Subject: CVE-2024-41083: netfs: Fix netfs_page_mkwrite() to check folio->mapping is valid

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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

netfs: Fix netfs_page_mkwrite() to check folio->mapping is valid

Fix netfs_page_mkwrite() to check that folio->mapping is valid once it has
taken the folio lock (as filemap_page_mkwrite() does).  Without this,
generic/247 occasionally oopses with something like the following:

    BUG: kernel NULL pointer dereference, address: 0000000000000000
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page

    RIP: 0010:trace_event_raw_event_netfs_folio+0x61/0xc0
    ...
    Call Trace:
     <TASK>
     ? __die_body+0x1a/0x60
     ? page_fault_oops+0x6e/0xa0
     ? exc_page_fault+0xc2/0xe0
     ? asm_exc_page_fault+0x22/0x30
     ? trace_event_raw_event_netfs_folio+0x61/0xc0
     trace_netfs_folio+0x39/0x40
     netfs_page_mkwrite+0x14c/0x1d0
     do_page_mkwrite+0x50/0x90
     do_pte_missing+0x184/0x200
     __handle_mm_fault+0x42d/0x500
     handle_mm_fault+0x121/0x1f0
     do_user_addr_fault+0x23e/0x3c0
     exc_page_fault+0xc2/0xe0
     asm_exc_page_fault+0x22/0x30

This is due to the invalidate_inode_pages2_range() issued at the end of the
DIO write interfering with the mmap'd writes.

The Linux kernel CVE team has assigned CVE-2024-41083 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.8 with commit 102a7e2c598c and fixed in 6.9.8 with commit 3473eb87afd4
	Issue introduced in 6.8 with commit 102a7e2c598c and fixed in 6.10 with commit a81c98bfa40c

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-41083
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	fs/netfs/buffered_write.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/3473eb87afd402e415a8ca885b284ea0420dde25
	https://git.kernel.org/stable/c/a81c98bfa40c11f8ea79b5a9b3f5fda73bfbb4d2