CVE-2024-26868

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.