xfs: check for deleted cursors when revalidating two btrees

In the Linux kernel, the following vulnerability has been resolved:

xfs: check for deleted cursors when revalidating two btrees

The free space and inode btree repair functions will rebuild both btrees

at the same time, after which it needs to evaluate both btrees to

confirm that the corruptions are gone.

However, Jiaming Zhang ran syzbot and produced a crash in the second

xchk_allocbt call. His root-cause analysis is as follows (with minor

corrections):

In xrep_revalidate_allocbt(), xchk_allocbt() is called twice (first

for BNOBT, second for CNTBT). The cause of this issue is that the

first call nullified the cursor required by the second call.

Let's first enter xrep_revalidate_allocbt() via following call chain:

xfs_file_ioctl() ->

xfs_ioc_scrubv_metadata() ->

xfs_scrub_metadata() ->

`sc->ops->repair_eval(sc)` ->

xrep_revalidate_allocbt()

xchk_allocbt() is called twice in this function. In the first call:

/* Note that sc->sm->sm_type is XFS_SCRUB_TYPE_BNOPT now */

xchk_allocbt() ->

xchk_btree() ->

`bs->scrub_rec(bs, recp)` ->

xchk_allocbt_rec() ->

xchk_allocbt_xref() ->

xchk_allocbt_xref_other()

since sm_type is XFS_SCRUB_TYPE_BNOBT, pur is set to &sc->sa.cnt_cur.

Kernel called xfs_alloc_get_rec() and returned -EFSCORRUPTED. Call

chain:

xfs_alloc_get_rec() ->

xfs_btree_get_rec() ->

xfs_btree_check_block() ->

(XFS_IS_CORRUPT || XFS_TEST_ERROR), the former is false and the latter

is true, return -EFSCORRUPTED. This should be caused by

ioctl$XFS_IOC_ERROR_INJECTION I guess.

Back to xchk_allocbt_xref_other(), after receiving -EFSCORRUPTED from

xfs_alloc_get_rec(), kernel called xchk_should_check_xref(). In this

function, *curpp (points to sc->sa.cnt_cur) is nullified.

Back to xrep_revalidate_allocbt(), since sc->sa.cnt_cur has been

nullified, it then triggered null-ptr-deref via xchk_allocbt() (second

call) -> xchk_btree().

So. The bnobt revalidation failed on a cross-reference attempt, so we

deleted the cntbt cursor, and then crashed when we tried to revalidate

the cntbt. Therefore, check for a null cntbt cursor before that

revalidation, and mark the repair incomplete. Also we can ignore the

second tree entirely if the first tree was rebuilt but is already

corrupt.

Apply the same fix to xrep_revalidate_iallocbt because it has the same

problem.