On Sat, Aug 28, 2021 at 08:28:17PM +0100, Al Viro wrote:
> AFAICS, a48b73eca4ce "btrfs: fix potential deadlock in the search ioctl"
> has introduced a bug at least on arm64.
>
> Relevant bits: in search_ioctl() we have
> while (1) {
> ret = fault_in_pages_writeable(ubuf + sk_offset,
> *buf_size - sk_offset);
> if (ret)
> break;
>
> ret = btrfs_search_forward(root, &key, path, sk->min_transid);
> if (ret != 0) {
> if (ret > 0)
> ret = 0;
> goto err;
> }
> ret = copy_to_sk(path, &key, sk, buf_size, ubuf,
> &sk_offset, &num_found);
> btrfs_release_path(path);
> if (ret)
> break;
>
> }
> and in copy_to_sk() -
> sh.objectid = key->objectid;
> sh.offset = key->offset;
> sh.type = key->type;
> sh.len = item_len;
> sh.transid = found_transid;
>
> /*
> * Copy search result header. If we fault then loop again so
> we
> * can fault in the pages and -EFAULT there if there's a
> * problem. Otherwise we'll fault and then copy the buffer in
> * properly this next time through
> */
> if (copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh)))
> {
> ret = 0;
> goto out;
> }
> with sk_offset left unchanged if the very first copy_to_user_nofault() fails.
>
> Now, consider a situation on arm64 where ubuf points to the beginning of page,
> ubuf[0] can be accessed, but ubuf[16] can not (possible with MTE, AFAICS).
> We do
> fault_in_pages_writeable(), which succeeds. When we get to
> copy_to_user_nofault()
> we fail as soon as it gets past the first 16 bytes. And we repeat everything
> from
> scratch, with no progress made, since short copies are treated as "discard and
> repeat" here.
So if copy_to_user_nofault() returns -EFAULT, copy_to_sk() returns 0
(following commit a48b73eca4ce). I think you are right, search_ioctl()
can get into an infinite loop attempting to write to user if the
architecture can trigger faults at smaller granularity than the page
boundary. fault_in_pages_writeable() won't fix it if ubuf[0] is
writable and doesn't trigger an MTE tag check fault.
An arm64-specific workaround would be for pagefault_disable() to disable
tag checking. It's a pretty big hammer, weakening the out of bounds
access detection of MTE. My preference would be a fix in the btrfs code.
A btrfs option would be for copy_to_sk() to return an indication of
where the fault occurred and get fault_in_pages_writeable() to check
that location, even if the copying would restart from an earlier offset
(this requires open-coding copy_to_user_nofault()). An attempt below,
untested and does not cover read_extent_buffer_to_user_nofault():
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 0ba98e08a029..9e74ba1c955d 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2079,6 +2079,7 @@ static noinline int copy_to_sk(struct btrfs_path *path,
size_t *buf_size,
char __user *ubuf,
unsigned long *sk_offset,
+ unsigned long *fault_offset,
int *num_found)
{
u64 found_transid;
@@ -2143,7 +2144,11 @@ static noinline int copy_to_sk(struct btrfs_path *path,
* problem. Otherwise we'll fault and then copy the buffer in
* properly this next time through
*/
- if (copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh))) {
+ pagefault_disable();
+ ret = __copy_to_user_inatomic(ubuf + *sk_offset, &sh,
sizeof(sh));
+ pagefault_enable();
+ *fault_offset = *sk_offset + sizeof(sh) - ret;
+ if (ret) {
ret = 0;
goto out;
}
@@ -2218,6 +2223,7 @@ static noinline int search_ioctl(struct inode *inode,
int ret;
int num_found = 0;
unsigned long sk_offset = 0;
+ unsigned long fault_offset = 0;
if (*buf_size < sizeof(struct btrfs_ioctl_search_header)) {
*buf_size = sizeof(struct btrfs_ioctl_search_header);
@@ -2244,8 +2250,8 @@ static noinline int search_ioctl(struct inode *inode,
key.offset = sk->min_offset;
while (1) {
- ret = fault_in_pages_writeable(ubuf + sk_offset,
- *buf_size - sk_offset);
+ ret = fault_in_pages_writeable(ubuf + fault_offset,
+ *buf_size - fault_offset);
if (ret)
break;
@@ -2256,7 +2262,7 @@ static noinline int search_ioctl(struct inode *inode,
goto err;
}
ret = copy_to_sk(path, &key, sk, buf_size, ubuf,
- &sk_offset, &num_found);
+ &sk_offset, &fault_offset, &num_found);
btrfs_release_path(path);
if (ret)
break;
--
Catalin
