CVE-2025-39723

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

netfs: Fix unbuffered write error handling

If all the subrequests in an unbuffered write stream fail, the subrequest

collector doesn't update the stream->transferred value and it retains its

initial LONG_MAX value. Unfortunately, if all active streams fail, then we

take the smallest value of { LONG_MAX, LONG_MAX, ... } as the value to set

in wreq->transferred - which is then returned from ->write_iter().

LONG_MAX was chosen as the initial value so that all the streams can be

quickly assessed by taking the smallest value of all stream->transferred -

but this only works if we've set any of them.

Fix this by adding a flag to indicate whether the value in

stream->transferred is valid and checking that when we integrate the

values. stream->transferred can then be initialised to zero.

This was found by running the generic/750 xfstest against cifs with

cache=none. It splices data to the target file. Once (if) it has used up

all the available scratch space, the writes start failing with ENOSPC.

This causes ->write_iter() to fail. However, it was returning

wreq->transferred, i.e. LONG_MAX, rather than an error (because it thought

the amount transferred was non-zero) and iter_file_splice_write() would

then try to clean up that amount of pipe bufferage - leading to an oops

when it overran. The kernel log showed:

CIFS: VFS: Send error in write = -28

followed by:

BUG: kernel NULL pointer dereference, address: 0000000000000008

with:

RIP: 0010:iter_file_splice_write+0x3a4/0x520

do_splice+0x197/0x4e0

or:

RIP: 0010:pipe_buf_release (include/linux/pipe_fs_i.h:282)

iter_file_splice_write (fs/splice.c:755)

Also put a warning check into splice to announce if ->write_iter() returned

that it had written more than it was asked to.