tpm: verify that it's actually an event log header before parsing

PATCH Superseded Fabian Vogt 2 participants 4 messages Jun 5, 2020 – Jun 12, 2020

Subsystems (2)

EFI TPM device driver

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Fabian Vogt

author

2020-06-05 14:22 UTC #0 lore raw

PATCH 1/1 tpm: verify that it's actually an event log header before parsing

It's possible that the first event in the log is not actually a log header at all, but rather a normal event. This leads to the cast in __calc_tpm2_event_size being an invalid conversion, which means that the values read are effectively garbage. Depending on the first event's contents, this leads either to apparently normal behaviour, a crash or a freeze.

While this behaviour of the firmware is not in accordance with the TCG Client EFI Specification, this happens on a Dell Precision 5510 with the TPM enabled but hidden from the OS ("TPM On" disabled, state otherwise untouched). The EFI claims that the TPM is present and active
and supports the TCG 2.0 event log format.

Fortunately, this can be worked around by simply checking the header of the first event and the event log header signature itself.

Commit b4f1874c6216 ("tpm: check event log version before reading final events") addressed a similar issue also found on Dell models.

Fixes 6b0326190205 ("efi: Attempt to get the TCG2 event log in the boot stub")

Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1165773

Signed-off-by Fabian Vogt <[email protected]>

---
 include/linux/tpm_eventlog.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

include/linux/tpm_eventlog.h +13 -1

index 4f8c90c93c29..b913faeedcb5 100644
@@ -81,6 +81,8 @@ struct tcg_efi_specid_event_algs {
 	u16 digest_size;
 } __packed;
 
+#define TCG_SPECID_SIG "Spec ID Event03"
+
 struct tcg_efi_specid_event_head {
 	u8 signature[16];
 	u32 platform_class;

@@ -171,6 +173,7 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
 	int i;
 	int j;
 	u32 count, event_type;
+	const u8 zero_digest[sizeof(event_header->digest)] = {0};
 
 	marker = event;
 	marker_start = marker;

@@ -198,10 +201,19 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
 	count = READ_ONCE(event->count);
 	event_type = READ_ONCE(event->event_type);
 
+	/* Verify that it's the log header */
+	if (READ_ONCE(event_header->pcr_idx) != 0 ||
+	    READ_ONCE(event_header->event_type) != NO_ACTION ||
+	    memcmp(event_header->digest, zero_digest, sizeof(zero_digest))) {
+		size = 0;
+		goto out;
+	}
+
 	efispecid = (struct tcg_efi_specid_event_head *)event_header->event;
 
 	/* Check if event is malformed. */
-	if (count > efispecid->num_algs) {
+	if (memcmp(efispecid->signature, TCG_SPECID_SIG,
+		   sizeof(TCG_SPECID_SIG)) || count > efispecid->num_algs) {
 		size = 0;
 		goto out;
 	}

Signature · 1 line

2.25.1

Ard Biesheuvel

maintainer

2020-06-12 11:01 UTC #1 lore raw

Quoted 49 lines · Fabian Vogt

It's possible that the first event in the log is not actually a log header at all, but rather a normal event. This leads to the cast in __calc_tpm2_event_size being an invalid conversion, which means that the values read are effectively garbage. Depending on the first event's contents, this leads either to apparently normal behaviour, a crash or a freeze.
While this behaviour of the firmware is not in accordance with the TCG Client EFI Specification, this happens on a Dell Precision 5510 with the TPM enabled but hidden from the OS ("TPM On" disabled, state otherwise untouched). The EFI claims that the TPM is present and active
and supports the TCG 2.0 event log format.
Fortunately, this can be worked around by simply checking the header of the first event and the event log header signature itself.
Commit b4f1874c6216 ("tpm: check event log version before reading final events") addressed a similar issue also found on Dell models.
Fixes 6b0326190205 ("efi: Attempt to get the TCG2 event log in the boot stub")
Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1165773
Signed-off-by Fabian Vogt <[email protected]>
---
 include/linux/tpm_eventlog.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)
include/linux/tpm_eventlog.h +3 -0
index 4f8c90c93c29..b913faeedcb5 100644
@@ -81,6 +81,8 @@ struct tcg_efi_specid_event_algs {
        u16 digest_size;
 } __packed;
 
