fs/buffer: add alert in try_to_free_buffers() for folios without buffers

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: fs/buffer: add alert in try_to_free_buffers() for folios without buffers try_to_free_buffers() can be called on folios with no buffers attached when filemap_release_folio() is invoked on a folio belonging to a mapping with AS_RELEASE_ALWAYS set but no release_folio operation defined. In such cases, folio_needs_release() returns true because of the AS_RELEASE_ALWAYS flag, but the folio has no private buffer data. This causes try_to_free_buffers() to call drop_buffers() on a folio with no buffers, leading to a null pointer dereference. Adding a check in try_to_free_buffers() to return early if the folio has no buffers attached, with WARN_ON_ONCE() to alert about the misconfiguration. This provides defensive hardening.

References

• https://git.kernel.org/stable/c/1b111a69a6e33a922622bf9870e4e63fb2b649c8
• https://git.kernel.org/stable/c/c1b6227555c52781178132b7a06466711855795c
• https://git.kernel.org/stable/c/727e5140e0cf83b4ce6a11b89bb73bff5d96f8f3
• https://git.kernel.org/stable/c/42c32d7571ccd8ef32351cac506f00b0fae99fd2
• https://git.kernel.org/stable/c/c6246ca15999053d2632fbcc7b86e6eef7f077cb
• https://git.kernel.org/stable/c/b68f91ef3b3fe82ad78c417de71b675699a8467c