net/9p: fix uninit-value in p9_client_rpc()

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

net/9p: fix uninit-value in p9_client_rpc()

Syzbot with the help of KMSAN reported the following error:

BUG: KMSAN: uninit-value in trace_9p_client_res include/trace/events/9p.h:146 [inline]

BUG: KMSAN: uninit-value in p9_client_rpc+0x1314/0x1340 net/9p/client.c:754

trace_9p_client_res include/trace/events/9p.h:146 [inline]

p9_client_rpc+0x1314/0x1340 net/9p/client.c:754

p9_client_create+0x1551/0x1ff0 net/9p/client.c:1031

v9fs_session_init+0x1b9/0x28e0 fs/9p/v9fs.c:410

v9fs_mount+0xe2/0x12b0 fs/9p/vfs_super.c:122

legacy_get_tree+0x114/0x290 fs/fs_context.c:662

vfs_get_tree+0xa7/0x570 fs/super.c:1797

do_new_mount+0x71f/0x15e0 fs/namespace.c:3352

path_mount+0x742/0x1f20 fs/namespace.c:3679

do_mount fs/namespace.c:3692 [inline]

__do_sys_mount fs/namespace.c:3898 [inline]

__se_sys_mount+0x725/0x810 fs/namespace.c:3875

__x64_sys_mount+0xe4/0x150 fs/namespace.c:3875

do_syscall_64+0xd5/0x1f0

entry_SYSCALL_64_after_hwframe+0x6d/0x75

Uninit was created at:

__alloc_pages+0x9d6/0xe70 mm/page_alloc.c:4598

__alloc_pages_node include/linux/gfp.h:238 [inline]

alloc_pages_node include/linux/gfp.h:261 [inline]

alloc_slab_page mm/slub.c:2175 [inline]

allocate_slab mm/slub.c:2338 [inline]

new_slab+0x2de/0x1400 mm/slub.c:2391

___slab_alloc+0x1184/0x33d0 mm/slub.c:3525

__slab_alloc mm/slub.c:3610 [inline]

__slab_alloc_node mm/slub.c:3663 [inline]

slab_alloc_node mm/slub.c:3835 [inline]

kmem_cache_alloc+0x6d3/0xbe0 mm/slub.c:3852

p9_tag_alloc net/9p/client.c:278 [inline]

p9_client_prepare_req+0x20a/0x1770 net/9p/client.c:641

p9_client_rpc+0x27e/0x1340 net/9p/client.c:688

p9_client_create+0x1551/0x1ff0 net/9p/client.c:1031

v9fs_session_init+0x1b9/0x28e0 fs/9p/v9fs.c:410

v9fs_mount+0xe2/0x12b0 fs/9p/vfs_super.c:122

legacy_get_tree+0x114/0x290 fs/fs_context.c:662

vfs_get_tree+0xa7/0x570 fs/super.c:1797

do_new_mount+0x71f/0x15e0 fs/namespace.c:3352

path_mount+0x742/0x1f20 fs/namespace.c:3679

do_mount fs/namespace.c:3692 [inline]

__do_sys_mount fs/namespace.c:3898 [inline]

__se_sys_mount+0x725/0x810 fs/namespace.c:3875

__x64_sys_mount+0xe4/0x150 fs/namespace.c:3875

do_syscall_64+0xd5/0x1f0

entry_SYSCALL_64_after_hwframe+0x6d/0x75

If p9_check_errors() fails early in p9_client_rpc(), req->rc.tag

will not be properly initialized. However, trace_9p_client_res()

ends up trying to print it out anyway before p9_client_rpc()

finishes.

Fix this issue by assigning default values to p9_fcall fields

such as 'tag' and (just in case KMSAN unearths something new) 'id'

during the tag allocation stage.