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

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

[Gustavo A. R. Silva]

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>
---
Changes in v2:
 - Update changelog text.
 - Make use of the struct_size() helper.

 drivers/char/tpm/eventlog/tpm2.c | 12 +++++-------
 include/linux/tpm_eventlog.h     |  2 +-
 2 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/char/tpm/eventlog/tpm2.c b/drivers/char/tpm/eventlog/tpm2.c
index e741b1157525..37a05800980c 100644
--- a/drivers/char/tpm/eventlog/tpm2.c
+++ b/drivers/char/tpm/eventlog/tpm2.c
@@ -51,8 +51,7 @@ static void *tpm2_bios_measurements_start(struct seq_file *m, loff_t *pos)
 	int i;
 
 	event_header = addr;
-	size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
-		+ event_header->event_size;
+	size = struct_size(event_header, event, event_header->event_size);
 
 	if (*pos == 0) {
 		if (addr + size < limit) {
@@ -98,8 +97,8 @@ static void *tpm2_bios_measurements_next(struct seq_file *m, void *v,
 	event_header = log->bios_event_log;
 
 	if (v == SEQ_START_TOKEN) {
-		event_size = sizeof(struct tcg_pcr_event) -
-			sizeof(event_header->event) + event_header->event_size;
+		event_size = struct_size(event_header, event,
+					 event_header->event_size);
 		marker = event_header;
 	} else {
 		event = v;
@@ -136,9 +135,8 @@ static int tpm2_binary_bios_measurements_show(struct seq_file *m, void *v)
 	size_t size;
 
 	if (v == SEQ_START_TOKEN) {
-		size = sizeof(struct tcg_pcr_event) -
-			sizeof(event_header->event) + event_header->event_size;
-
+		size = struct_size(event_header, event,
+				   event_header->event_size);
 		temp_ptr = event_header;
 
 		if (size > 0)
diff --git a/include/linux/tpm_eventlog.h b/include/linux/tpm_eventlog.h
index c253461b1c4e..4f8c90c93c29 100644
--- a/include/linux/tpm_eventlog.h
+++ b/include/linux/tpm_eventlog.h
@@ -97,7 +97,7 @@ struct tcg_pcr_event {
 	u32 event_type;
 	u8 digest[20];
 	u32 event_size;
-	u8 event[0];
+	u8 event[];
 } __packed;
 
 struct tcg_event_field {
-- 
2.26.2

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

[Kees Cook]

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

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

[Gustavo A. R. Silva]

On Mon, May 11, 2020 at 10:24:10PM -0700, Kees Cook wrote:
> 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>
> 

Thanks, Kees.

--
Gustavo

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

[Jarkko Sakkinen]

On Fri, 2020-05-08 at 11:38 -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: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

/Jarkko

> ---
> Changes in v2:
>  - Update changelog text.
>  - Make use of the struct_size() helper.
> 
>  drivers/char/tpm/eventlog/tpm2.c | 12 +++++-------
>  include/linux/tpm_eventlog.h     |  2 +-
>  2 files changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/char/tpm/eventlog/tpm2.c b/drivers/char/tpm/eventlog/tpm2.c
> index e741b1157525..37a05800980c 100644
> --- a/drivers/char/tpm/eventlog/tpm2.c
> +++ b/drivers/char/tpm/eventlog/tpm2.c
> @@ -51,8 +51,7 @@ static void *tpm2_bios_measurements_start(struct seq_file *m, loff_t *pos)
>  	int i;
>  
>  	event_header = addr;
> -	size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event)
> -		+ event_header->event_size;
> +	size = struct_size(event_header, event, event_header->event_size);
>  
>  	if (*pos == 0) {
>  		if (addr + size < limit) {
> @@ -98,8 +97,8 @@ static void *tpm2_bios_measurements_next(struct seq_file *m, void *v,
>  	event_header = log->bios_event_log;
>  
>  	if (v == SEQ_START_TOKEN) {
> -		event_size = sizeof(struct tcg_pcr_event) -
> -			sizeof(event_header->event) + event_header->event_size;
> +		event_size = struct_size(event_header, event,
> +					 event_header->event_size);
>  		marker = event_header;
>  	} else {
>  		event = v;
> @@ -136,9 +135,8 @@ static int tpm2_binary_bios_measurements_show(struct seq_file *m, void *v)
>  	size_t size;
>  
>  	if (v == SEQ_START_TOKEN) {
> -		size = sizeof(struct tcg_pcr_event) -
> -			sizeof(event_header->event) + event_header->event_size;
> -
> +		size = struct_size(event_header, event,
> +				   event_header->event_size);
>  		temp_ptr = event_header;
>  
>  		if (size > 0)
> diff --git a/include/linux/tpm_eventlog.h b/include/linux/tpm_eventlog.h
> index c253461b1c4e..4f8c90c93c29 100644
> --- a/include/linux/tpm_eventlog.h
> +++ b/include/linux/tpm_eventlog.h
> @@ -97,7 +97,7 @@ struct tcg_pcr_event {
>  	u32 event_type;
>  	u8 digest[20];
>  	u32 event_size;
> -	u8 event[0];
> +	u8 event[];
>  } __packed;
>  
>  struct tcg_event_field {

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

[Jarkko Sakkinen]

On Mon, 2020-05-11 at 22:24 -0700, Kees Cook wrote:
> 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>

Thank you.

I applied this patch, will include it to the next PR.

/Jarkko

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

[Gustavo A. R. Silva]

On Thu, May 14, 2020 at 03:08:58AM +0300, Jarkko Sakkinen wrote:
> > > 
> > > Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> > 
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> 
> Thank you.
> 
> I applied this patch, will include it to the next PR.
> 

Awesome. :)

Thanks, Jarkko.
--
Gustavo

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

[Jarkko Sakkinen]

On Wed, 2020-05-13 at 19:23 -0500, Gustavo A. R. Silva wrote:
> On Thu, May 14, 2020 at 03:08:58AM +0300, Jarkko Sakkinen wrote:
> > > > Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> > > 
> > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > 
> > Thank you.
> > 
> > I applied this patch, will include it to the next PR.
> > 
> 
> Awesome. :)

NP, sorry for the latencies! 

Right now SGX is very near getting pulled. That might cause some
*temporary* latencies in my review queue during the phase of this
month and maybe early next month. After that things should return
to normal.

/Jarkko