[PATCH] mm/slab: reject unsupported kmalloc sizes

[PATCH] mm/slab: reject unsupported kmalloc sizes

[Zi Yan]

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 <ziy@nvidia.com>
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")
---
 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

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

[Alan Stern]

On Mon, Aug 17, 2026 at 04:40:18PM -0400, 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 <ziy@nvidia.com>
> 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")

Thanks for doing this.  If you like, I can write a follow-up patch to 
remove the __GFP_NOWARN added in 4f2629ea67e72, now that it isn't needed 
any more.

Alan Stern