ANDROID: fuse: Add null terminator to path in canonical path to avoid…

… issue

page allocated in fuse_dentry_canonical_path to be handled in
fuse_dev_do_write is allocated using __get_free_pages(GFP_KERNEL).
This may not return a page with data filled with 0. Now this
page may not have a null terminator at all.
If this happens and userspace fuse daemon screws up by passing a string
to kernel which is not NULL terminated (or did not fill anything),
then inside fuse driver in kernel when we try to do
strlen(fuse_dev_write->kern_path->getname_kernel)
on that page data -> it may give us issue with kernel paging request.

Unable to handle kernel paging request at virtual address
------------[ cut here ]------------
<..>
PC is at strlen+0x10/0x90
LR is at getname_kernel+0x2c/0xf4
<..>
strlen+0x10/0x90
kern_path+0x28/0x4c
fuse_dev_do_write+0x5b8/0x694
fuse_dev_write+0x74/0x94
do_iter_readv_writev+0x80/0xb8
do_readv_writev+0xec/0x1cc
vfs_writev+0x54/0x64
SyS_writev+0x64/0xe4
el0_svc_naked+0x24/0x28

To avoid this we should ensure in case of FUSE_CANONICAL_PATH,
the page is null terminated.

Change-Id: I33ca7cc76b4472eaa982c67bb20685df451121f5
Signed-off-by: Ritesh Harjani <[email protected]>
Bug: 75984715
[Daniel - small edit, using args size ]
Signed-off-by: Daniel Rosenberg <[email protected]>

Signed-off-by: Nathan Chancellor <[email protected]>

fs/fuse/dev.c

@@ -1947,8 +1947,10 @@ static ssize_t fuse_dev_do_write(struct fuse_dev *fud,
 
 	err = copy_out_args(cs, &req->out, nbytes);
 	if (req->in.h.opcode == FUSE_CANONICAL_PATH) {
-		req->out.h.error = kern_path((char *)req->out.args[0].value, 0,
-							req->canonical_path);
+		char *path = (char *)req->out.args[0].value;
+
+		path[req->out.args[0].size - 1] = 0;
+		req->out.h.error = kern_path(path, 0, req->canonical_path);
 	}
 	fuse_copy_finish(cs);