Re: [PATCH v2] nfs: fix panic when nfs4_ff_layout_prepare_ds() fails

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2024-03-11 at 11:11 -0400, Josef Bacik wrote:
> 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.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> v1->v2:
> - My bad, I missed the formatting of the drgn output and it looked mangled.
> 
>  fs/nfs/flexfilelayout/flexfilelayout.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c b/fs/nfs/flexfilelayout/flexfilelayout.c
> index ef817a0475ff..3e724cb7ef01 100644
> --- a/fs/nfs/flexfilelayout/flexfilelayout.c
> +++ b/fs/nfs/flexfilelayout/flexfilelayout.c
> @@ -2016,7 +2016,7 @@ static void ff_layout_cancel_io(struct pnfs_layout_segment *lseg)
>  	for (idx = 0; idx < flseg->mirror_array_cnt; idx++) {
>  		mirror = flseg->mirror_array[idx];
>  		mirror_ds = mirror->mirror_ds;
> -		if (!mirror_ds)
> +		if (IS_ERR_OR_NULL(mirror_ds))
>  			continue;
>  		ds = mirror->mirror_ds->ds;
>  		if (!ds)

Nice catch:

Reviewed-by: Jeff Layton <jlayton@kernel.org>