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

[Harry Yoo]

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? 

> 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/

-- 
Cheers,
Harry / Hyeonggon