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[198.0.35.241]) by smtp.gmail.com with ESMTPSA id d12-20020a170902c18c00b001b694140d96sm11363171pld.170.2023.09.06.11.16.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 Sep 2023 11:16:31 -0700 (PDT) Date: Wed, 6 Sep 2023 11:16:30 -0700 From: Kees Cook To: David Laight Cc: "linux-kernel@vger.kernel.org" , "'linux-mm@kvack.org'" , 'Vlastimil Babka' , 'Christoph Lameter' , 'Pekka Enberg' , 'David Rientjes' , 'Joonsoo Kim' , 'Andrew Morton' , 'Eric Dumazet' Subject: Re: [PATCH] slab: kmalloc_size_roundup() must not return 0 for non-zero size Message-ID: <202309061106.C0690BDBB@keescook> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: A63AE40014 X-Rspam-User: X-Stat-Signature: fqhdc963xt8j4p5ohz57masj7j4rtr4t X-Rspamd-Server: rspam03 X-HE-Tag: 1694024193-761728 X-HE-Meta: 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 8FoDPWps 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 06, 2023 at 08:18:21AM +0000, David Laight wrote: > The typical use of kmalloc_size_roundup() is: > ptr = kmalloc(sz = kmalloc_size_roundup(size), ...); > if (!ptr) return -ENOMEM. > This means it is vitally important that the returned value isn't > less than the argument even if the argument is insane. > In particular if kmalloc_slab() fails or the value is above > (MAX_ULONG - PAGE_SIZE) zero is returned and kmalloc() will return > it's single zero-length buffer. > > Fix by returning the input size on error or if the size exceeds > a 'sanity' limit. > kmalloc() will then return NULL is the size really is too big. > > Signed-off-by: David Laight > Fixes: 05a940656e1eb ("slab: Introduce kmalloc_size_roundup()") > --- > The 'sanity limit' value doesn't really matter (even if too small) > It could be 'MAX_ORDER + PAGE_SHIFT' but one ppc64 has MAX_ORDER 16 > and I don't know if that also has large pages. > Maybe it could be 1ul << 30 on 64bit, but it really doesn't matter > if it is too big. I agree that returning 0 for an (impossible to reach) non-zero is wrong, but the problem seen in netdev was that a truncation happened for a value returned by kmalloc_size_roundup(). So, for the first, it shouldn't be possible for "c" to ever be NULL here: c = kmalloc_slab(size, GFP_KERNEL, 0); return c ? c->object_size : 0; But sure, we can return KMALLOC_MAX_SIZE for that. The pathological case was this: unsigned int truncated; size_t fullsize = UINT_MAX + 1; ptr = kmalloc(truncated = kmalloc_size_roundup(fullsize), ...); Should the logic be changed to return KMALLOC_MAX_SIZE for anything larger than KMALLOC_MAX_SIZE? This seems like a different kind of foot-gun. Everything else in the allocator sanity checking (e.g. struct_size(), etc) uses SIZE_MAX as the saturation value, which is why kmalloc_size_roundup() did too. -- Kees Cook