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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id g18sm6353691pgn.47.2020.05.29.01.31.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 29 May 2020 01:31:56 -0700 (PDT) Date: Fri, 29 May 2020 01:31:54 -0700 From: Kees Cook To: "Gustavo A. R. Silva" Cc: Ard Biesheuvel , linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, "Gustavo A. R. Silva" Subject: Re: [PATCH] efi: Replace zero-length array and use struct_size() helper Message-ID: <202005290131.4B104937C@keescook> References: <20200527171425.GA4053@embeddedor> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200527171425.GA4053@embeddedor> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 27, 2020 at 12:14:25PM -0500, Gustavo A. R. Silva wrote: > The current codebase makes use of the zero-length array language > extension to the C90 standard, but the preferred mechanism to declare > variable-length types such as these ones is a flexible array member[1][2], > introduced in C99: > > struct foo { > int stuff; > struct boo array[]; > }; > > By making use of the mechanism above, we will get a compiler warning > in case the flexible array does not occur last in the structure, which > will help us prevent some kind of undefined behavior bugs from being > inadvertently introduced[3] to the codebase from now on. > > Also, notice that, dynamic memory allocations won't be affected by > this change: > > "Flexible array members have incomplete type, and so the sizeof operator > may not be applied. As a quirk of the original implementation of > zero-length arrays, sizeof evaluates to zero."[1] > > sizeof(flexible-array-member) triggers a warning because flexible array > members have incomplete type[1]. There are some instances of code in > which the sizeof operator is being incorrectly/erroneously applied to > zero-length arrays and the result is zero. Such instances may be hiding > some bugs. So, this work (flexible-array member conversions) will also > help to get completely rid of those sorts of issues. > > Lastly, make use of the sizeof_field() helper instead of an open-coded > version. > > This issue was found with the help of Coccinelle and audited _manually_. > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html > [2] https://github.com/KSPP/linux/issues/21 > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour") > > Signed-off-by: Gustavo A. R. Silva Reviewed-by: Kees Cook > --- > drivers/firmware/efi/efi.c | 3 ++- > include/linux/efi.h | 7 ++----- > 2 files changed, 4 insertions(+), 6 deletions(-) > > diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c > index 7f1657b6c30df..edc5d36caf54e 100644 > --- a/drivers/firmware/efi/efi.c > +++ b/drivers/firmware/efi/efi.c > @@ -622,7 +622,8 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables, > rsv = (void *)(p + prsv % PAGE_SIZE); > > /* reserve the entry itself */ > - memblock_reserve(prsv, EFI_MEMRESERVE_SIZE(rsv->size)); > + memblock_reserve(prsv, > + struct_size(rsv, entry, rsv->size)); > > for (i = 0; i < atomic_read(&rsv->count); i++) { > memblock_reserve(rsv->entry[i].base, > diff --git a/include/linux/efi.h b/include/linux/efi.h > index c45ac969ea4eb..328cc52a5fd45 100644 > --- a/include/linux/efi.h > +++ b/include/linux/efi.h > @@ -1234,14 +1234,11 @@ struct linux_efi_memreserve { > struct { > phys_addr_t base; > phys_addr_t size; > - } entry[0]; > + } entry[]; > }; > > -#define EFI_MEMRESERVE_SIZE(count) (sizeof(struct linux_efi_memreserve) + \ > - (count) * sizeof(((struct linux_efi_memreserve *)0)->entry[0])) > - > #define EFI_MEMRESERVE_COUNT(size) (((size) - sizeof(struct linux_efi_memreserve)) \ > - / sizeof(((struct linux_efi_memreserve *)0)->entry[0])) > + / sizeof_field(struct linux_efi_memreserve, entry[0])) Whoa. This is kind of a "reverse struct_size()". I wonder if any other places in the kernel do a similar calculation? -- Kees Cook