5.4-stable review patch. If anyone has any objections, please let me know.
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From: Krister Johansen kjlx@templeofstupid.com
commit 80f130bfad1dab93b95683fc39b87235682b8f72 upstream.
The documentation in rculist.h explains the absence of list_empty_rcu() and cautions programmers against relying on a list_empty() -> list_first() sequence in RCU safe code. This is because each of these functions performs its own READ_ONCE() of the list head. This can lead to a situation where the list_empty() sees a valid list entry, but the subsequent list_first() sees a different view of list head state after a modification.
In the case of dm-thin, this author had a production box crash from a GP fault in the process_deferred_bios path. This function saw a valid list head in get_first_thin() but when it subsequently dereferenced that and turned it into a thin_c, it got the inside of the struct pool, since the list was now empty and referring to itself. The kernel on which this occurred printed both a warning about a refcount_t being saturated, and a UBSAN error for an out-of-bounds cpuid access in the queued spinlock, prior to the fault itself. When the resulting kdump was examined, it was possible to see another thread patiently waiting in thin_dtr's synchronize_rcu.
The thin_dtr call managed to pull the thin_c out of the active thins list (and have it be the last entry in the active_thins list) at just the wrong moment which lead to this crash.
Fortunately, the fix here is straight forward. Switch get_first_thin() function to use list_first_or_null_rcu() which performs just a single READ_ONCE() and returns NULL if the list is already empty.
This was run against the devicemapper test suite's thin-provisioning suites for delete and suspend and no regressions were observed.
Signed-off-by: Krister Johansen kjlx@templeofstupid.com Fixes: b10ebd34ccca ("dm thin: fix rcu_read_lock being held in code that can sleep") Cc: stable@vger.kernel.org Acked-by: Ming-Hung Tsai mtsai@redhat.com Signed-off-by: Mikulas Patocka mpatocka@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/md/dm-thin.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)
--- a/drivers/md/dm-thin.c +++ b/drivers/md/dm-thin.c @@ -2325,10 +2325,9 @@ static struct thin_c *get_first_thin(str struct thin_c *tc = NULL;
rcu_read_lock(); - if (!list_empty(&pool->active_thins)) { - tc = list_entry_rcu(pool->active_thins.next, struct thin_c, list); + tc = list_first_or_null_rcu(&pool->active_thins, struct thin_c, list); + if (tc) thin_get(tc); - } rcu_read_unlock();
return tc;