padata: Fix possible divide-by-0 panic in padata_mt_helper()

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From: Waiman Long <longman@xxxxxxxxxx>

commit 6d45e1c948a8b7ed6ceddb14319af69424db730c upstream.

We are hit with a not easily reproducible divide-by-0 panic in padata.c at
bootup time.

  [   10.017908] Oops: divide error: 0000 1 PREEMPT SMP NOPTI
  [   10.017908] CPU: 26 PID: 2627 Comm: kworker/u1666:1 Not tainted 6.10.0-15.el10.x86_64 #1
  [   10.017908] Hardware name: Lenovo ThinkSystem SR950 [7X12CTO1WW]/[7X12CTO1WW], BIOS [PSE140J-2.30] 07/20/2021
  [   10.017908] Workqueue: events_unbound padata_mt_helper
  [   10.017908] RIP: 0010:padata_mt_helper+0x39/0xb0
    :
  [   10.017963] Call Trace:
  [   10.017968]  <TASK>
  [   10.018004]  ? padata_mt_helper+0x39/0xb0
  [   10.018084]  process_one_work+0x174/0x330
  [   10.018093]  worker_thread+0x266/0x3a0
  [   10.018111]  kthread+0xcf/0x100
  [   10.018124]  ret_from_fork+0x31/0x50
  [   10.018138]  ret_from_fork_asm+0x1a/0x30
  [   10.018147]  </TASK>

Looking at the padata_mt_helper() function, the only way a divide-by-0
panic can happen is when ps->chunk_size is 0.  The way that chunk_size is
initialized in padata_do_multithreaded(), chunk_size can be 0 when the
min_chunk in the passed-in padata_mt_job structure is 0.

Fix this divide-by-0 panic by making sure that chunk_size will be at least
1 no matter what the input parameters are.

Link: https://lkml.kernel.org/r/20240806174647.1050398-1-longman@xxxxxxxxxx
Fixes: 004ed42638f4 ("padata: add basic support for multithreaded jobs")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
Cc: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/padata.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -521,6 +521,13 @@ void __init padata_do_multithreaded(stru
 	ps.chunk_size = max(ps.chunk_size, job->min_chunk);
 	ps.chunk_size = roundup(ps.chunk_size, job->align);
 
+	/*
+	 * chunk_size can be 0 if the caller sets min_chunk to 0. So force it
+	 * to at least 1 to prevent divide-by-0 panic in padata_mt_helper().`
+	 */
+	if (!ps.chunk_size)
+		ps.chunk_size = 1U;
+
 	list_for_each_entry(pw, &works, pw_list)
 		queue_work(system_unbound_wq, &pw->pw_work);
 


Patches currently in stable-queue which might be from longman@xxxxxxxxxx are

queue-5.10/padata-fix-possible-divide-by-0-panic-in-padata_mt_helper.patch




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