+#define TCG_SPECID_SIG "Spec ID Event03"
+
 struct tcg_efi_specid_event_head {
        u8 signature[16];
        u32 platform_class;
@@ -171,6 +173,7 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
        int i;
        int j;
        u32 count, event_type;
+       const u8 zero_digest[sizeof(event_header->digest)] = {0};
 
        marker = event;
        marker_start = marker;

include/linux/tpm_eventlog.h static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event, · line 206

Quoted 6 lines

include/linux/tpm_eventlog.h +3 -0

@@ -198,10 +201,19 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
        count = READ_ONCE(event->count);
        event_type = READ_ONCE(event->event_type);
 
+       /* Verify that it's the log header */
+       if (READ_ONCE(event_header->pcr_idx) != 0 ||
+           READ_ONCE(event_header->event_type) != NO_ACTION ||

Is the READ_ONCE() necessary here?

include/linux/tpm_eventlog.h static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event, · line 216

Quoted 11 lines

include/linux/tpm_eventlog.h +7 -1

+           memcmp(event_header->digest, zero_digest, sizeof(zero_digest))) {
+               size = 0;
+               goto out;
+       }
+
        efispecid = (struct tcg_efi_specid_event_head *)event_header->event;
 
        /* Check if event is malformed. */
-       if (count > efispecid->num_algs) {
+       if (memcmp(efispecid->signature, TCG_SPECID_SIG,
+                  sizeof(TCG_SPECID_SIG)) || count > efispecid->num_algs) {

So is it guaranteed that every well formed event starts with TCG_SPECID_SIG?

Quoted 9 lines

include/linux/tpm_eventlog.h +0 -1

                size = 0;
                goto out;
        }
--

2.25.1

Fabian Vogt

author

2020-06-12 11:58 UTC #2 lore raw

Am Freitag, 12. Juni 2020, 13:01:33 CEST schrieb Ard Biesheuvel:

Quoted 53 lines · Fabian Vogt

Quoted 52 lines
It's possible that the first event in the log is not actually a log header at all, but rather a normal event. This leads to the cast in __calc_tpm2_event_size being an invalid conversion, which means that the values read are effectively garbage. Depending on the first event's contents, this leads either to apparently normal behaviour, a crash or a freeze.
While this behaviour of the firmware is not in accordance with the TCG Client EFI Specification, this happens on a Dell Precision 5510 with the TPM enabled but hidden from the OS ("TPM On" disabled, state otherwise untouched). The EFI claims that the TPM is present and active
and supports the TCG 2.0 event log format.
Fortunately, this can be worked around by simply checking the header of the first event and the event log header signature itself.
Commit b4f1874c6216 ("tpm: check event log version before reading final events") addressed a similar issue also found on Dell models.
Fixes 6b0326190205 ("efi: Attempt to get the TCG2 event log in the boot stub")
Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1165773
Signed-off-by Fabian Vogt <[email protected]>
---
 include/linux/tpm_eventlog.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)
include/linux/tpm_eventlog.h +3 -0
index 4f8c90c93c29..b913faeedcb5 100644
@@ -81,6 +81,8 @@ struct tcg_efi_specid_event_algs {
        u16 digest_size;
 } __packed;
 
+#define TCG_SPECID_SIG "Spec ID Event03"
+
 struct tcg_efi_specid_event_head {
        u8 signature[16];
        u32 platform_class;
@@ -171,6 +173,7 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
        int i;
        int j;
        u32 count, event_type;
+       const u8 zero_digest[sizeof(event_header->digest)] = {0};
 
        marker = event;
        marker_start = marker;
@@ -198,10 +201,19 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
        count = READ_ONCE(event->count);
        event_type = READ_ONCE(event->event_type);
 

Quoted 5 lines

include/linux/tpm_eventlog.h static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event, · line 206
Quoted 3 lines
include/linux/tpm_eventlog.h +3 -0
+       /* Verify that it's the log header */
+       if (READ_ONCE(event_header->pcr_idx) != 0 ||
+           READ_ONCE(event_header->event_type) != NO_ACTION ||
Is the READ_ONCE() necessary here?

