[PATCH] net: skb: guard kmalloc_reserve() against oversized allocations

[PATCH] net: skb: guard kmalloc_reserve() against oversized allocations

[Kriish Sharma]

Add an explicit size check in kmalloc_reserve() to reject requests
larger than KMALLOC_MAX_SIZE before they reach the allocator.

syzbot reported warnings triggered by attempts to allocate buffers
with an object size exceeding KMALLOC_MAX_SIZE. While the existing
code relies on kmalloc() failure and a comment states that truncation
is "harmless", in practice this causes high-order allocation warnings
and noisy kernel logs that interfere with testing.

This patch introduces an early guard in kmalloc_reserve() that returns
NULL if obj_size exceeds KMALLOC_MAX_SIZE. This ensures impossible
requests fail fast and silently, avoiding allocator warnings while
keeping the intended semantics unchanged.

Fixes: 7fa4d8dc380f ("Add linux-next specific files for 20250821")
Reported-by: syzbot+5a2250fd91b28106c37b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5a2250fd91b28106c37b

Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
---
 net/core/skbuff.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index ee0274417948..98efa95ea038 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -591,6 +591,8 @@ static void *kmalloc_reserve(unsigned int *size, gfp_t flags, int node,
 	/* The following cast might truncate high-order bits of obj_size, this
 	 * is harmless because kmalloc(obj_size >= 2^32) will fail anyway.
 	 */
+	if (unlikely(obj_size > KMALLOC_MAX_SIZE))
+		return NULL;
 	*size = (unsigned int)obj_size;
 
 	/*
-- 
2.34.1

Re: [PATCH] net: skb: guard kmalloc_reserve() against oversized allocations

[Andrew Lunn]

On Sat, Sep 20, 2025 at 11:37:23AM +0000, Kriish Sharma wrote:
> Add an explicit size check in kmalloc_reserve() to reject requests
> larger than KMALLOC_MAX_SIZE before they reach the allocator.
> 
> syzbot reported warnings triggered by attempts to allocate buffers
> with an object size exceeding KMALLOC_MAX_SIZE. While the existing
> code relies on kmalloc() failure and a comment states that truncation
> is "harmless", in practice this causes high-order allocation warnings
> and noisy kernel logs that interfere with testing.
> 
> This patch introduces an early guard in kmalloc_reserve() that returns
> NULL if obj_size exceeds KMALLOC_MAX_SIZE. This ensures impossible
> requests fail fast and silently, avoiding allocator warnings while
> keeping the intended semantics unchanged.

What is the value of KMALLOC_MAX_SIZE? Do we actually want a
warn_once() here because it indicates a driver bug, or missing
validation of user space parameters?

	   Andrew