Re: [syzbot] [mm?] WARNING in ep_write_iter

On Sun Aug 16, 2026 at 4:52 PM EDT, Andrew Morton wrote:
> On Sun, 16 Aug 2026 12:25:48 -0700 syzbot <[email protected]> wrote:
>
>> Hello,
>> 
>> syzbot found the following issue on:
>> 
>> HEAD commit:    3d6d817622b0 Merge tag 'scsi-fixes' of git://git.kernel.or..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=15927479580000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=a59830cba91a1981
>> dashboard link: https://syzkaller.appspot.com/bug?extid=805630f1453e490427fa
>> compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
>> 
>> Unfortunately, I don't have any reproducer for this issue yet.
>> 
>> Downloadable assets:
>> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-3d6d8176.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/d19e0514c02a/vmlinux-3d6d8176.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/f6da706811f4/bzImage-3d6d8176.xz
>> 
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+805630f1453e490427fa-Pl5Pbv+GP7P466ipTTIvnc23WoclnBCfAL8bYrjMMd8@public.gmane.org
>> 
>> gadgetfs: bound to dummy_udc driver
>> ------------[ cut here ]------------
>> 1
>> WARNING: mm/page_alloc.c:5280 at __alloc_frozen_pages_noprof+0x2ce/0x380 mm/page_alloc.c:5280, CPU#0: syz.0.0/5319
>
> Thanks.  drivers/usb/gadget is the offender.
>
> Gemini sums it up well.  "ep_write_iter() needs a bounds check prior to
> memory allocation".  https://share.gemini.google/5NzjyttO0ULc
>
> I expect an easy fix would be

Maybe it is better to stop asking kmalloc for unreasonable len:


diff --git a/drivers/usb/gadget/legacy/inode.c b/drivers/usb/gadget/legacy/inode.c
index d87a8ab515107..20905f254968f 100644
--- a/drivers/usb/gadget/legacy/inode.c
+++ b/drivers/usb/gadget/legacy/inode.c
@@ -645,6 +645,9 @@ ep_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	ssize_t value;
 	char *buf;
 
+	if (len > KMALLOC_MAX_SIZE)
+		return -EINVAL;
+
 	if ((value = get_ready_ep(file->f_flags, epdata, true)) < 0)
 		return value;

>
> --- a/drivers/usb/gadget/legacy/inode.c~a
> +++ a/drivers/usb/gadget/legacy/inode.c
> @@ -666,7 +666,7 @@ ep_write_iter(struct kiocb *iocb, struct
>  		return -EBADMSG;
>  	}
>  
> -	buf = kmalloc(len, GFP_KERNEL);
> +	buf = kmalloc(len, GFP_KERNEL|__GFP_NOWARN);
>  	if (unlikely(!buf)) {
>  		mutex_unlock(&epdata->lock);
>  		return -ENOMEM;
>
> or do what Gemini said.  Me, I'll add some cc's and run away.
>
>
>> Modules linked in:
>> CPU: 0 UID: 0 PID: 5319 Comm: syz.0.0 Not tainted syzkaller #0 PREEMPT(full) 
>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
>> RIP: 0010:__alloc_frozen_pages_noprof+0x2ce/0x380 mm/page_alloc.c:5280
>> Code: 74 10 4c 89 e7 89 54 24 0c e8 0e 2c 0e 00 8b 54 24 0c 49 83 3c 24 00 0f 85 ab fe ff ff e9 ac fe ff ff c6 05 95 d2 18 0e 01 90 <0f> 0b 90 e9 17 ff ff ff a9 00 00 08 00 48 8b 4c 24 10 4c 8d 44 24
>> RSP: 0018:ffffc9000e13fa20 EFLAGS: 00010246
>> RAX: ffffc9000e13fa00 RBX: 0000000000000013 RCX: 0000000000000000
>> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffc9000e13fa88
>> RBP: ffffc9000e13fb08 R08: ffffc9000e13fa87 R09: 0000000000000000
>> R10: ffffc9000e13fa60 R11: fffff52001c27f51 R12: 0000000000000000
>> R13: 1ffff92001c27f48 R14: 0000000000040cc0 R15: dffffc0000000000
>> FS:  00007f6f4a58c6c0(0000) GS:ffff88808c549000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00002000000010a0 CR3: 000000004323d000 CR4: 0000000000352ef0
>> Call Trace:
>>  <TASK>
>>  alloc_pages_mpol+0x212/0x380 mm/mempolicy.c:2490
>>  ___kmalloc_large_node+0x4c/0x120 mm/slub.c:5274
>>  __kmalloc_large_node_noprof+0x18/0x90 mm/slub.c:5305
>>  __do_kmalloc_node mm/slub.c:5322 [inline]
>>  __kmalloc_noprof+0x405/0x720 mm/slub.c:5359
>>  _kmalloc_noprof include/linux/slab.h:992 [inline]
>>  ep_write_iter+0x1bc/0x500 drivers/usb/gadget/legacy/inode.c:669
>>  new_sync_write fs/read_write.c:595 [inline]
>>  vfs_write+0x612/0xba0 fs/read_write.c:687
>>  ksys_write+0x150/0x270 fs/read_write.c:739
>>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>>  do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
>>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>> RIP: 0033:0x7f6f4979e0d9
>> Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
>> RSP: 002b:00007f6f4a58bfe8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
>> RAX: ffffffffffffffda RBX: 00007f6f49a25fa0 RCX: 00007f6f4979e0d9
>> RDX: 00000000ffffffb9 RSI: 0000000000000000 RDI: 0000000000000004
>> RBP: 00007f6f49835024 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>> R13: 00007f6f49a26038 R14: 00007f6f49a25fa0 R15: 00007ffc6cf49328
>>  </TASK>
>> 




-- 
Best Regards,
Yan, Zi