I'm actually not sure about that. What's confusing me is that both event and event_header are pointers returned from EFI calls, but only event is passed through TPM_MEMREMAP if do_mapping is true. event_header is directly dereferenced without any mapping in any case. I assume it is an unwritten precondition that event_header is a pointer to mapped memory already? In that case READ_ONCE might be unnecessary indeed.

Quoted 7 lines

include/linux/tpm_eventlog.h +5 -0

+           memcmp(event_header->digest, zero_digest, sizeof(zero_digest))) {
+               size = 0;
+               goto out;
+       }
+
        efispecid = (struct tcg_efi_specid_event_head *)event_header->event;

Quoted 6 lines

include/linux/tpm_eventlog.h static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event, · line 216
Quoted 4 lines
include/linux/tpm_eventlog.h +2 -1
        /* Check if event is malformed. */
-       if (count > efispecid->num_algs) {
+       if (memcmp(efispecid->signature, TCG_SPECID_SIG,
+                  sizeof(TCG_SPECID_SIG)) || count > efispecid->num_algs) {
So is it guaranteed that every well formed event starts with TCG_SPECID_SIG?

I assume you mean "event log header" here.
This is defined by the TCG EFI Protocol Spec in 5.3 Event Log Header:

A parser may read the first event [...]. The fields of the event log header are defined to be PCRIndex of 0, EventType of EV_NO_ACTION, Digest of 20 bytes of 0, and Event content defined as TCG_EfiSpecIDEventStruct. This first event is the event log header.

Table 3 then says about the first 16 bytes, the signature field:

The null terminated ASCII string “Spec ID Event03”.
SHALL be set to {0x53, 0x70, 0x65, 0x63, 0x20, 0x49, 0x44, 0x20, 0x45, 0x76, 0x65, 0x6e, 0x74, 0x30, 0x33, 0x00}.

It references the TCG PC Client Specific TIS, where it says about Table 5:

The 16 bytes Signature for the TCG_EfiSpecIdEvent as defined in this specification is the NUL-terminated ASCII string “Spec ID Event03”. Based on this string an evaluator can determine that this log is formatted as crypto agile log and that the TCG_EfiSpecIdEvent contains information about recorded digests.

In the case that the event header indicates that it is the log header, it is still possible that this header is in a format which is not compatible with struct tcg_efi_specid_event_head. Verification of this signature makes sure that it is. The way I read the PC Client Specific TIS, only for this specific string it is guaranteed that it contains the desired information.

Quoted 5 lines

include/linux/tpm_eventlog.h +0 -1

                size = 0;
                goto out;
        }
--

2.25.1

Fabian Vogt

author

2020-06-12 12:16 UTC #3 lore raw

Am Freitag, 12. Juni 2020, 13:58:51 CEST schrieb Fabian Vogt:

Quoted 60 lines

Am Freitag, 12. Juni 2020, 13:01:33 CEST schrieb Ard Biesheuvel:
Quoted 53 lines · Fabian Vogt
Quoted 52 lines
It's possible that the first event in the log is not actually a log header at all, but rather a normal event. This leads to the cast in __calc_tpm2_event_size being an invalid conversion, which means that the values read are effectively garbage. Depending on the first event's contents, this leads either to apparently normal behaviour, a crash or a freeze.
While this behaviour of the firmware is not in accordance with the TCG Client EFI Specification, this happens on a Dell Precision 5510 with the TPM enabled but hidden from the OS ("TPM On" disabled, state otherwise untouched). The EFI claims that the TPM is present and active
and supports the TCG 2.0 event log format.
Fortunately, this can be worked around by simply checking the header of the first event and the event log header signature itself.
Commit b4f1874c6216 ("tpm: check event log version before reading final events") addressed a similar issue also found on Dell models.
Fixes 6b0326190205 ("efi: Attempt to get the TCG2 event log in the boot stub")
Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1165773
Signed-off-by Fabian Vogt <[email protected]>
---
 include/linux/tpm_eventlog.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)
include/linux/tpm_eventlog.h +3 -0
index 4f8c90c93c29..b913faeedcb5 100644
@@ -81,6 +81,8 @@ struct tcg_efi_specid_event_algs {
        u16 digest_size;
 } __packed;
 
