From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C86FF3BD651; Wed, 26 Aug 2026 09:28:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787736499; cv=none; b=hRse3eLiMa7YsRm/pTSGxLo6XfktgM4ugV2MMehiUtt0jiMwiCAD6Fe7MtIrzv5QGAPLm/C4+9bNO4+ZUzRJpK4MfskOmgxJPOTFimQJLveFLGwKXtTclzjXaMRlygm4jBWdZLIcsVLZ6gx2huoNiF2XXJeLa1PNwsQsVUfrHHM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787736499; c=relaxed/simple; bh=zOlZYyvRs9ChzGU73C832xzR4WXH1lB3xuJGvoCrFpc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OFjgfMzBgEUCJrtpPh7KO9sGuHfHu+AVtFxlaFJqIcoV4bdgMI5kNrnkqsC9pt79dIVtgRVF4M/vXgd5P08lpdl5dOA2qMJ4KlHIhw8aYqwOEdoDAkYIxlzGUwDvsey8M5Z11p584nX1z4QWXuVLVm/wom5qja55/z9CBxso1WA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RkFXSijR; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="RkFXSijR" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 751711F01561; Wed, 26 Aug 2026 09:28:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787736497; bh=VkPDzgtqpJZ+C+pFu3IpQfmXypArccQSfZj/VF6NMbQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=RkFXSijRbA6EM3ZrjDU8tSwuriXKTTOKjpopi5CErpNWTeCRYBX/qsSUpyn8Ggw/R kPodnkEfLkODpYExzkYsht60y+Ht4NVpAtwCjHkGW071tyXfWzKONDRORVx0+2QE2h zAUsNR84Y0VJkVKEYg+xwJ/PsQmf2mhC46R5H69eX6Rj0okfWJkL0JaWA24/a/ue6K 8S/3idW0QkFMKJ3nRlZj8zfw3V9bg3e/B1zBxkMQNBUagKF5Fq7fD2KGSvyp8gNOXB 7N8bQJJqeIvVJDE0sVvbKo0VkfJSliyfgJII+5KVQG+v9W2NAhWYpNB1mlsc33s3gV W90ZgP8IBSMLQ== Message-ID: <82f138f5-11d4-4839-94a3-c226fd982715@kernel.org> Date: Wed, 26 Aug 2026 11:28:12 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/slab: reject unsupported kmalloc sizes To: Zi Yan , Harry Yoo , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Alan Stern , Greg Kroah-Hartman Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org, syzbot+805630f1453e490427fa@syzkaller.appspotmail.com, stable@vger.kernel.org References: <20260817-limit_kmalloc_size-v1-1-5bef487701cc@nvidia.com> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/17/26 22:40, Zi Yan wrote: > 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. Systems > with panic_on_warn=1 crash because of this WARN. Fix it by rejecting any > kmalloc size bigger than KMALLOC_MAX_SIZE. > > Fixes: aadb4bc4a1f9 ("SLUB: direct pass through of page size or higher kmalloc requests") > Reported-by: syzbot+805630f1453e490427fa@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/all/6a820ebc.9ebadd4d.20b15e.001b.GAE@google.com/ > Tested-by: syzbot+805630f1453e490427fa@syzkaller.appspotmail.com > Signed-off-by: Zi Yan > Cc: stable@vger.kernel.org > --- > It fixes a page allocator warning (order > MAX_PAGE_ORDER) when gadgetfs > requests excessively large memory from kmalloc. Instead of adding > __GFP_NOWARN to suppress the warning, as was done for usbfs[1], change > kmalloc to return NULL without a warning for this specific issue. > > [1] commit 4f2629ea67e72 ("USB: usbfs: Don't WARN about excessively large memory allocations") So I checked and for kvmalloc() we have in __kvmalloc_node_noprof() /* Don't even allow crazy sizes */ if (unlikely(size > INT_MAX)) { WARN_ON_ONCE(!(flags & __GFP_NOWARN)); return NULL; } This comes from Linus in commit 7661809d493b4. I'd do the same thing here then. Thus there would be a useful warning for e.g. development mistakes resulting in the size to be unexpectedly high. Callers passing size that comes from userspace or similar untrusted source can either pass __GFP_NOWARN or sanitize the size to what they expect to be sane (which is context dependent and I assume actually way lower than kmalloc limits in practice). Note that passing even sizes within but close to/at the limit, trusting blindly some external source, can succeed the allocations but effectively DoS the system with heavy reclaim/compaction. So caller sanitization should still be preferred IMHO. Some of the recent arguments from Linus [1] would apply to this too, I think. [1] https://lore.kernel.org/all/CAHk-=wiSmgwwLKCqJwGS-dVHnSLU8W+7q1UQq-G9=TBGGZbuhQ@mail.gmail.com/ > --- > mm/slub.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 0337e60db5ace..a3071f4ef1945 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -5263,7 +5263,12 @@ static void *___kmalloc_large_node(size_t size, gfp_t flags, int node) > { > struct page *page; > void *ptr = NULL; > - unsigned int order = get_order(size); > + unsigned int order; > + > + if (size > KMALLOC_MAX_SIZE) > + return NULL; > + > + order = get_order(size); > > if (unlikely(flags & GFP_SLAB_BUG_MASK)) > flags = kmalloc_fix_flags(flags); > > --- > base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f > change-id: 20260817-limit_kmalloc_size-3a4a2c73beac > > Best regards, > -- > Yan, Zi >