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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id q15-20020a056a00084f00b0050ab60bf37fsm5166814pfk.22.2022.04.20.11.11.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 20 Apr 2022 11:11:27 -0700 (PDT) Date: Wed, 20 Apr 2022 11:11:26 -0700 From: Kees Cook To: Jann Horn Cc: Johan Hovold , kernel test robot , Greg Kroah-Hartman , linux-usb@vger.kernel.org, stable@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH] USB: serial: Fix heap overflow in WHITEHEAT_GET_DTR_RTS Message-ID: <202204201056.5A1A6BAE04@keescook> References: <20220419041742.4117026-1-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-usb@vger.kernel.org On Wed, Apr 20, 2022 at 02:33:06PM +0200, Jann Horn wrote: > On Wed, Apr 20, 2022 at 10:14 AM Johan Hovold wrote: > > On Mon, Apr 18, 2022 at 09:17:42PM -0700, Kees Cook wrote: > > > This looks like it's harmless, as both the source and the destinations are > > > currently the same allocation size (4 bytes) and don't use their padding, > > > but if anything were to ever be added after the "mcr" member in "struct > > > whiteheat_private", it would be overwritten. The structs both have a > > > single u8 "mcr" member, but are 4 bytes in padded size. The memcpy() > > > destination was explicitly targeting the u8 member (size 1) with the > > > length of the whole structure (size 4), triggering the memcpy buffer > > > overflow warning: > > > > Ehh... No. The size of a structure with a single u8 is 1, not 4. There's > > nothing wrong with the current code even if the use of memcpy for this > > is a bit odd. I thought that was surprising too, and suspected it was something specific to the build (as Jann also suggested). I tracked it down[1] to "-mabi=apcs-gnu", which is from CONFIG_AEABI=n. whiteheat_private { __u8 mcr; /* 0 1 */ /* size: 4, cachelines: 1, members: 1 */ /* padding: 3 */ /* last cacheline: 4 bytes */ }; > > > > > In file included from include/linux/string.h:253, > > > from include/linux/bitmap.h:11, > > > from include/linux/cpumask.h:12, > > > from include/linux/smp.h:13, > > > from include/linux/lockdep.h:14, > > > from include/linux/spinlock.h:62, > > > from include/linux/mmzone.h:8, > > > from include/linux/gfp.h:6, > > > from include/linux/slab.h:15, > > > from drivers/usb/serial/whiteheat.c:17: > > > In function 'fortify_memcpy_chk', > > > inlined from 'firm_send_command' at drivers/usb/serial/whiteheat.c:587:4: > > > include/linux/fortify-string.h:328:25: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning] > > > 328 | __write_overflow_field(p_size_field, size); > > > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > So something is confused here. > > So something's going wrong in fortify_memcpy_chk()? It looks like it > is called with constant "size" equal to 1, and the condition sizeof(info->mcr) is 1. sizeof(struct whiteheat_dr_info) (with CONFIG_AEABI=n) is 4. Given nothing actually uses "struct whiteheat_dr_info", except for the sizing (har har), I suspect the better solution is just to do: info->mcr = command_info->result_buffer[0]; -Kees [1] https://godbolt.org/z/3YvM1MYWW -- Kees Cook