814c ("i40e: debugfs interface").

Both of these debugfs files provide a read handler which is mostly useless,

and which is implemented with questionable logic. They both use a static

256 byte buffer which is initialized to the empty string. In the case of

the 'command' file this buffer is literally never used and simply wastes

space. In the case of the 'netdev_ops' file, the last command written is

saved here.

On read, the files contents are presented as the name of the device

followed by a colon and then the contents of their respective static

buffer. For 'command' this will always be "<device>: ". For 'netdev_ops',

this will be "<device>: <last command written>". But note the buffer is

shared between all devices operated by this module. At best, it is mostly

meaningless information, and at worse it could be accessed simultaneously

as there doesn't appear to be any locking mechanism.

We have also recently received multiple reports for both read functions

about their use of snprintf and potential overflow that could result in

reading arbitrary kernel memory. For the 'command' file, this is definitely

impossible, since the static buffer is always zero and never written to.

For the 'netdev_ops' file, it does appear to be possible, if the user

carefully crafts the command input, it will be copied into the buffer,

which could be large enough to cause snprintf to truncate, which then

causes the copy_to_user to read beyond the length of the buffer allocated

by kzalloc.

A minimal fix would be to replace snprintf() with scnprintf() which would

cap the return to the number of bytes written, preventing an overflow. A

more involved fix would be to drop the mostly useless static buffers,

saving 512 bytes and modifying the read functions to stop needing those as

input.

Instead, lets just completely drop the read access to these files. These

are debug interfaces exposed as part of debugfs, and I don't believe that

dropping read access will break any script, as the provided output is

pretty useless. You can find the netdev name through other more standard

interfaces, and the 'netdev_ops' interface can easily result in garbage if

you issue simultaneous writes to multiple devices at once.

In order to properly remove the i40e_dbg_netdev_ops_buf, we need to

refactor its write function to avoid using the static buffer. Instead, use

the same logic as the i40e_dbg_command_write, with an allocated buffer.

Update the code to use this instead of the static buffer, and ensure we

free the buffer on exit. This fixes simultaneous writes to 'netdev_ops' on

multiple devices, and allows us to remove the now unused static buffer

along with removing the read access.