[PATCH v2] padata: validate cpumask without removed CPU during offline

[Daniel Jordan]

Configuring an instance's parallel mask without any online CPUs...

  echo 2 > /sys/kernel/pcrypt/pencrypt/parallel_cpumask
  echo 0 > /sys/devices/system/cpu/cpu1/online

...crashes like this:

  divide error: 0000 [#1] SMP PTI
  CPU: 4 PID: 281 Comm: modprobe Not tainted 5.2.0-padata-base+ #25
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-<snip>
  RIP: 0010:padata_do_parallel+0xf1/0x270
  ...
  Call Trace:
   pcrypt_do_parallel+0xed/0x1e0 [pcrypt]
   pcrypt_aead_encrypt+0xbf/0xd0 [pcrypt]
   do_mult_aead_op+0x68/0x112 [tcrypt]
   test_mb_aead_speed.constprop.0.cold+0x21a/0x55a [tcrypt]
   do_test+0x2280/0x4ca2 [tcrypt]
   tcrypt_mod_init+0x55/0x1000 [tcrypt]
   ...

The cpumask_weight call in padata_cpu_hash returns 0, causing the
division error, because the mask has no CPUs, which is expected in this
situation.  The problem is __padata_remove_cpu doesn't mark the instance
PADATA_INVALID as expected, which would have made padata_do_parallel
return error before doing the division, because it checks for valid
masks too early.

Fix by moving the checks after the masks have been adjusted for the
offlined CPU.  Only do the second check if the first succeeded to avoid
inadvertently clearing PADATA_INVALID.

Stop the instance unconditionally and start again if the masks are
valid.  Stopping the instance only after an invalid mask is found risks
this div-by-0 crash since a padata_do_parallel call in another task
could happen between cpumask_clear_cpu and padata_validate_cpumask.

Fixes: 33e54450683c ("padata: Handle empty padata cpumasks")
Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Cc: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
Cc: linux-crypto@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---

v2: Don't leave the instance stopped if the masks are valid.

 kernel/padata.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/padata.c b/kernel/padata.c
index d056276a96ce..01460ea1d160 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -702,10 +702,7 @@ static int __padata_remove_cpu(struct padata_instance *pinst, int cpu)
 	struct parallel_data *pd = NULL;
 
 	if (cpumask_test_cpu(cpu, cpu_online_mask)) {
-
-		if (!padata_validate_cpumask(pinst, pinst->cpumask.pcpu) ||
-		    !padata_validate_cpumask(pinst, pinst->cpumask.cbcpu))
-			__padata_stop(pinst);
+		__padata_stop(pinst);
 
 		pd = padata_alloc_pd(pinst, pinst->cpumask.pcpu,
 				     pinst->cpumask.cbcpu);
@@ -716,6 +713,9 @@ static int __padata_remove_cpu(struct padata_instance *pinst, int cpu)
 
 		cpumask_clear_cpu(cpu, pd->cpumask.cbcpu);
 		cpumask_clear_cpu(cpu, pd->cpumask.pcpu);
+		if (padata_validate_cpumask(pinst, pd->cpumask.pcpu) &&
+		    padata_validate_cpumask(pinst, pd->cpumask.cbcpu))
+			__padata_start(pinst);
 	}
 
 	return 0;
-- 
2.22.0