9P FS: Fix wild-memory-access write in v9fs_get_acl

KASAN reported the following issue:
[   36.825817][ T5923] BUG: KASAN: wild-memory-access in v9fs_get_acl+0x1a4/0x390
[   36.827479][ T5923] Write of size 4 at addr 9fffeb37f97f1c00 by task syz-executor798/5923
[   36.829303][ T5923]
[   36.829846][ T5923] CPU: 0 PID: 5923 Comm: syz-executor798 Not tainted 6.2.0-syzkaller-18302-g596b6b709632 #0
[   36.832110][ T5923] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/21/2023
[   36.834464][ T5923] Call trace:
[   36.835196][ T5923]  dump_backtrace+0x1c8/0x1f4
[   36.836229][ T5923]  show_stack+0x2c/0x3c
[   36.837100][ T5923]  dump_stack_lvl+0xd0/0x124
[   36.838103][ T5923]  print_report+0xe4/0x4c0
[   36.839068][ T5923]  kasan_report+0xd4/0x130
[   36.840052][ T5923]  kasan_check_range+0x264/0x2a4
[   36.841199][ T5923]  __kasan_check_write+0x2c/0x3c
[   36.842216][ T5923]  v9fs_get_acl+0x1a4/0x390
[   36.843232][ T5923]  v9fs_mount+0x77c/0xa5c
[   36.844163][ T5923]  legacy_get_tree+0xd4/0x16c
[   36.845173][ T5923]  vfs_get_tree+0x90/0x274
[   36.846137][ T5923]  do_new_mount+0x25c/0x8c8
[   36.847066][ T5923]  path_mount+0x590/0xe58
[   36.848147][ T5923]  __arm64_sys_mount+0x45c/0x594
[   36.849273][ T5923]  invoke_syscall+0x98/0x2c0
[   36.850421][ T5923]  el0_svc_common+0x138/0x258
[   36.851397][ T5923]  do_el0_svc+0x64/0x198
[   36.852398][ T5923]  el0_svc+0x58/0x168
[   36.853224][ T5923]  el0t_64_sync_handler+0x84/0xf0
[   36.854293][ T5923]  el0t_64_sync+0x190/0x194

Calling '__v9fs_get_acl' method in 'v9fs_get_acl' creates the
following chain of function calls:

__v9fs_get_acl
	v9fs_fid_get_acl
		v9fs_fid_xattr_get
			p9_client_xattrwalk

Function p9_client_xattrwalk accepts a pointer to u64-typed
variable attr_size and puts some u64 value into it. However,
after the executing the p9_client_xattrwalk, in some circumstances
we assign the value of u64-typed variable 'attr_size' to the
variable 'retval', which we will return. However, the type of
'retval' is ssize_t, and if the value of attr_size is larger
than SSIZE_MAX, we will face the signed type overflow. If the
overflow occurs, the result of v9fs_fid_xattr_get may be
negative, but not classified as an error. When we try to allocate
an acl with 'broken' size we receive an error, but don't process
it. When we try to free this acl, we face the 'wild-memory-access'
error (because it wasn't allocated).

This patch will add new condition to the 'v9fs_fid_xattr_get'
function, so it will return an EOVERFLOW error if the 'attr_size'
is larger than SSIZE_MAX.

In this version of the patch I simplified the condition.

In previous (v2) version of the patch I removed explicit type conversion
and added separate condition to check the possible overflow and return
an error (in v1 version I've just modified the existing condition).

Tested via syzkaller.

Suggested-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Reported-by: syzbot+cb1d16facb3cc90de5fb@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=fbbef66d9e4d096242f3617de5d14d12705b4659
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>

fs/9p/xattr.c

@@ -35,10 +35,12 @@ ssize_t v9fs_fid_xattr_get(struct p9_fid *fid, const char *name,
 		return retval;
 	}
 	if (attr_size > buffer_size) {
-		if (!buffer_size) /* request to get the attr_size */
-			retval = attr_size;
-		else
+		if (buffer_size)
 			retval = -ERANGE;
+		else if (attr_size > SSIZE_MAX)
+			retval = -EOVERFLOW;
+		else /* request to get the attr_size */
+			retval = attr_size;
 	} else {
 		iov_iter_truncate(&to, attr_size);
 		retval = p9_client_read(attr_fid, 0, &to, &err);