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: [email protected]
>> 
>> 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
>
> --- 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.

Let’s do this instead of suppressing kmalloc WARNs. Since a similar WARN[1]
showed up yesterday and that WARN is better handled by a bound check,
I do not think we want to lose the WARN from kmalloc/the page allocator.

[1] https://lore.kernel.org/all/[email protected]/

Like the patch below:

From 732ea7074854ac3495bbceb274cdd01966118e57 Mon Sep 17 00:00:00 2001
From: Zi Yan <[email protected]>
Date: Thu, 20 Aug 2026 09:19:12 -0400
Subject: [PATCH] USB: gadgetfs: do not WARN about excessively large memory
 allocations

GadgetFS passes an excessively large user input len to kmalloc and kmalloc
gives a WARN (see below for details). Suppress it by passing __GFP_NOWARN
to kmalloc used by both ep_write_iter() and ep_read_iter(). Follow the same
method as commit 4f2629ea67e72 ("USB: usbfs: Don't WARN about excessively
large memory allocations").

kmalloc is used to allocate physically contiguous memory for kernel
allocations. For requests larger than KMALLOC_MAX_CACHE_SIZE, kmalloc uses
the page allocator and can only support up to KMALLOC_MAX_SIZE. For request
sizes bigger than KMALLOC_MAX_SIZE, the page allocator can emit a WARN
because kmalloc allocates an order greater than MAX_PAGE_ORDER.

Fixes: b3c466ce5129 ("page allocator: do not sanity check order in the fast path")
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Tested-by: [email protected]
Signed-off-by: Zi Yan <[email protected]>
Cc: [email protected]
---
 drivers/usb/gadget/legacy/inode.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/legacy/inode.c b/drivers/usb/gadget/legacy/inode.c
index d87a8ab515107..278a0a2b39f4c 100644
--- a/drivers/usb/gadget/legacy/inode.c
+++ b/drivers/usb/gadget/legacy/inode.c
@@ -604,7 +604,7 @@ ep_read_iter(struct kiocb *iocb, struct iov_iter *to)
 		return -EBADMSG;
 	}
 
-	buf = kmalloc(len, GFP_KERNEL);
+	buf = kmalloc(len, GFP_KERNEL | __GFP_NOWARN);
 	if (unlikely(!buf)) {
 		mutex_unlock(&epdata->lock);
 		return -ENOMEM;
@@ -666,7 +666,7 @@ ep_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		return -EBADMSG;
 	}
 
-	buf = kmalloc(len, GFP_KERNEL);
+	buf = kmalloc(len, GFP_KERNEL | __GFP_NOWARN);
 	if (unlikely(!buf)) {
 		mutex_unlock(&epdata->lock);
 		return -ENOMEM;
-- 
2.53.0






-- 
Best Regards,
Yan, Zi