+#define TCG_SPECID_SIG "Spec ID Event03"
+
 struct tcg_efi_specid_event_head {
        u8 signature[16];
        u32 platform_class;
@@ -171,6 +173,7 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
        int i;
        int j;
        u32 count, event_type;
+       const u8 zero_digest[sizeof(event_header->digest)] = {0};
 
        marker = event;
        marker_start = marker;
@@ -198,10 +201,19 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
        count = READ_ONCE(event->count);
        event_type = READ_ONCE(event->event_type);
 
Quoted 5 lines
include/linux/tpm_eventlog.h static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event, · line 206
Quoted 3 lines
include/linux/tpm_eventlog.h +3 -0
+       /* Verify that it's the log header */
+       if (READ_ONCE(event_header->pcr_idx) != 0 ||
+           READ_ONCE(event_header->event_type) != NO_ACTION ||
Is the READ_ONCE() necessary here?

Quoted 6 lines

I'm actually not sure about that. What's confusing me is that both event and event_header are pointers returned from EFI calls, but only event is passed through TPM_MEMREMAP if do_mapping is true. event_header is directly dereferenced without any mapping in any case. I assume it is an unwritten precondition that event_header is a pointer to mapped memory already? In that case READ_ONCE might be unnecessary indeed.

Nevermind, I just missed VVVVVVVVV
@do_mapping: Whether or not the event needs to be mapped

Will send V2 without READ_ONCE if the rest of the patch is fine.

Quoted 47 lines

Quoted 7 lines
Quoted 7 lines
include/linux/tpm_eventlog.h +5 -0
+           memcmp(event_header->digest, zero_digest, sizeof(zero_digest))) {
+               size = 0;
+               goto out;
+       }
+
        efispecid = (struct tcg_efi_specid_event_head *)event_header->event;
 
Quoted 6 lines
include/linux/tpm_eventlog.h static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event, · line 216
Quoted 4 lines
include/linux/tpm_eventlog.h +2 -1
        /* Check if event is malformed. */
-       if (count > efispecid->num_algs) {
+       if (memcmp(efispecid->signature, TCG_SPECID_SIG,
+                  sizeof(TCG_SPECID_SIG)) || count > efispecid->num_algs) {
So is it guaranteed that every well formed event starts with TCG_SPECID_SIG?
I assume you mean "event log header" here.
This is defined by the TCG EFI Protocol Spec in 5.3 Event Log Header:
A parser may read the first event [...]. The fields of the event log header are defined to be PCRIndex of 0, EventType of EV_NO_ACTION, Digest of 20 bytes of 0, and Event content defined as TCG_EfiSpecIDEventStruct. This first event is the event log header.
Table 3 then says about the first 16 bytes, the signature field:
The null terminated ASCII string “Spec ID Event03”.
SHALL be set to {0x53, 0x70, 0x65, 0x63, 0x20, 0x49, 0x44, 0x20, 0x45, 0x76, 0x65, 0x6e, 0x74, 0x30, 0x33, 0x00}.
It references the TCG PC Client Specific TIS, where it says about Table 5:
The 16 bytes Signature for the TCG_EfiSpecIdEvent as defined in this specification is the NUL-terminated ASCII string “Spec ID Event03”. Based on this string an evaluator can determine that this log is formatted as crypto agile log and that the TCG_EfiSpecIdEvent contains information about recorded digests.
In the case that the event header indicates that it is the log header, it is still possible that this header is in a format which is not compatible with struct tcg_efi_specid_event_head. Verification of this signature makes sure that it is. The way I read the PC Client Specific TIS, only for this specific string it is guaranteed that it contains the desired information.
Quoted 5 lines
Quoted 5 lines
include/linux/tpm_eventlog.h +0 -1
                size = 0;
                goto out;
        }
--
2.25.1