On 3/17/19 11:49 PM, syzbot wrote:

syzbot has bisected this bug to:

commit c981f254cc82f50f8cb864ce6432097b23195b9c
Author: Al Viro [off-list ref]
Date:   Sun Jan 7 18:19:09 2018 +0000

    sctp: use vmemdup_user() rather than badly open-coding memdup_user()

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=137bcecf200000
start commit:   c981f254 sctp: use vmemdup_user() rather than badly open-c..
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=10fbcecf200000
console output: https://syzkaller.appspot.com/x/log.txt?x=177bcecf200000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5e7dc790609552d7
dashboard link: https://syzkaller.appspot.com/bug?extid=ec1b7575afef85a0e5ca
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16a9a84b400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17199bb3400000

Reported-by: syzbot+ec1b7575afef85a0e5ca@syzkaller.appspotmail.com
Fixes: c981f254 ("sctp: use vmemdup_user() rather than badly open-coding memdup_user()")

From bisection log:

	testing release v4.17
	testing commit 29dcea88779c856c7dc92040a0c01233263101d4 with gcc (GCC) 8.1.0
	run #0: crashed: kernel panic: corrupted stack end in wb_workfn
	run #1: crashed: kernel panic: corrupted stack end in worker_thread
	run #2: crashed: kernel panic: Out of memory and no killable processes...
	run #3: crashed: kernel panic: corrupted stack end in wb_workfn
	run #4: crashed: kernel panic: corrupted stack end in wb_workfn
	run #5: crashed: kernel panic: corrupted stack end in wb_workfn
	run #6: crashed: kernel panic: corrupted stack end in wb_workfn
	run #7: crashed: kernel panic: corrupted stack end in wb_workfn
	run #8: crashed: kernel panic: Out of memory and no killable processes...
	run #9: crashed: kernel panic: corrupted stack end in wb_workfn
	testing release v4.16
	testing commit 0adb32858b0bddf4ada5f364a84ed60b196dbcda with gcc (GCC) 8.1.0
	run #0: OK
	run #1: OK
	run #2: OK
	run #3: OK
	run #4: OK
	run #5: crashed: kernel panic: Out of memory and no killable processes...
	run #6: OK
	run #7: crashed: kernel panic: Out of memory and no killable processes...
	run #8: OK
	run #9: OK
	testing release v4.15
	testing commit d8a5b80568a9cb66810e75b182018e9edb68e8ff with gcc (GCC) 8.1.0
	all runs: OK
	# git bisect start v4.16 v4.15

Why bisect started between 4.16 4.15 instead of 4.17 4.16?


	testing commit c14376de3a1befa70d9811ca2872d47367b48767 with gcc (GCC) 8.1.0
	run #0: crashed: kernel panic: Out of memory and no killable processes...
	run #1: crashed: kernel panic: Out of memory and no killable processes...
	run #2: crashed: kernel panic: Out of memory and no killable processes...
	run #3: crashed: kernel panic: Out of memory and no killable processes...
	run #4: OK
	run #5: OK
	run #6: crashed: WARNING: ODEBUG bug in netdev_freemem
	run #7: crashed: no output from test machine
	run #8: OK
	run #9: OK
	# git bisect bad c14376de3a1befa70d9811ca2872d47367b48767

Why c14376de3a1befa70d9811ca2872d47367b48767 is bad? There was no stack corruption.
It looks like the syzbot were bisecting a different bug - "kernel panic: Out of memory and no killable processes..."
And bisection for that bug seems to be correct. kvmalloc() in vmemdup_user() may eat up all memory unlike kmalloc which is limited by KMALLOC_MAX_SIZE (4MB usually).