pipe: preserve existing fasync readers on writer setup failure

Overview

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.

Affected

• Linux >= 76398425bb06b07cc3a3b1ce169c67dc9d6874ed < 4c42ce499ce4b1c787e61082e3b13df4f30c4eaa
• Linux >= 76398425bb06b07cc3a3b1ce169c67dc9d6874ed < 2259809e5b7043c4ba20af7b14ba3e471cf55521
• Linux >= 76398425bb06b07cc3a3b1ce169c67dc9d6874ed < 40bb03d3ea6f6a915218508819ecc4d7a7fa260d
• Linux 2.6.30

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.