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 <[email protected]>

Reported-by [email protected]

Link https://syzkaller.appspot.com/bug?id=fbbef66d9e4d096242f3617de5d14d12705b4659

Signed-off-by Ivan Orlov <[email protected]>

---
 fs/9p/xattr.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

fs/9p/xattr.c +5 -3

index 50f7f3f6b55e..1974a38bce20 100644
@@ -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);