CVE-2026-99203

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

pipe: preserve existing fasync readers on writer setup failure

F SETFL and FIOASYNC decide whether to invoke ->fasync() before pipe fasync() takes pipe->mutex. Two enables of one O RDWR FIFO can therefore both enter the callback.

fasync helper() 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 f owner. An unprivileged memory-pressure reproducer triggers a KASAN slab-use-after-free in send sigio() 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 fa fd as before.