Re: [PATCH] mm/slab: reject unsupported kmalloc sizes

[Zi Yan]

On Wed Aug 26, 2026 at 5:47 PM EDT, Harry Yoo wrote:
> On Wed, Aug 26, 2026 at 11:28:12AM +0000, Vlastimil Babka (SUSE) wrote:
>> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>> > Closes: https://lore.kernel.org/all/6a820ebc.9ebadd4d.20b15e.001b.GAE@xxxxxxxxxx/
>> > Tested-by: syzbot+805630f1453e490427fa@xxxxxxxxxxxxxxxxxxxxxxxxx
>> > Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> > Cc: stable@xxxxxxxxxxxxxxx
>> > ---
>> > 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.
>
> But the purpose of this patch is to avoid the warning in the page
> allocator. Should we fix this in the caller (gadgetfs) then? 

It is fixed by: https://lore.kernel.org/all/20260820223719.A4A3C1F000E9@xxxxxxxxxxxxxxx/

Please disregard this patch, but we can keep the discussion going.


>
>> 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@xxxxxxxxxxxxxx/




-- 
Best Regards,
Yan, Zi