Re: [PATCH v3] tpm: Actually fail on TPM errors during "get random"

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On 4/3/19 1:52 PM, Jarkko Sakkinen wrote:
On Tue, Apr 02, 2019 at 07:13:52PM +0000, Winkler, Tomas wrote:


On Tue, Apr 02, 2019 at 02:46:25AM +0300, Jarkko Sakkinen wrote:
On Mon, Apr 01, 2019 at 12:06:07PM -0700, Kees Cook wrote:
A "get random" may fail with a TPM error, but those codes were
returned as-is to the caller, which assumed the result was the
number of bytes that had been written to the target buffer, which
could lead to a kernel heap memory exposure and over-read.

This fixes tpm1_get_random() to mask positive TPM errors into -EIO,
as before.

[   18.092103] tpm tpm0: A TPM error (379) occurred attempting get
random
[   18.092106] usercopy: Kernel memory exposure attempt detected from
SLUB object 'kmalloc-64' (offset 0, size 379)!

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1650989
Reported-by: Phil Baker <baker1tex@xxxxxxxxx>
Reported-by: Craig Robson <craig@xxxxxxxxx>
Fixes: 7aee9c52d7ac ("tpm: tpm1: rewrite tpm1_get_random() using
tpm_buf structure")
Cc: Laura Abbott <labbott@xxxxxxxxxx>
Cc: Tomas Winkler <tomas.winkler@xxxxxxxxx>
Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v3: fix never-succeed, limit checks to tpm cmd return (James, Jason)
v2: also fix tpm2 implementation (Jason Gunthorpe)
---
  drivers/char/tpm/tpm1-cmd.c | 7 +++++--
drivers/char/tpm/tpm2-cmd.c | 7 +++++--
  2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/char/tpm/tpm1-cmd.c
b/drivers/char/tpm/tpm1-cmd.c index 85dcf2654d11..faacbe1ffa1a
100644
--- a/drivers/char/tpm/tpm1-cmd.c
+++ b/drivers/char/tpm/tpm1-cmd.c
@@ -510,7 +510,7 @@ struct tpm1_get_random_out {
   *
   * Return:
   * *  number of bytes read
- * * -errno or a TPM return code otherwise
+ * * -errno (positive TPM return codes are masked to -EIO)
   */
  int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)  {
@@ -531,8 +531,11 @@ int tpm1_get_random(struct tpm_chip *chip, u8
*dest, size_t max)

  		rc = tpm_transmit_cmd(chip, &buf, sizeof(out->rng_data_len),
  				      "attempting get random");
-		if (rc)
+		if (rc) {
+			if (rc > 0)
+				rc = -EIO;
  			goto out;
+		}

  		out = (struct tpm1_get_random_out
*)&buf.data[TPM_HEADER_SIZE];

diff --git a/drivers/char/tpm/tpm2-cmd.c
b/drivers/char/tpm/tpm2-cmd.c index e74c5b7b64bf..8ffa6af61580
100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -301,7 +301,7 @@ struct tpm2_get_random_out {
   *
   * Return:
   *   size of the buffer on success,
- *   -errno otherwise
+ *   -errno otherwise ((positive TPM return codes are masked to -EIO)
   */
  int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)  {
@@ -328,8 +328,11 @@ int tpm2_get_random(struct tpm_chip *chip, u8
*dest, size_t max)
  				       offsetof(struct tpm2_get_random_out,
  						buffer),
  				       "attempting get random");
-		if (err)
+		if (err) {
+			if (err > 0)
+				err = -EIO;
  			goto out;
+		}

  		out = (struct tpm2_get_random_out *)
  			&buf.data[TPM_HEADER_SIZE];
--
2.17.1


--
Kees Cook

Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>

Applied to my master branch. Jason, Tomas, do you want me to add reviewed-
by's?
Sure, it fixes my patch.

Great, I'll add it. Thank you. Just want to be explicit with these
things as I consider them as if I was asking a signature from someone
:-)

/Jarkko

Was this intended to go in for 5.2? I still don't see it in the tree.

Thanks,
Laura



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