Re: [PATCH v2] tpm: eventlog: Replace zero-length array with flexible-array member

Kees Cook
2020-05-12T05:24:15Z

On Fri, May 08, 2020 at 11:38:26AM -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.
>
> Also, the following issue shows up due to the flexible-array member
> having incomplete type[4]:
>
> drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_start’:
> drivers/char/tpm/eventlog/tpm2.c:54:46: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
> 54 | size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
> | ^
> drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_next’:
> drivers/char/tpm/eventlog/tpm2.c:102:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
> 102 | sizeof(event_header->event) + event_header->event_size;
> | ^
> drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_binary_bios_measurements_show’:
> drivers/char/tpm/eventlog/tpm2.c:140:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’}
> 140 | sizeof(event_header->event) + event_header->event_size;
> | ^
> scripts/Makefile.build:266: recipe for target 'drivers/char/tpm/eventlog/tpm2.o' failed
> make[3]: *** [drivers/char/tpm/eventlog/tpm2.o] Error 1
>
> As mentioned above: "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] As
> in "sizeof(event_header->event) always evaluated to 0, so removing it
> has no effect".
>
> Lastly, make use of the struct_size() helper to deal with the
> flexible array member and its host structure.
>
> This issue was found with the help of Coccinelle.
>
> [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")
> [4] https://github.com/KSPP/linux/issues/43
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook