CVE-2026-99203: pipe: preserve existing fasync readers on writer setup failure

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

pipe: preserve existing fasync readers on writer setup failure

FSETFL and FIOASYNC decide whether to invoke ->fasync() before pipefasync() takes pipe->mutex. Two enables of one ORDWR FIFO can therefore both enter the callback.

fasynchelper() allocates before checking for an existing entry. If the first enable installs both entries, a second enable can update the reader entry and then fail the allocation for the writer entry. The error path removes the pre-existing reader entry, clearing FASYNC while the writer entry remains registered. fput() then skips ->fasync(), so a later pipe notification can dereference the freed file or its fowner. An unprivileged memory-pressure reproducer triggers a KASAN slab-use-after-free in sendsigio() through this path.

Keep the reader helper's result and undo the registration only if this call added it. The pipe mutex serializes changes to both lists, so a failed new registration still removes its reader, while a failed duplicate preserves the existing registrations and FASYNC. Successful duplicate enables continue to update fafd as before.