9p/fd: Fix write overflow in p9_read_work

This error was reported while fuzzing:

BUG: KASAN: slab-out-of-bounds in _copy_to_iter+0xd35/0x1190
Write of size 4043 at addr ffff888008724eb1 by task kworker/1:1/24

CPU: 1 PID: 24 Comm: kworker/1:1 Not tainted 6.1.0-rc5-00002-g1adf73218daa-dirty #223
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014
Workqueue: events p9_read_work
Call Trace:
 <TASK>
 dump_stack_lvl+0x4c/0x64
 print_report+0x178/0x4b0
 kasan_report+0xae/0x130
 kasan_check_range+0x179/0x1e0
 memcpy+0x38/0x60
 _copy_to_iter+0xd35/0x1190
 copy_page_to_iter+0x1d5/0xb00
 pipe_read+0x3a1/0xd90
 __kernel_read+0x2a5/0x760
 kernel_read+0x47/0x60
 p9_read_work+0x463/0x780
 process_one_work+0x91d/0x1300
 worker_thread+0x8c/0x1210
 kthread+0x280/0x330
 ret_from_fork+0x22/0x30
 </TASK>

Allocated by task 457:
 kasan_save_stack+0x1c/0x40
 kasan_set_track+0x21/0x30
 __kasan_kmalloc+0x7e/0x90
 __kmalloc+0x59/0x140
 p9_fcall_init.isra.11+0x5d/0x1c0
 p9_tag_alloc+0x251/0x550
 p9_client_prepare_req+0x162/0x350
 p9_client_rpc+0x18d/0xa90
 p9_client_create+0x670/0x14e0
 v9fs_session_init+0x1fd/0x14f0
 v9fs_mount+0xd7/0xaf0
 legacy_get_tree+0xf3/0x1f0
 vfs_get_tree+0x86/0x2c0
 path_mount+0x885/0x1940
 do_mount+0xec/0x100
 __x64_sys_mount+0x1a0/0x1e0
 do_syscall_64+0x3a/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

This BUG pops up when trying to reproduce
https://syzkaller.appspot.com/bug?id=6c7cd46c7bdd0e86f95d26ec3153208ad186f9fa
The callstack is different but the issue is valid and re-producable with
the same re-producer in the link.

The root cause of this issue is that we check the size of the message
received against the msize of the client in p9_read_work. However, it
turns out that capacity is no longer consistent with msize. Thus,
the message size should be checked against sdata capacity.

As the msize is non-consistant with the capacity of the tag and as we
are now checking message size against capacity directly, there is no
point checking message size against msize. So remove it.

Link: https://lkml.kernel.org/r/20221117091159.31533-2-guozihua@huawei.com
Link: https://lkml.kernel.org/r/20221117091159.31533-3-guozihua@huawei.com


Reported-by:  <syzbot+0f89bd13eaceccc0e126@syzkaller.appspotmail.com>
Fixes: 60ece083 ("net/9p: allocate appropriate reduced message buffers")
Signed-off-by: GUO Zihua <guozihua@huawei.com>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
[Dominique: squash patches 1 & 2 and fix size including header part]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>