tpm: eventlog: Replace zero-length array with flexible-array member

PATCH Applied Gustavo A. R. Silva 4 participants 7 messages May 8, 2020 – May 14, 2020

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Keys-trusted TPM device driver

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Gustavo A. R. Silva

author

2020-05-08 16:34 UTC #0 lore raw

PATCH v2 1/1 tpm: eventlog: Replace zero-length array with flexible-array member

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 <[email protected]>

---
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(-)

drivers/char/tpm/eventlog/tpm2.c +5 -7

index e741b1157525..37a05800980c 100644
@@ -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)

include/linux/tpm_eventlog.h +1 -1

index c253461b1c4e..4f8c90c93c29 100644
@@ -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 {

Signature · 1 line

2.26.2

Kees Cook

Reviewed-by on v2 1/1

2020-05-12 05:24 UTC #1 lore raw

Quoted 56 lines · 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.

Quoted 8 lines

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 <[email protected]>

Reviewed-by Kees Cook <[email protected]>

Signature · 1 line

Kees Cook

Gustavo A. R. Silva

2020-05-12 18:06 UTC #2 lore raw

Quoted 59 lines · Kees Cook

Quoted 58 lines · 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.

Quoted 9 lines

Quoted 6 lines
[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 <[email protected]>
Reviewed-by Kees Cook <[email protected]>

Thanks, Kees.

--
Gustavo

Jarkko Sakkinen

Applied

2020-05-14 00:09 UTC #3 lore raw

In reply to Kees Cook

Quoted 59 lines · Kees Cook

Quoted 58 lines · 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.

Quoted 8 lines

Quoted 6 lines
[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 <[email protected]>
Reviewed-by Kees Cook <[email protected]>

Thank you.

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

/Jarkko

Gustavo A. R. Silva

2020-05-14 00:19 UTC #4 lore raw

Quoted 9 lines · Jarkko Sakkinen

Quoted 4 lines
Quoted 2 lines
Signed-off-by Gustavo A. R. Silva <[email protected]>
Reviewed-by Kees Cook <[email protected]>
Thank you.
I applied this patch, will include it to the next PR.

Awesome. :)

Thanks, Jarkko.
--
Gustavo

Jarkko Sakkinen

2020-05-14 11:07 UTC #5 lore raw

Quoted 11 lines · Gustavo A. R. Silva

Quoted 8 lines · Jarkko Sakkinen
Quoted 3 lines
Quoted 1 line
Signed-off-by Gustavo A. R. Silva <[email protected]>
Reviewed-by Kees Cook <[email protected]>
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

Jarkko Sakkinen

Reviewed-by on v2 1/1

2020-05-13 23:17 UTC #6 lore raw

In reply to Gustavo A. R. Silva

Quoted 56 lines · 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.

Quoted 8 lines

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 <[email protected]>

Reviewed-by Jarkko Sakkinen <[email protected]>

/Jarkko

Quoted 59 lines

---
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(-)

drivers/char/tpm/eventlog/tpm2.c +5 -7

index e741b1157525..37a05800980c 100644
@@ -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)

include/linux/tpm_eventlog.h +1 -1

index c253461b1c4e..4f8c90c93c29 100644
@@ -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 {