Subject: CVE-2024-26868: nfs: fix panic when nfs4_ff_layout_prepare_ds() fails

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

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

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

nfs: fix panic when nfs4_ff_layout_prepare_ds() fails

We've been seeing the following panic in production

BUG: kernel NULL pointer dereference, address: 0000000000000065
PGD 2f485f067 P4D 2f485f067 PUD 2cc5d8067 PMD 0
RIP: 0010:ff_layout_cancel_io+0x3a/0x90 [nfs_layout_flexfiles]
Call Trace:
 <TASK>
 ? __die+0x78/0xc0
 ? page_fault_oops+0x286/0x380
 ? __rpc_execute+0x2c3/0x470 [sunrpc]
 ? rpc_new_task+0x42/0x1c0 [sunrpc]
 ? exc_page_fault+0x5d/0x110
 ? asm_exc_page_fault+0x22/0x30
 ? ff_layout_free_layoutreturn+0x110/0x110 [nfs_layout_flexfiles]
 ? ff_layout_cancel_io+0x3a/0x90 [nfs_layout_flexfiles]
 ? ff_layout_cancel_io+0x6f/0x90 [nfs_layout_flexfiles]
 pnfs_mark_matching_lsegs_return+0x1b0/0x360 [nfsv4]
 pnfs_error_mark_layout_for_return+0x9e/0x110 [nfsv4]
 ? ff_layout_send_layouterror+0x50/0x160 [nfs_layout_flexfiles]
 nfs4_ff_layout_prepare_ds+0x11f/0x290 [nfs_layout_flexfiles]
 ff_layout_pg_init_write+0xf0/0x1f0 [nfs_layout_flexfiles]
 __nfs_pageio_add_request+0x154/0x6c0 [nfs]
 nfs_pageio_add_request+0x26b/0x380 [nfs]
 nfs_do_writepage+0x111/0x1e0 [nfs]
 nfs_writepages_callback+0xf/0x30 [nfs]
 write_cache_pages+0x17f/0x380
 ? nfs_pageio_init_write+0x50/0x50 [nfs]
 ? nfs_writepages+0x6d/0x210 [nfs]
 ? nfs_writepages+0x6d/0x210 [nfs]
 nfs_writepages+0x125/0x210 [nfs]
 do_writepages+0x67/0x220
 ? generic_perform_write+0x14b/0x210
 filemap_fdatawrite_wbc+0x5b/0x80
 file_write_and_wait_range+0x6d/0xc0
 nfs_file_fsync+0x81/0x170 [nfs]
 ? nfs_file_mmap+0x60/0x60 [nfs]
 __x64_sys_fsync+0x53/0x90
 do_syscall_64+0x3d/0x90
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

Inspecting the core with drgn I was able to pull this

  >>> prog.crashed_thread().stack_trace()[0]
  #0 at 0xffffffffa079657a (ff_layout_cancel_io+0x3a/0x84) in ff_layout_cancel_io at fs/nfs/flexfilelayout/flexfilelayout.c:2021:27
  >>> prog.crashed_thread().stack_trace()[0]['idx']
  (u32)1
  >>> prog.crashed_thread().stack_trace()[0]['flseg'].mirror_array[1].mirror_ds
  (struct nfs4_ff_layout_ds *)0xffffffffffffffed

This is clear from the stack trace, we call nfs4_ff_layout_prepare_ds()
which could error out initializing the mirror_ds, and then we go to
clean it all up and our check is only for if (!mirror->mirror_ds).  This
is inconsistent with the rest of the users of mirror_ds, which have

  if (IS_ERR_OR_NULL(mirror_ds))

to keep from tripping over this exact scenario.  Fix this up in
ff_layout_cancel_io() to make sure we don't panic when we get an error.
I also spot checked all the other instances of checking mirror_ds and we
appear to be doing the correct checks everywhere, only unconditionally
dereferencing mirror_ds when we know it would be valid.

The Linux kernel CVE team has assigned CVE-2024-26868 to this issue.

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

	Issue introduced in 6.1 with commit b739a5bd9d9f and fixed in 6.1.83 with commit 31db25e3141b
	Issue introduced in 6.1 with commit b739a5bd9d9f and fixed in 6.6.23 with commit 7ca651b4ec4a
	Issue introduced in 6.1 with commit b739a5bd9d9f and fixed in 6.7.11 with commit 5ada9016b121
	Issue introduced in 6.1 with commit b739a5bd9d9f and fixed in 6.8.2 with commit dac068f164ad
	Issue introduced in 6.1 with commit b739a5bd9d9f and fixed in 6.9 with commit 719fcafe07c1

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-2024-26868
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/nfs/flexfilelayout/flexfilelayout.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/31db25e3141b20e2a76a9f219eeca52e3cab126c
	https://git.kernel.org/stable/c/7ca651b4ec4a049f5a46a0e5ff921b86b91c47c5
	https://git.kernel.org/stable/c/5ada9016b1217498fad876a3d5b07645cc955608
	https://git.kernel.org/stable/c/dac068f164ad05b35e7c0be13f138c3f6adca58f
	https://git.kernel.org/stable/c/719fcafe07c12646691bd62d7f8d94d657fa0766